Mastering Rebreathers Second Ed

Table of contents :
Dedication......Page 12
Preface......Page 13
Foreword (First Edition)......Page 19
Foreword (Second Edition)......Page 23
Acknowledgements......Page 26
Credits (First Edition)......Page 30
Credits (Second Edition)......Page 34
About the Author......Page 39
Your Responsibilities as a Rebreather Diver......Page 42
Disclaimer......Page 43
chapter 1: Introduction to Rebreathers......Page 44
Why Rebreathers?......Page 46
What is a Rebreather?......Page 49
Rebreather Training......Page 55
Evolving Rebreather Standards......Page 62
Chapter 2: History of Rebreathers......Page 68
Early Concepts......Page 69
Rebreathers at War......Page 74
Post War Developments......Page 81
Commercial Applications......Page 86
Recreational Market Developments......Page 91
Chapter 3: Types of Rebreathers......Page 102
What is a Rebreather?......Page 103
Oxygen Closed-Circuit Rebreathers......Page 104
Constant Mass Flow (Active SCR)......Page 106
Respiratory Minute Volume Keyed (Passive SCR, Quasi-Constant FO2)......Page 108
Constant Ratio (Quasi-Constant PO2)......Page 109
Mixed-Gas Closed -Circuit Rebreathers......Page 110
Choosing a Rebreather- Criteria......Page 113
Chapter 4: Diving Physics......Page 125
Pressure and Depth......Page 126
Pressure and Volume......Page 130
Breathing Gas Composition......Page 132
Maximum Operating Depth......Page 134
Equivalent Air Depth......Page 136
Gas Consumption......Page 137
Cylinder Gas Volumes......Page 143
Summary......Page 146
Chapter 5: Physiology......Page 153
Oxygen Metabolism......Page 155
Oxygen Exposure......Page 157
Hypoxia......Page 160
CNS Oxygen Toxicity......Page 163
Pulmonary Oxygen Toxicity......Page 170
Hyperoxic Myopia......Page 176
Hypercapnia......Page 177
Decompression Sickness......Page 179
Inert Gas Narcosis......Page 180
Arterial Gas Embolism......Page 182
Squeezes......Page 183
Middle Ear Oxygen Absorption Syndrome......Page 185
Disease Transmission......Page 189
Allergic Reactions......Page 190
Kinesthetic Considerations......Page 192
Psychological Considerations......Page 194
Chapter 6: Theory......Page 202
Mechanics and Oxygen Addition......Page 205
Depth Limits and Dive Tables......Page 209
Mechanics......Page 210
Oxygen Addition......Page 212
Depth Limits......Page 216
Dive Tables......Page 220
Mechanics......Page 222
Oxygen Addition......Page 225
Depth Limits......Page 233
Dive Tables......Page 234
Mechanics......Page 237
Oxygen Addition......Page 240
Depth Limits......Page 248
Dive Tables......Page 253
Constant PO2 Tables......Page 259
Absorbents......Page 266
Chapter 7: Rebreather Design......Page 280
Scrubber Canisters......Page 282
Counterlungs......Page 287
Water Traps......Page 291
Mouthpieces......Page 294
Bailout Valves......Page 300
Automatic Diluent Valve......Page 301
Automatic Diluent Bailout Valve......Page 302
Hoses......Page 303
Gas Supply Apparatus......Page 305
Gas Supply Cylinders......Page 306
Absorbent......Page 307
Oxygen Sensors......Page 314
Displays......Page 320
Carbon Dioxide Monitoring......Page 323
Full-Face Masks......Page 324
Communication Equipment......Page 325
Dive Computers......Page 327
Case, Harness, and Buoyancy Compensator......Page 330
Chapter 8: Preparing for the Dive......Page 337
Pre-Dive Checks......Page 339
Canister Filling......Page 340
Gas Supply System......Page 346
Electronics......Page 351
Pressure Checks......Page 354
General......Page 356
Altitude Diving......Page 357
Dive Planning......Page 358
Diving with an OC Buddy......Page 360
Immediate Pre-Dive Checks......Page 362
Chapter 9: Diving Techniques......Page 371
Mouthpiece Protocol......Page 373
Surface Use......Page 375
Gas Supply......Page 376
Buoyancy and Trim......Page 377
Mouthpiece Clearing......Page 381
Communication......Page 383
Gauge Monitoring......Page 391
Counterlung Gas Addition......Page 392
Nose Breathing......Page 393
Reflux......Page 394
Bubble Checks......Page 395
Work Rates......Page 397
Ascent......Page 398
Chapter 10: Post-Dive Procedures......Page 405
Cleaning and Disinfecting......Page 406
Scrubber Canister......Page 411
Electronics......Page 412
Post Dive Maintenance......Page 415
Log Your Dive......Page 416
Summary......Page 418
Chapter 11: Long-Term Maintenance......Page 424
Monthly Maintenance......Page 426
Annual Maintenance......Page 430
Longer-Term Maintenance......Page 432
Storage......Page 434
Chapter 12: Emergency Procedures......Page 441
Philosophies of Emergencies......Page 443
Loss of Gas Supply......Page 445
Hypoxia......Page 447
Decompression Illness......Page 448
Scrubber Failure......Page 449
Oxygen Toxicity......Page 450
Flooding......Page 451
Caustic Cocktail......Page 454
Oxygen Sensor Failure......Page 457
Buoyancy Loss......Page 458
Bailout......Page 460
SCR Mode......Page 461
Allergic Reactions......Page 462
Chapter 13: Travel......Page 469
Travel by Automobile......Page 471
Travel by Boat......Page 472
Travel by Plane......Page 475
Travel in a Post-9/11 World......Page 478
While Away From Home......Page 482
International Travel......Page 484
Common Sense......Page 486
Chapter 14: Where Do You Go From Here?......Page 491
Keep Diving......Page 492
Future Training......Page 497
Mastering Rebreathers, Volume 2......Page 499
Staying Current......Page 500
The Adventure Continues......Page 503
Rebreather Supporters......Page 505
Appendices......Page 533
Appendix A Dive Tables......Page 536
Appendix B AP Diving......Page 624
Appendix C B & E Manufacturing......Page 646
Appendix D Dive Rite......Page 654
Appendix E Dräger......Page 674
Appendix F Halcyon......Page 733
Appendix G Hollis......Page 748
Appendix H Hydrospace Engineering......Page 759
Appendix I Innerspace Systems......Page 764
Appendix J Jetsam Technology......Page 771
Appendix K MDEA......Page 801
Appendix L Poseidon......Page 813
Appendix M Rebreathers Australia......Page 828
Appendix N rEvo Rebreathers......Page 833
Appendix O Titan Dive Gear......Page 855
Appendix P VR Technology......Page 873
Appendix Q Documentation/Regulation......Page 877
Appendix R References......Page 925
Appendix S Glossary......Page 930
Appendix T Index......Page 965

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Cover photograph credits: Front, Jeffrey Bozanic Back, Author (upper left), Eric Hanauer Back (upper right), Jeffrey Bozanic Back (lower right), Sten Johansson Copyright © 2010 by Best Publishing Company All Rights Reserved While the author believes that the information and guidance given in this work is correct, all parties must rely on their own skill and judgment when making use of it. The author and publisher assume no liability to anyone for any loss, damage, or injury caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed. Apart from any fair dealing for the purposes of research and private study, or criticism or review, as permitted under the Copyright, Designs, and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any forms or by any means, only with prior permission in writing from the publisher. Inquiries concerning reproduction outside these terms should be sent to the publisher at the address below. ISBN 978-1-930536-57-9 Library of Congress Catalog Number 00-110676

Best Publishing Company 2355 North Steves Boulevard P.O. Box 30100 Flagstaff, AZ 86003-0100 USA Email Address: [email protected]

Contents Dedication Preface Foreword (First Edition) Foreword (Second Edition) Acknowledgements Credits (First Edition) Credits (Second Edition) About the Author Your Responsibilities as a Rebreather Diver Disclaimer

Chapter 1 Introduction to Rebreathers Why Rebreathers? What is a Rebreather? Rebreather Training Evolving Rebreather Standards Chapter 2 History of Rebreathers Early Concepts Rebreathers at War Post War Developments Commercial Applications Recreational Market Developments Chapter 3 Types of Rebreathers What is a Rebreather? Oxygen Closed-Circuit Rebreathers Semi-Closed Circuit Rebreathers Constant Mass Flow (Active SCR)

Respiratory Minute Volume Keyed (Passive SCR, QuasiConstant FO2) Constant Ratio (Quasi-Constant PO2) Electronically Controlled Longer, Quieter Dives Mixed-Gas Closed -Circuit Rebreathers Choosing a Rebreather- Criteria Chapter 4 Diving Physics Pressure and Depth Pressure and Volume Breathing Gas Composition Maximum Operating Depth Equivalent Air Depth Gas Consumption Cylinder Gas Volumes Summary Chapter 5 Physiology Oxygen Metabolism Oxygen Exposure Hypoxia CNS Oxygen Toxicity Pulmonary Oxygen Toxicity Hyperoxic Myopia Hypercapnia Decompression Sickness Inert Gas Narcosis Arterial Gas Embolism Squeezes Middle Ear Oxygen Absorption Syndrome Disease Transmission

Allergic Reactions Carbon Monoxide Kinesthetic Considerations Psychological Considerations Chapter 6 Theory Oxygen Closed-Circuit Rebreathers Mechanics and Oxygen Addition Depth Limits and Dive Tables Mass Flow Controlled Semi-Closed Rebreathers Mechanics Oxygen Addition Depth Limits Dive Tables RMV-Keyed Semi-Closed Rebreathers Mechanics Oxygen Addition Depth Limits Dive Tables Mixed-Gas Closed-Circuit Rebreathers Mechanics Oxygen Addition Depth Limits Dive Tables Constant PO2 Tables Absorbents Chapter 7 Rebreather Design Scrubber Canisters Counterlungs Water Traps

Mouthpieces Bailout Valves Automatic Diluent Valve Automatic Diluent Bailout Valve Hoses Gas Supply Apparatus Gas Supply Cylinders Absorbent Lubricants Oxygen Sensors Displays Carbon Dioxide Monitoring Full-Face Masks Communication Equipment Dive Computers Case, Harness, and Buoyancy Compensator Chapter 8 Preparing for the Dive Pre-Dive Checks Canister Filling Gas Supply System Electronics Buoyancy System Pressure Checks General Altitude Diving Dive Planning Diving with an OC Buddy Immediate Pre-Dive Checks Chapter 9 Diving Techniques Mouthpiece Protocol

Surface Use Gas Supply Descent Buoyancy and Trim Mouthpiece Clearing Mouthpiece Recovery Communication Gauge Monitoring Counterlung Gas Addition Nose Breathing Reflux Mask Clearing Bubble Checks Work Rates Ascent Chapter 10 Post-Dive Procedures Cleaning and Disinfecting Scrubber Canister Electronics Reassembly Bailout Systems Post-Dive Barotrauma Post Dive Maintenance Log Your Dive Summary Chapter 11 Long-Term Maintenance Why More Frequent Maintenance Monthly Maintenance Annual Maintenance Longer-Term Maintenance

Storage Chapter 12 Emergency Procedures Philosophies of Emergencies Loss of Gas Supply Hypoxia Decompression Illness Scrubber Failure Oxygen Toxicity Flooding Caustic Cocktail Electronics Failure Oxygen Sensor Failure Buoyancy Loss Bailout SCR Mode Allergic Reactions Chapter 13 Travel General Travel Hints Travel by Automobile Travel by Boat Travel by Plane Travel in a Post-9/11 World While Away From Home International Travel Common Sense Chapter 14 Where Do You Go From Here? Keep Diving Future Training Mastering Rebreathers, Volume 2 Staying Current

The Adventure Continues Rebreather Supporters Appendices Appendix A Dive Tables Appendix B AP Diving Appendix C B & E Manufacturing Appendix D Dive Rite Appendix E Dräger Appendix F Halcyon Appendix G Hollis Appendix H Hydrospace Engineering Appendix I Innerspace Systems Appendix J Jetsam Technology Appendix K MDEA Appendix L Poseidon Appendix M Rebreathers Australia Appendix N rEvo Rebreathers Appendix O Titan Dive Gear Appendix P VR Technology Appendix Q Documentation/Regulation Appendix R References Appendix S Glossary Appendix T Index

Preface Why a second edition? This text was written to help educate divers and non-divers alike about rebreathers and their use. Technology and its availability have changed tremendously in the past decade. One of the most significant changes is that rebreathers no longer interest just the "out-on-the-edge" technical diver, but appeal to the average advanced recreational diver. I have even had non-divers approach me to begin their dive training not on open circuit equipment, but using a rebreather. I see this trend as continuing to grow, necessitating a different mind-set with regards to rebreather education. Likewise, in the time since the first edition was published, I have found that some content was insufficiently explored, omitted, or just did not exist at the time. Some information and recommendations have changed. I have augmented the subject matter of this text where it was indicated. Mastering Rebreathers is a comprehensive text covering the major aspects of using rebreathers for recreational diving. The chapters cover types and history of rebreathers; diving physics and physiology as they apply to rebreathers; theory as it pertains to different general designs of units; pre-dive, dive, post-dive and maintenance procedures; and emergency procedures used with rebreathers. Considerations for traveling with this equipment are also discussed. This text was written as a training aid for the student enrolled in a rebreather diving course. Many training agencies have a variety of rebreather courses; some are discussed in Chapter 1. It is not meant to be a self-study course. Any person wishing to learn to dive using rebreathers MUST do so from an approved rebreather instructor! No text can replace the knowledge and expertise of a competent instructor.

Each chapter begins with a list of specific learning objectives. These provide your "road map," telling you what you will be learning in the chapter. Read them before and after the chapter, to ensure you have obtained the desired knowledge. Each chapter ends with a series of review questions. The answers to those questions are provided at the bottom of the page, printed upside down. Read and answer the questions, then check your responses for accuracy (don't peek!). If you miss any, re-read the part of the chapter that discusses that information. While the text has been written in a logical, sequential manner (or at least one that seemed that way to me!), your instructor may structure your rebreather course differently. Accordingly, your instructor may have you read the chapters in a non-linear order, to match the course. While the text may be read in any order, some terms are presented and defined in earlier chapters, and used in later chapters assuming that the reader understands the terms. A glossary is included after the main text body to assist you in identifying terms or acronyms that you may have missed, or do not remember. You will note frequent references to other parts of the text, which allow intra-text reference, without digression from main points. These assist in aiding the reader who is not reading the text sequentially. New terms are introduced in bold face type. Finally, sidebars set aside with a colored background describe actual diving experiences, which illustrate or emphasize points under discussion. Your instructor may also have you skip certain portions of the text that may not pertain to the units upon which you will be trained. Chapter 6 in particular covers multiple types of rebreathers. It is likely that you will be asked to read only two parts of this chapter: the section dealing with your rebreather design, and the part on absorbents. The questions for that chapter are also divided into sections, so that you are checked only on the rebreather design you are using. It is OK to skip those design sections that you will not be using.

The appendices provide detailed information that will benefit the user. Material Safety Data Sheets and excerpts from the Code of Federal Regulations contain details on handling carbon dioxide absorbents and some rebreather components. While these were current at the time the manuscript was written, they are subject to change. Thus, the reader is cautioned not to rely on this data, but to contact the manufacturer and/or the appropriate governing body for current information and regulations as needed. Other appendices provide summary checklists for specific rebreathers. Detailed pre- and post-dive procedures accompany some of these checklists. All of the currently operating rebreather manufacturers listed in Table 2.1 were asked to supply checklists, procedures, and photographs for this volume. Many manufacturers declined to respond or participate. I included whatever information I was sent, in as much detail as provided. Thus, the level of detail in these appendices is variable, ranging from nothing to quite explicit instructions. These appendices are provided to assist the diver in using their rebreather. Again, while the information was current at the time of submission, manufacturers may change their units or procedures. Any changes may invalidate portions or all of the information presented in these appendices. In all cases, rely on the information provided by the manufacturer of your rebreather for proper use of your system. In some portions of the text, occasional references to gender are made as "he." This is not meant to imply that only males may dive, or dive using rebreathers. It is meant as a universal reference to both males and females, as the English language has no commonly held convention for a general, non-gender specific pronoun (nobody wants to be an "it!"). I apologize in advance if this distresses anyone. All quantities and measurements used in the text are imperial units, with metric equivalents provided in parentheses following. In some cases, the equivalent values may be rounded to facilitate ease in reading and comprehension. When cylinder volumes are cited, the imperial convention of furnishing the released gas volume is used in

the primary text, while the European convention of providing internal cylinder capacity is noted in the following parentheses. Other texts refer to individual breathing consumption rates as SCR, or surface consumption rate. In this text, SCR is used as an abbreviation for semi-closed circuit rebreather. Therefore, I have taken the liberty of referring to individual gas consumption rates as SAC, or surface air consumption. The abbreviation atm is used for atmospheres when discussing partial pressures of specific gases, or gauge pressures. Other texts use the convention of discussing pressure in terms of ATMabs, or atmospheres absolute. This text uses ata to specify the same unit. The nearest metric equivalent is bar. While one bar is not exactly equal to an atmosphere, it is used in this manner in this text to increase comprehension for those readers familiar with the unit. There are many possible ways to portray some of the equations and variables used to describe diving physics and physiology. All may be correct. In the first edition of this text, I opted to use the conventions and presentation that seemed to make the most sense. In the intervening years of instruction since the publication of that edition, I have found that comprehension was improved if I used a different presentation of the equations or variables for some topics. Thus, this edition has been modified based on that experience. Readers who are familiar with the content of the first edition may find that while the theoretical concepts have not changed, the arrangement of some of the equations has. You may notice in the text references to Mastering Rebreathers, Volume 2. This is a work-in progress. It will cover advanced rebreather usage, such as for helium-based (trimix) diluents, decompression diving, and other overhead environments like caves, wrecks, and ice. While it is not available as of the time of the initial publication of this edition of Mastering Rebreathers, it does exist in partially competed manuscript form, and will hopefully be available soon.

Many different individuals have reviewed this text on many separate occasions. Even so, it is likely that minor errors have not been identified. The fault for any such errors is entirely mine, even though the credit for this book belongs to all who helped with its writing and production. If you locate such an error, I would appreciate your assistance. Please send any comments and feedback to me in care of Best Publishing Company at the address on the publisher's title page, or via e-mail to [email protected]. Any questions you have may be addressed in the same manner. As mentioned previously, new information, procedures, rebreathers, and rebreather accessories are being introduced, almost monthly it seems. It is impossible to delay publication of this text to allow incorporating this information as rapidly as the field is changing. Therefore, it is not too soon to begin work on the third edition! If you have suggestions or feedback that might be of interest, please send it to be via the methods provided in the previous paragraph. In the near future, it is possible that Best Publishing Company will have an electronic version of this text. If so, it is anticipated that updated information will be published and provided to registered users of that text as a service once it becomes available.

Many rebreather divers have contributed their experience in helping make this book a reality...Thank you! Jeff Bozanic Photo by Dennis Ratcliffe

Foreword (First Edition) I first met Jeff Bozanic in 1991, at the 11th annual scientific diving symposium of the American Academy of Underwater Sciences, which was held in Honolulu. Even before the meeting began, Jeff's name caught my eye on the schedule of presentations for that symposium, because his was one of the only names that appeared on more than one presentation. In fact, Jeff presented three different talks, on three different topics! I thought to myself, "Who is this guy?" As I got to know him better over the years (when we'd see each other at various meetings, or when he'd come through Hawaii on his way to some more exotic destination), I couldn't help but admire (and envy) the broad spectrum of diving projects that Jeff had been involved with: cave exploration, diving under Antarctic ice, collecting algae at remote islands, underwater archaeological projects, leading groups to spectacular dive destinations, mixed-gas diving, rebreather diving, dive instruction of all sorts....the list goes on and on. If I had to choose one word to describe Jeff in the context of diving, that word would have to be "dedicated." Jeff is dedicated to diving at all levels, ranging from his own diving to his enthusiasm to share information about diving with others. An example of Jeff's dedication to his personal diving that comes to mind happened during one of his visits to Hawaii a number of years ago. I took Jeff for a few dives on the prototype Cis-Lunar MK4P rebreather, which I was using at the time. On the last of these dives, Jeff wanted me to take a picture of him interacting with a large moray eel. He was "twiddling" the fingers of his left hand mere inches in front of the eel's mouth, and as I was adjusting the settings on my camera I thought to myself, "Be careful, Jeff. Those things do bite..." Sure enough, when I looked up from my camera, I saw Jeff clutching his hands to his chest amid a big green cloud of blood. When he showed me the gaping wound on his finger, I failed to notice that the bite was on his right hand – while he was engaging the first eel, a second eel had come up and bit his other hand while he wasn't watching! I assumed by the severity of the bite and the

amount of blood in the water that he would want to end the dive; but when I suggested that we surface, he shrugged and indicated that he would prefer to continue the dive, if that was OK by me. While that alone is impressive, the real sign of dedication came the next day. I was too busy to dive that day, but there was a full set of double tanks in my garage. Rather than relax and recuperate from the previous day's ordeal, Jeff asked if he could use the doubles to make a dive in front of my house (in Kailua Bay on Oahu). Despite my warnings that there was nothing to see but sand out there, Jeff nevertheless humped the doubles down the quarter-mile walk to the beach, got in the water, swam for 45 minutes straight seaward over barren nothingness, only to be rewarded by one small rock with a few fish at the grand depth of about 25 feet. After another 45-minute swim back to the beach, and the long walk (with heavy tanks) back to my house, finger wrapped in a sodden bandage all the while, Jeff still returned with a big grin on his face. If that's not a sign of being dedicated to diving, I don't know what is! To see an example of Jeff's enthusiasm for sharing information about diving, one need look no further than the pages of this book. An experienced and talented writer, his style is characterized by clarity and eloquence. Complex concepts about rebreathers are explained in a straightforward, logical way that would make sense to any diver (or non-diver, for that matter). Moreover, he's managed to assemble a broad spectrum of information related to rebreather diving without succumbing to the omnipresent "politics" inherent to any emerging technology (or, in this case, emerging popular application of an old technology). Jeff has been careful to afford "equal time" to many different types and brands of rebreathers, objectively describing the relative costs and benefits of each. I admire this book because it leaves the reader with the impression that different types and brands of rebreathers are to be viewed as different tools, optimally suited for different tasks and by different sorts of divers. This is, in my opinion, an accurate representation of reality.

Rebreathers are an exciting development in recreational diving. They allow for enormously increased breathing gas efficiency (especially as the depth increases); they offer a quieter underwater experience (to a greater or lesser extent, depending on the specific type of rebreather used); and, in the case of fully closed-circuit mixed-gas systems, they optimize the oxygen content of the breathing gas mixture throughout the dive. Many of us who regularly dive with rebreathers also find such dives to be substantially more pleasant and peaceful – breathing warm moist gas, rather than cold dry gas. For me, learning how to dive with rebreathers brought back the exact same thrill and excitement I had when I first learned to dive. Even the most mundane dive sites can seem positively exhilarating when dived with a rebreather for the first time. Anyone who has spent any time on a rebreather will agree: this is definitely not scuba! But this simple truth works both ways: a diver's competence in scuba diving does not automatically translate to competence in rebreather diving. There is really only one way to become proficient and safe at rebreather diving: that is, to dive with rebreathers. While an experience-base of multiple thousands of open-circuit scuba dives will certainly enhance one's comfort level while being underwater (an important issue for any sort of human underwater intervention), such robust experience will not necessarily prepare a person to be ready to jump right into rebreathers with the same impunity as, say, diving with a new open-circuit regulator. This is why I implore you, the reader, to approach rebreather diving in general (and this book in particular) with the attitude of an eager young student, open-minded and willing to learn new things. If, instead, you find yourself thinking, "Yeah, yeah; sure, sure; whatever... I'm just after a C-card, and I only bought this book because it's a required text for the course...I've been diving for so long that I can handle rebreathers in my sleep..."; then I'd say you should think again. For the sake of your loved ones, for the sake, of the diving community as a whole, and for your own sake, if this is your attitude about rebreather diving, then do us all a favor and find another hobby to pursue.

For the rest of you – those who would like to experience the thrill and excitement of expanding your diving skills and re-invigorating your passion for diving – you hold in your hands a magnificent tool to help you get to where you want to go. In well-trained hands, a rebreather (any rebreather) will not only enhance the capabilities and enjoyment of diving, but will increase safety as well. As you read through this book, take Jeff's words to heart, because that's from where they were written. The information you need is here for you to acquire. Whether you are new to rebreathers and are using this book as part of a formal training course, or you've been diving with rebreathers for years and have bought this book to add to your library, or even if you're just curious to learn more about this exciting form of underwater life-support; you will find something of value in the pages that follow. I know I have. Now, get ready to be "in the loop," mind the PO2, and tame the complacency demon. And above all else, please remember to dive safely! Richard L. Pyle, 1991 Honolulu

Foreword (Second Edition) What else is there to say? As I read through the Foreword for the first edition of this fine book, I find myself nodding in agreement with what I wrote nearly a decade ago. The word "dedicated" is still the one that, in my mind, best characterizes Jeff's commitment to the art of diving and related education – both generally, and for rebreathers in particular. But there is another word that keeps popping into my head to describe Jeff as I leaf through these pages and see the enormous amount of well-organized and clearly-presented information contained therein. That word is wisdom. The first edition of this book was published at a critical time in diving history, when rebreathers were transitioning from an unusual novelty among non-commercial civilian divers, into something much more commonplace. Throughout the 1990s, the use of rebreathers was automatically regarded as "technical" diving. Indeed, it was the needs of technical divers that fuelled the rebreather renaissance of the late twentieth century. But over the first decade of this century, as the number of active rebreather divers grew from hundreds to thousands, and as the kinds of diving conducted with rebreathers shifted increasingly from technical to recreational, there was a pressing need to assemble the collective information and knowledge related to rebreather diving into a single volume. From Jeff's unwavering dedication to diving emerged such a volume – the first edition of this book. As we now enter the second decade of the 21st century, I believe we are on the brink of another critical time in diving history, and another major transition in the role of rebreathers in recreational diving in general. To illustrate the nature of this new transition, I'll invoke an analogy that I've used many times over the years to compare rebreather diving with conventional open-circuit SCUBA diving. The amount of training required to become a proficient opencircuit SCUBA diver is roughly equivalent to that required to learn how to drive an automobile. By contrast, so the analogy goes, the

training and dedication necessary to achieve proficiency with rebreathers is roughly equivalent to that required to learn how to pilot a small private aircraft. However, I'm not sure how much longer this analogy will continue to ring true. When I look at the spectrum of emerging rebreathers today, the general pattern is clearly indicative of a shift more and more towards recreational diving, and away from technical diving. Certainly, technical divers will continue to use rebreathers to fulfill their advanced diving needs. But increasingly, with reduced costs (both in terms of initial purchase price and in terms of consumables), increased reliability of electronic control systems, reduced complexity of operation, and more widespread support for rebreathers among dive shops and facilities, rebreather diving will begin to encroach further and further into the realm of recreational open-circuit SCUBA diving. Indeed, the threshold for becoming a proficient rebreather diver might begin to look less and less like flying a small aircraft, and more and more like driving an automobile. I find this notion of rebreathers going "mainstream" both exciting and terrifying. It's exciting because I believe that rebreathers can dramatically increase both the enjoyment and the overall safety of recreational diving. It's terrifying because I see how people drive their automobiles on the streets and highways, and the thought of those same people flying small private aircraft overhead simply sends shivers down my spine. No doubt, these same feelings of trepidation were shared by the first generation of SCUBA divers, as open-circuit diving began to go mainstream. Regardless of whether you embrace the emerging brave new world of "rebreathers for the masses" as an important step forward, or are a skeptic who is appalled by that notion, there is no question that rebreather technology will continue to evolve, and that the population of rebreather divers in the world will continue to expand. One thing is clear: we're now beyond the point where mere knowledge will be sufficient to shape and foster the expanding rebreather market. What we need now is something much more substantial: wisdom.

In the pages of this book, you will find that wisdom. Richard L. Pyle, PhD Honolulu, July 1 2010

Acknowledgements No textbook is ever written without lots of outside help. This text is no exception. Many people have provided information and assistance and given freely of their time during this project. I would like to acknowledge and thank all of these individuals for their generous contributions. This is a far better product because of their help. The first edition of the book was completed in 2002. Over the course of the five years it took to finish the book, many people assisted with information and content, editing and review, and photography. Their contributions are again recognized by their inclusion in the Credits from the 1st Edition section. In particular, I would again like to thank several persons who were particularly instrumental: Peter and Sharon Readey, William Thackrey, Mike Williams, and Richard Pyle all provided substantial support throughout the process of writing and editing multiple drafts of the original edition of this book. I am forever indebted for their efforts. One would think that simply revising a book would be much easier and faster than beginning from scratch. In some ways it is. Even so, from the decision to begin the revision to completion took more than three years. During that time, the Best Publishing Company staff were always available and extremely supportive in all aspects of the process. They acted as a sounding board, offered suggestions for content inclusion and format, and edited drafts of the revised manuscript and figures. They were always receptive to new ideas, and often struggled to incorporate these when it would have been much easier to just ignore them. Best Publishing Company's contributions to the entire diving industry will continue to live on not just in this book, but in the many others they have published. Mike Williams edited both editions of Mastering Rebreathers. He read and commented on several drafts of each final product, an extraordinary effort that resulted in text that was much more

comprehendible than it would have otherwise been. Mike is a NAUI instructor, currently retired, who has never used a rebreather. I believe that his perspective and insight added immensely to the readability of the book. Mike did all of the editing in his free time, after hours when he was still working, continuing on into his retirement when he could have been out riding his bike or enjoying his many other interests. I cannot adequately express how appreciative I am of his commitment to this project. Edward Tu accepted the unenviable task of correlating the hand and ink editorial changes with the interim manuscript layouts, helping to ensure that all of the modifications were correctly incorporated. This is an extremely tedious task, one that Edward accomplished diligently and with a high degree of enthusiasm. His efforts found numerous errors of omission that otherwise probably would not have been identified. Many readers of the original text identified errors, offered suggestions, and brought new information to my attention. I have listed these individuals in the Credits from the second edition. Bob Hicks and Bob Crawford in particular furnished extremely detailed reviews, noting everything from misspellings to punctuation errors to suggestions for grammatical improvements. While one always hates to have their noses rubbed in their deficiencies, both gentlemen offered their feedback in the best of humor and a true spirit of cooperation, with the goal of not only helping make this a better book, but of improving the safety of the dive community in general. The photographs in this edition came from many sources. Above all others, Elaine Jobin's dedication and effort were instrumental to providing the imagery that augments the text. She spent weeks of her time beach diving, on dive boats, and on shore capturing images for the book. Her efforts did not stop at merely taking the pictures. Elaine was meticulous in her post-shoot processing of the images, spending hours upon hours in cleaning and formatting. I am sincerely grateful for her labor, concern, and friendship.

Other photographers who went well out of their way to provide images include Brett Seymour, of the U.S. National Park Service, and Aleph Alighieri. Brett has been a friend for many years, and when I asked him for imagery he dug through his immense photographic library looking for images that portrayed exactly what I was looking for. Aleph's photographs were provided by Nautilus America, and portray the wonder of rebreather diving. Cover photography is always an important issue. Elaine Ferritto was my model for the shot of the dolphins on the front cover, as well as the image on the upper right side of the back cover. She endured hours of nudging, nit-picking, and shivering during the photographic shoots. Atlantis Resort, Paradise Island (Nassau, Bahamas) generously opened their Dolphin Cay location for the cover shoot. AJ Penny assisted us on site, while Beth Watson-Jones, Stuart Cove, Earl Miller, and the Bahamas Island Tourism Board were instrumental in assisting with the arrangements. Sometimes photography is a matter of being in the right place at the right time. Sten Johansson captured the image of me being visited by two humpback whales during a decompression stop while diving off the Nautilus Explorer at Roca Partida, Mexico. What you do not see are the dozens of images he has of me with my back to the whales, as they looked me over before I knew they were there! He graciously allowed me to use the photograph as one of the cover images. Eric Hanauer took the author photograph on the back cover during a dive trip off San Diego, California. Many of the changes in this book resulted from things I learned from students. In the course of teaching classes, concepts in which students were confused or had difficulties identified areas in which content could be modified, improved, or added. I continue to believe that one of the best ways an instructor has of learning is to listen to those who they try to teach.

Again I would like to thank those companies and individuals who supported this project by advertising in the book. The advertisers were solicited with the thought that their services and products would be of benefit to rebreather users. Their financial contribution has helped allow the publication of this text in a professional format at a reasonable price. Finally, I would like to thank my family. Projects like this entail a great deal of time spent away from home. While I am off "having fun," my children and wife are those who suffer in my absence. It is especially difficult for my wife, Rebekah, whose hobby is diving, and would love to be with me while I am off working. She remains supportive and helpful, even when it means being a single parent for frequent, extended stretches of time. My three children, Evan, JohnAaron, and Taleah, all know they have a father who cares, even when he cannot be at the functions important to them. And my nieces Taylor Bianchino and Brenna Bozanic, nephew Brandon Bianchino, mother Maxine Bozanic, and mother-in-law Lois "Grandma FooFoo" Flynn have all bent over backwards to help with kid duty when I am away. Undoubtedly, I have omitted someone from the list. The omission was not intentional, but just a result of residual narcosis caused by too many hours spent under water! I still appreciate your contributions, even if unrecognized. From the depths of my heart, thank you all! Jeffrey Bozanic Fountain Valley, CA July 20, 2010

Credits (First Edition) Author: Jeffrey Bozanic Contributors: Peter Readey William Thackrey Sharon Readey Cartoons and Diagrams: Marlon Rojas, DoInk! Debra Schechter Halcyon Principal Photography: Jeffrey Bozanic William Thackrey Additional Photography: Dr. Christian Lambertsen Dräger Mike Pelissier Dr. Robert Lanello Richard Pyle Martyn Farr Nick Icorn Wes Skiles Marty Snyderman Michelle Hall Mark Conlin Dan Burton Bo Molder Steve Reimers U.S. Navy Stuart Clough

Clark Presswood Bill Elliott George Irvine DCIEM Jack Kellon Robert Carmichael Desco Company Hans Hass Siebe Gorman and Company Carleton Life Support Technologies Ocean Technology Systems Fullerton-Sherwood Engineering Innerspace Systems Corporation Halcyon WKPP Irene Hashimoto Hank Tonnemacher Simon Fraser University Steve Millard Gene Melton Chris Parrett Irene Lewis Kelly Bracken Scott Cassell Dennis Ratcliffe XL Washington Historical Diving Society Lynn Lee David Rhea Ron Scorese NAUI Ed Buie Micropore Principal Reviewers: Peter Readey

Timothy O'Leary Michael Williams Other Reviewers: NAUI Rebreather Advisory Council IANTD Board of Directors IANTD International Advisory Board Frank K. Butler, M.D. Marie Knafelc, M.D. R.W. Bill Hamilton, Ph.D. Jed Livingstone Jan Neal John Brooks R. Eldridge Hicks, Esq. Billy Deans Gord Boivin Joe Dituri Bill Elliot Jack Kellon Rebekah Halpern, PA-C George Irvine Dr. Christian Lambertsen Dr. Sam Miller III Leslie Leaney Scott Cassell Jeanne Bear Sleeper Special Thanks to: IANTD Jim Brown Bonnie Davis, NEDU Mike Ange, Dräger Mary Sloan, Dräger Jürgen Tillmann, Dräger Paul Davis, Dräger Tim Curtis, Dräger Russ Orlowski, Dräger

Kevin Christianson, Dräger Dan Wible, Aura Leon Scamahom, ISC Steven Stolen, ISC Aquarium of the Pacific Peter Pehl, AOP Dick Long, DUI Ed Rodgers, AquaFlite Wetsuits Jim Joiner, Best Publishing Company Historical Diving Society, USA Scuba Luv Abysmal Diving Hydrospace Engineering Halcyon Brownie's Third Lung Company Laguna Sea Sports Sport Chalet Pacific Wilderness Sea & Sea, USA AquaFlite Wetsuits Dan Miccio, O.C. Lugo Company Mike Hansson, Beach City Scuba Jeff Mack, W.R. Grace Company Fred Colburn Jeff Smith Jim Buch Ray Ortiz NEC NAUI Ed Buie

Credits (Second Edition) Author: Jeffrey Bozanic Principal Reviewers: Michael Williams Edward Chih-Han Tu Other Reviewers: Richard Vann, Ph.D., DAN Gene Smith, NOAA Jim Joiner J. Ian Martin R. Eldridge Hicks, Esq. Lars Erik Frimann Jeanne Bear Sleeper Feedback from 1st Edition: Robert Crawford, Esq. Gene Melton Dudley Crosson, Ph.D. David Pence David Sawatzky, M.D. Jared Hires Kevin Bentz Conrad Daubanton Kent Rockwell Randy Gross, Ph.D. Jerry Smith, WIES Joerg Hess, Ph.D. Chris Walker Steve Mercer, NIWAR Steve Taylor Bob Mankoff Danny Huton

Duncan M. Price, Ph.D., Loughborough University Michael Ross Marty Steinberg Michael Clarke, Ph.D., Molecular Products Doug Ferrier Janwillem Bech Steve Clark Dan Dunfee Mark Fyvie Martin Parker, AP Diving Tino deRijk Cam Banks Dan Warkander, Ph.D. Matthew Hahn Walter Ciscato Richard Pyle, Ph.D. Web Jessup Diagrams: Debra Moysychyn, CMI Principal Photography: Jeffrey Bozanic Elaine Jobin Additional Photography: Brett Seymour, National Park Service Aleph Alighieri Sten Johansson (Back Cover, whales) Jill Heinerth David Pence Ramon Llaneza Warren Miller Eric Hanauer (Back Cover, author) Pascal Eeckhoudt Cameron Etezadi Jill Heinerth

Andrew Sallmon, SeaIt.com Jonas Brandt, Poseidon David Rhea Kim Smith, Jetsam Technology Bruce Partridge, Shearwater Research Kevin Lee Doug Ebersole Jarrod Jablonski Darren Fox, Ocean Legends Pete Seupel, Aquanauts Grenada Rex Rolston Myfanwy Rowlands Wakatobi Dive Resort Special Thanks to: Richard Pyle, Ph.D. Jamie Brisbin Dive Rite Ocean Management Systems (OMS) Ocean Reef Stuart Cove Elaine Ferritto Derek Smith Debi vanZyl Adrian "AJ" Penny Beth Watson-Jones Atlantis Resort (Dolphin Cay) Bahamas Tourism Board Molecular Products Hydraulics International LiquiVision Barry Coleman, RAID Robert Landreth, TecRB.com Poseidon Titan Dive Gear Jonas Brandt, Poseidon Halcyon Manufacturing

Kim Smith, Jetsam Technology Lamar Hires, Dive Rite Jerry Murphy Catherine Morris, Best Publishing Company Travis Moore, Best Publishing Company Mike Frick, Best Publishing Company Nigel Hestor Nautilus Explorer David Bostic Casey Omholt, Nautilus America Kurt Sjoblom, Poseidon Rick Stratton, Dive News Network MDEA Joe Porter, Wreck Diving Magazine Pete Murray, Scubaboard Craig De Wit, Dolphin Enterprises Kathleen Byars, Dive Gear Express Kevin Sakuda, Immersion Diving Technology Sonya Tittle, Halcyon Henrik Rosen, Wakatobi Dive Resort Geoffrey May Martin Parker, AP Diving Sean Harrison, TDI David Thompson Kathleen Byars Chauncey Chapman Hollis Gear Wayne Quarberg Gene Melton, Hydrospace Engineering Leon Scamahorn, Innerspace Systems Corp. Sea & Sea Aerospace Lubricants Tom Mount, IANTD Joe Dituri, IANTD Rusty Berry, Scuba Schools of America Gregg Stanton, Wakulla Diving Center Max Pro

KME Drysuits Ocean Technology Systems Les Turner Brian Carney, TDI Mattias Lachner Jeff Gourley Mark Gibello, XS Scuba Richard Graff, Xtreme Scuba Tom McKenna, Micropore Doug McKenna, Micropore Ian Harrington, Micropore Joe Wythoff, OmniSwivel Joe DeWit, OmniSwivel John and JJ Womack, Otter Drysuits Hilary and Paul Child, Weezle Ed Rogers, Aquaflite Wetsuits Interspiro David Lodwick Curt Harpold, Rebreathers Australia Nick Clark Mitch Dexter, Nuvair Michael Chan Lt. Greg Garnett, NEDU Mike Lever Mary Anne Lever David Burroughs Patti Mount, IANTD rEvo Rebreathers Paul Raymaekers Curt Bowen Rebreather World Advanced Diver Magazine (online) Karl Huggins Evan Bozanic Sam Myers

About the Author Jeff is an active diving educator and author who has been involved in many management positions in the recreational diving community. Jeff was certified as an Instructor by NAUI in 1978, the NSS-CDS in 1983, NACD in 1988, HSA in 1988, IANTD in 1993, and TDI in 1994. He has been teaching technical diving since 1983, and holds Instructor Trainer credentials for levels up to and including Rebreather and Advanced Trimix. Jeff's formal education includes degrees in Underwater Occupations (AA), Geology (BS), Environmental Education (MA), International Marketing (MBA), and Education (PhD). In addition, he completed several years of graduate level studies in natural resource management and marine geochemistry (oceanography). He has published and lectured extensively on diving education topics, with heavy emphasis on cave diving safety techniques. His work includes over 250 diving related articles, papers, or texts in conference symposia and magazines including: Alert Diver, Immersed, Scuba Times, Pressure, Advanced Diver, Western Diver, NDA News, Dive Log New Zealand, NSS News, Sources, New Zealand Speleological Bulletin, Underwater Speleology, NACD Journal, and others. He is a co-author of the Antarctic Scientific Diving Manual, and a contributor to The NSS Cave Diving Manual, Advanced Diving: Technology and Techniques, The Art of Safe Cave Diving, Cave Diving: Articles and Opinions, and other diving texts. Jeff serves as a National Advisor Diving to the U.S. National Park Service, a supervisor at the Catalina Hyperbaric Chamber, Research Associate for the Natural History Museum of Los Angeles, member of the IANTD International Board of Advisors, Chairman of the NSSCDS Accident Committee, IUCRR Incident Data Base Coordinator, TDI Rebreather Advisory Committee member, and Diving Safety Officer of Island Caves Research Center. In the past he has held positions as Diving Safety Officer for the University of Southern

California, Dive Technician for the U.S. Antarctic Program, Member of NAUI's Technical Diving Standards Committee, and as Training Director for Steam Machines, manufacturers of the SM1600 and PRISM Topaz rebreathers. Jeff either currently or in the past has provided consulting services for a wide variety of companies involved with rebreathers, including Poseidon, Titan Dive Gear, Steam Machines, Micropore, BluVu Oil and Gas Exploration, Deltron, Radikal, Dive Rite, B&E Engineering (Nautilus America), and Teledyne. Jeff has served on several Boards of Directors in the diving community, including as Chairman of the NSS-CDS, Vice-Chairman of NAUI, President of the International Underwater Foundation, Director of IUCRR, and as Treasurer of the American Academy of Underwater Sciences (AAUS). He has received the NAUI Outstanding and Continuing Service Awards; the NACD Silver Wakulla, NSS-CDS Abe Davis, and Henry Nicholson Awards for safe cave diving; the NSS-CDS Chairman's Award; the SSI Platinum Pro 5000 Award; the Los Angeles County Scuba Educator's Award; and has been inducted into the NAUI Hall of Honor. In 2007 he was honored as the recipient of the DAN/Rolex Diver of the Year.

Dan Orr, President of Divers Alert Network, presenting the 2007 DAN/Rolex Diver of the Year Award to Jeff Bozanic.

Your Responsibilities as a Rebreather Diver Both standard open circuit scuba diving and rebreather diving are adventure activities with inherent risks of serious personal injury or death. Good training and good equipment can help to minimize those risks, but there is no guarantee that these risks can be completely eliminated. The code of the responsible Rebreather Diver states that: You must accept responsibility for your own actions and safety during every dive. You must dive within the limits of your ability and training. Evaluate the conditions before every dive; ensure that they fit your personal capabilities. Be familiar with and check all equipment before and during every dive. Personally analyze or directly observe the analysis of the breathing gas you will be using. Always dive with an alternate air source. Know your dive buddy's as well as your own ability level.

Disclaimer Diving involves inherent risks that must be accepted by any individual engaging in this activity. Neither the author nor publisher accepts responsibility for accidents or injuries resulting from the use of materials contained herein. The information in this book pertains to rebreather diving with various gas mixtures, and should be considered a supplement to an approved diving training program provided by a recognized certification agency. This book cannot take the place of training provided by a professional instructor. All divers should receive professional instruction and certification in diving before attempting to dive with any breathing mixtures including air. In addition to the information contained herein, all divers should follow safe diving rules, practices and procedures taught at every level of training. Professional training, certification, routine review of diving skills and knowledge, and adherence to conservative diving practices and procedures are necessary ingredients for safe diving, regardless of the breathing mixture used.

Introduction to Rebreathers Overview

If you are reading this book, you obviously have some interest in rebreathers. But is that in learning to dive rebreathers? Using them once or twice for the experience? Purchasing one for personal use? You may not even know the ultimate level of your interest; you may find that as you learn more you become more interested – or less. In any event, this text is designed to provide a comprehensive background on rebreathers – what they are, what they do, how they operate, and what is involved in learning to dive them. It is not meant to stand alone as a user training program, but may be incorporated into a training program under the guidance of a certified rebreather instructor. We hope you find the information interesting. In many cases it is unique among resources on the topic. Read and enjoy!

Objectives After reading this chapter, you will be able to: 1. Explain what a rebreather is. 2. State the difference between open circuit, semi-closed-circuit, and closed-circuit scuba systems. 3. List four advantages to using rebreathers. 4. Describe five functions a rebreather performs. 5. List six recreational agencies that offer rebreather training. 6. Outline the purpose and limitations of rebreather training courses.

7. Define "Type Rating," and explain its importance in rebreather use.

Why Rebreathers?

Figure 1.1: Meet Marvin. Marvin is an open circuit scuba diver. Meet Mary. Mary dives a rebreather.

Marvin and Mary are going to spend the weekend diving together. Being avid divers, they are traveling to a place where few divers have been, Paradise Cove on Valhalla Island, so unknown that there is not even a dive shop on the island. Partially because of that, the diving is wonderful: lots of huge grouper, sea turtles, manta rays, 15pound (7 kg) lobsters, and a beautiful, thriving coral reef. They are in for a phenomenal diving trip!

The trip begins with packing gear. Marvin starts with his diving cylinders. Three days, three dives per day... pretty simple math... nine 80 cf (11 L) aluminum tanks. Total weight: 315 pounds (143kg). Unfortunately, Marvin lives in a third floor apartment with no elevator. Up the stairs, down the stairs; two cylinders in the car. Up the stairs, down the stairs; four cylinders. Up, down. Huff, huff, huff.... Up, down. Pant, pant, pant.... Last trips up, down... Ugh! Mary, meanwhile, carries her two small rebreather flasks down to the car in one trip. For the nine dives, she needs one of air and one of oxygen. Total weight: 8 pounds (4 kg). Just to be safe, she carries an extra set (maybe she'll do some night dives!) Marvin packs his regulator, a spare, BC (buoyancy compensator), and other stuff. Because he gets cold, even in warm waters like the Nirvana Sea around Valhalla Island, he takes a wetsuit. Of course, with it go the lead weights. Another two trips up and down the stairs. Mary, because she is using a rebreather, breathes gas warmed by the system and requires no wetsuit or associated lead weights! The BC, regulators, and all of her accessories are integrated into her rebreather. She puts it on her back and she's ready to go! They arrive at the dock from which the boat will leave. Mary slips on her 48-pound (22 kg) rebreather, picks up her 20-pound (9 kg) gear bag (with her extra gas flasks and CO2 absorbent) in one hand, her overnight bag in the other, and trots up the two flights of stairs to the ticket counter. Poor Marvin! Two more flights of stairs with all those tanks. He is getting his exercise today, though! At the ticket counter, Mary buys her ticket and walks aboard. Marvin, groaning a bit now, buys his ticket and pays $275 excess baggage charges for all of the tanks, weights, and other junk (I mean gear). The boat ride is pleasant, and in a few hours they reach Valhalla Island. Their hotel, Blissful Manor, is but a few blocks from the boat dock. Of course, at such an unknown destination, taxis are few and far between so Mary walks through the open-air market to the hotel. Along the way, she stops to buy lunch, a delightful veggie patty for which the island is noted. Marvin gets lucky. He finds five kids to help

him haul his gear to the hotel. It takes them two trips. In exchange, he buys them all lunch. Nice guy. After all that hauling, Marvin is hot, sweaty, and tired, but ready for a dive. So is Mary (and she feels a lot more refreshed than Marvin!). So they suit up and walk from Blissful Manor to Paradise Cove, just a bit more than half a mile away. Marvin wears his wet suit, lead weights, and 80 cf (11 L) cylinder, and is roasting. Mary, in her rebreather, bikini and T-shirt, chats as they walk to the cove. "Look at the pretty butterfly!" she exclaims. Marvin just grunts. Fortunately, the diving is everything they expected. White sand beach, easy entry, 200-foot (60 m) visibility, warm water, lots of coral, fish and even a manatee! Mary sneaks up on the manatee and gets some nice, tight photographs. The manatee does not hear her because there are no bubbles from her rebreather. Marvin sneaks up behind her, but his bubbles scare it away. Too bad. But still, what a great dive! Marvin runs low on air at 45 minutes, and they surface. They exit together, water glistening on their gear. Mary drops her rebreather on a blanket, puts on some suntan oil, and lazes in the sun for her surface interval. Marvin drops his gear, picks up his empty cylinder, and walks the half-mile back to the hotel to trade it out for a full tank. When he returns, they dive again. Another ideal dive; another surface interval. Mary naps. Marvin walks. Dive. Mary reads. Marvin walks. Next day, more of the same. By the end of the second day, Marvin is wondering what possessed him to take up diving. Mary, on the other hand, is unwinding from the rush of the big city, and relaxing during the time she is not in the water. By the third day, Marvin has had it! He knows he has to schlep all those tanks back to the boat, from the boat to the parking lot, parking lot to the dive store (for fills), and then from the dive store to his apartment, and up those !@*&^%$#!! stairs. He's ready to buy a rebreather!

Figure 1.2: Marvin loaded down with his scuba equipment and falling behind Mary.

What is a Rebreather?

A rebreather, in its simplest form, is a machine that cleanses and conditions the gas a diver exhales so that it may be breathed again – hence the name rebreather. "Normal" scuba is termed OC (open circuit) scuba. As a diver breathes, the exhaled gas is vented to the ocean. These are the bubbles we see and hear when we exhale while diving with such units. Rebreathers are classified into two broad categories: semi-closed-circuit (SCR) and closed-circuit rebreathers (CCR). The difference between the two types is in the functional design. The closed-circuit systems are designed to retain all of the gas, while the semi-closed systems regularly vent some gas during use. These design differences will be discussed in greater detail later in this text. Generally, all rebreathers do similar things. They recirculate either part or all of the expired gas. This increases efficiency and gas economy. Because the expired gas is captured and reused,

rebreathers conserve inert gas and oxygen. Before the gas can be breathed again, the carbon dioxide (CO2) in it must be removed by chemical scrubbing. Simultaneously, enough oxygen must be added to replace the oxygen used by the diver, with oxygen partial pressure (PO2) maintained within safe limits. Finally, rebreathers must provide a reasonable breathing volume to fill your lungs throughout the dive. All of these functions will be discussed in detail later in this text.

Figure 1.3: Many animals that would normally avoid scuba divers because of their bubbles are often curious about rebreather divers, and may come quite close.

Table 1.1: Rebreather Function Recap

Rebreathers: • • • •

Recirculate expired gas Scrub carbon dioxide (CO2) Conserve inert gas Conserve oxygen (O2)

Maintain oxygen partial pressure (PO2) within safe limits • Maintain breathing volume •

The advantages offered by a rebreather primarily stem from the recirculation of the breathing gas. In our story, Mary was able to do all of her dives on 40 cf (5.5 L) of gas because none was wasted. Gas efficiency is generally cited as the primary advantage to using SCR or CCR apparatus. Along with gas efficiency comes reduced or eliminated bubble noise, as the bubbles are either reduced (with SCR) or eliminated (with CCR). Photographers and wildlife enthusiasts particularly appreciate this advantage. In fact, animal encounters are often quite remarkable when breathing from a rebreather in contrast to OC apparatus. Many new rebreather divers are astounded by the marine life they see while diving rebreathers, even in sites they have previously dived hundreds of hours. New fish, more fish, more juveniles and large adults, unusual wildlife behavior, and close exotic mammal encounters have all been reported by rebreather divers. The degree of such interactions is such that many rebreather divers have exclaimed, "It's a totally new world!" even after having done thousands of OC scuba dives.

Figure 1.4: Underwater photographers and videographers like rebreathers because they get better images. Courtesy of Elaine Jobin

Although more expensive than OC, rebreathers offer unique advantages (see Table 1.2). Some rebreathers will let the diver stay at depth for up to ten hours. Imagine ten hours on a single dive at 100 fsw (30.5 msw). You'd still have to decompress, but even these obligations are significantly reduced on some models. This is done with a unit smaller, lighter, and much more manageable than OC gear. On some dives, one CCR can replace more than fifty 80 cf (11 L) scuba cylinders. In particular, mixed gas CCRs provide several advantages both OC and SCR diving do not. Essentially, mixed gas CCRs are "on the fly" nitrox mixing machines, providing an optimal nitrox mix for your depth. Thus, they maximize your allowable "No Decompression Limit (NDL)" bottom time. As an example, using U.S. Navy air diving tables, your allowable bottom time at 56 fsw (17 msw) increases from 60 minutes to 232 minutes, an incredible fourfold increase! Figure 1.5 compares dive time limits for air, EAN32, and CCR diving. Table 1.2: Rebreather Benefits Gas economy

No or reduced bubbles Less noisy than OC Improved marine life interactions Increased bottom time Improved flying after diving considerations Warmer, moister breathing gas Cost effectiveness for helium based diving Extended operational capacity *Note that not all types of rebreathers offer all these advantages If you are a diver who enjoys traveling to remote destinations, CCRs may allow you to spend more time under water. Your CCR can be set to provide you with a high fraction nitrox mix. This allows you to do much of your surface interval time under water! As your OC traveling companions sit on the beach or boat for hours to off-gas excess dissolved nitrogen, you can be shooting underwater photographs, while eliminating inert gas at exactly the same rate they are. Diving CCRs can give you more than twice the underwater time as the next nearest OC diver, making vacations much less expensive on a cost per hour under water basis. This concept is covered more completely in Chapter 6, Part 4. Using the same concept, CCRs can also add diving days to your vacations. "Flying After Diving" rules no longer apply in the same way. By setting up your rebreather properly, you can climb out of the water and into an airplane, and fly with the same risk as if you had not been under water for the previous 24 hours. Your CCR instructor will discuss this with you in detail.

Figure 1.5: Dive time limits comparing OC Air, OC EAN32, and CCR diving modes, incorporating both no-decompression and oxygen toxicity time limits.

If you are a technical diver, CCRs offer many advantages. In deep diving, helium use is drastically reduced. Instead of using 100200 cf (2800-5600L) of helium per dive, you may use as little as 10 cf (283L). This can save you hundreds of dollars (or shekels, euros, or whatever currency you use)! They also offer logistical benefits, allowing you to dive deeper, longer, or further, especially for cave or wreck diving. Mastering Rebreathers, Volume 2 examines the use of CCRs in technical diving applications. Mary experienced another advantage on her dive. One of the side effects of the chemical removal of CO2 is generation of moisture and heat. This is imparted to the breathing gas, so the user gets warm, moist gas to breathe. This can be a big advantage over the cold, dry air an open circuit diver breathes, especially on long dives. Finally, many people really want to use rebreathers because they are new, exciting, and fun. Only recently have rebreathers become available to the recreational diver. Now you can find them for sale or

rent in dive stores and resorts throughout the world. However, prior to diving on any rebreather system, you must receive proper training. Paraphrasing what they say about cave diving, no amount of OC dive training or experience can prepare you to dive rebreathers. So let's join Marvin as he learns more about rebreathers in his training!

Rebreather Training

Many recreational scuba diving agencies offer rebreather training. Rebreather training in the recreational sector began with the "technical diving" agencies, and later was offered by the larger, more generic open water training agencies. As rebreathers become more common, it is expected that additional organizations will develop and offer training. A list of agencies that offer training is in Table 1.3 (pages 10 to 11). While not all courses are structured identically, there are basic similarities. Some agencies offer a single rebreather course, while others offer several types of rebreather training. Certification courses may train you to dive on either a SCR or a CCR system. In addition, rebreather training may be further divided into multiple levels for introductory and advanced use. Several training agencies now permit new divers to begin their dive training using rebreathers. They have no requirements that one must be certified first as an OC scuba diver. Because these students have no experience in scuba diving, these courses tend to be longer and more limiting than "crossover" or specialty programs for previously certified divers.

Figure 1.6: All rebreathers require specialized training before they are used

However, most people learning to dive rebreathers have been certified as OC divers first. Thus, most rebreather certification programs share some prerequisites. At a minimum, you must be certified as a scuba diver, have logged a minimum number of dives (6-75 depending on the agency and rebreather), and hold an Enriched Air Nitrox Diver Certification. Generally, advanced open water and nitrox certifications are advantageous, but not required. The rebreather certification program will build on knowledge you gained in your previous training, especially that from any nitrox courses. Bear in mind that rebreathers differ from open circuit scuba in that the breathing gas composition is always changing. They also differ in operational and emergency procedures. Table 1.3: Recreational Training Agencies Offering Rebreather Training American Nitrox Divers International (ANDI) 74 Woodcleft Avenue Freeport, NY 11520-3342 (800) 229-2634 • (516) 546-2026 www.andihq.com British Sub Aqua Club (BSAC) Telford's Quay, South Pier Road Ellesmere Port, Cheshire CH65 4FL United Kingdom (44) 0151 350 6200 www.bsac.com

Confederation Mondiale des Activites Subaquatiques (CMAS) www.cmas.org International Association of Nitrox and Technical Divers (IANTD) 2124 NE 123 Street, Suite 210 North Miami, FL 33181-2939 (786) 704-9722 www.iantd.com National Association of Underwater Instructors (NAUI) P.O. Box 89789 Tampa, FL 33689-0413 (800) 553-6284 • (813) 628-6284 www.nauiww.org National Speleological Society Cave Diving Section (NSS-CDS) 295 NW Commons Loop, Suite 115-317 Lake City, FL 32055 (386) 454-5550 www.nsscds.org Professional Association of Diving Instructors (PADI) 30151 Tomas Street Rancho Santa Margarita, CA 92688-2125 (800) 729-7234 • (949) 858-7234 www.padi.com Rebreather Advisory Board (RAB) Berliner Strasse 312 D-63067 Offen Bach, Germany 49 (0) 69-98 1902-0 www.rab-ev.de/english/siete_1.htm

Rebreather Association of International Divers (RAID) Mjolkekilsgatan 8 440 30 Marstrand, Sweden 46 (0) 303 605 64 www.diverraid.com Technical Diving International (TDI) 18 Elm Street Topsham, ME 04086 (888) 778-9073 • (207) 729-4201 www.tdisdi.com Rebreather entry level training programs provide the information you will need to begin diving with the course-specific rebreather to depths of 70-130 fsw (21-40 msw), using no-stop dive profiles. Course content includes basic rebreather design, rebreather selection, maintenance, use, and emergency procedures. Portions will deal with rebreather-specific physics, and physiological concerns, dive operations, and procedures. Advanced rebreather certification programs build on entry level skills and information, and qualify graduates to use SCRs or CCRs at depths below l30 fsw (40 msw), on dives requiring staged decompression stops, or dives using helium-based gas mixtures in the CCR. The prerequisites for these programs vary from agency to agency. As this level of training is beyond the scope of this text, further information on advanced rebreather training will not be addressed. Advanced topics are covered in Mastering Rebreathers, Volume 2. During your entry-level rebreather course, you may use rebreathers of more than one type. Depending on the time you spend actually preparing and diving the units, you will receive a type rating for the rebreather you used. The type rating is an important concept. Not all rebreathers operate the same. That is why most agencies offer two basic rebreather courses, one for semi-closed systems and another for closed-systems. Similarly, not all SCR or CCR apparatus is the same. Just because you train to use one manufacturer's unit you are not automatically qualified to use

another's. This is unlike open circuit equipment, where one regulator or BC generally operates like any other. The type rating states which unit(s) you qualified to use during your training, and is an integral part of your certification.

Figure 1.7: New rebreathers are regularly introduced to the market, so it is imperative that you continue to seek out new information to remain current. Courtesy of Hydrospace Engineering

Figure 1.8: Rebreather training includes theory, poolwork, and open water dives. Courtesy of Elaine Jobin

Figure 1.9: "Type Ratings" on rebreather certification cards define which systems the diver is trained to use.

Figure 1.10: Rebreather training is your first step to a new adventure. Enjoy!

Your basic SCR or CCR training program will not train you to do some things. It will not qualify you to use a CCR (if enrolled in a SCR program), or vice versa. Training agencies have different training programs for each unit. It is a recreational course. This limits diving to the standard recreational depth limit of 130 fsw (40 msw) or shallower and constrains the diver to follow traditional no-stop decompression diving procedures and dive profiles. That also rules out the use of helium-based inert gas mixtures. Several agencies

offer a variety of other courses for those interested in participating in advanced technical diving specialties.

Evolving Rebreather Standards

The recreational rebreather market is in rapid change. New certification programs are being developed, new units are being introduced, and innovative components are becoming available. As an example, one agency is trying to eliminate the type rating concept. To make this viable, they have set minimum equipment standards and design parameters to which rebreather manufacturers must adhere in order to have their units approved by the agency for instruction. The goal is to make all rebreathers sufficiently similar that training tailored to any given unit is not necessary. Establishing criteria for rebreather training is a matter open to significant interpretation and controversy. Experts in rebreathers have divergent opinions as to what to teach, and to what level of detail. Different agencies have established training programs, ranging from one day to several weeks or longer. In addition, always remember that rebreathers are not created equal. While all manufacturers perform some testing, not all rebreathers are evaluated in the same manner. More on this topic is presented in Chapter 3 of this book, and particularly in Mastering Rebreathers, Volume 2. There are no currently accepted industrywide standards for rebreather testing, training, or minimal safety components. In order to cope with this uncertainty, many of the training agencies have established special rebreather advisory boards to evaluate programs and new developments for their organizations. For example, NAUI has established the NAUI Rebreather Advisory Council, PADI works with their affiliate organization, Diving Science and Technology (DSAT), BSAC has their Rebreather Working Group, CMAS has the Rebreather Advisory Group, TDI has the Rebreather Committee, and IANTD utilizes their Board of Directors and International Board of Advisors to meet these needs.

When it comes to rebreathers, it is important to keep an open mind and learn from all available sources. Don't limit yourself to considering a single agency. Learn from all of them. Read the materials each has to offer, add them to your library, and when you complete your rebreather training, remember that change is rapid in today's environment. Continue to seek out new knowledge and information.

Figure 1.11: Rebreathers represent the dawn of a new era in recreationaland scientific diving. Courtesy of Elaine Jobin

1. Open circuit scuba differs from rebreathers in that: a. Open circuit scuba recycles the gas breathed by the diver. b. Rebreathers remove oxygen from the exhaled gas prior to venting the excess. c. Rebreathers allow the diver to reutilize the exhaled gas. d. All of the above.

2. Semi-closed and closed circuit rebreathers differ in that: a. Semi-closed systems vent some of the gas exhaled by the diver. b. Closed circuit rebreathers retain only part of the gas exhaled by the diver. c. Both of the above statements are true. d. None of the above are true.

3. All of the following are advantages of rebreathers except: a. The gas breathed by the diver is warm and moist. b. Rebreathers bubble more than scuba, keeping sharks away.

c. Breathing gas is reused, so the gas supply will last longer. d. A single rebreather can replace many standard scuba cylinders.

4. A rebreather: a. Conditions the breathing gas by removing carbon dioxide and oxygen, so the diver can reuse it. b. Maintains oxygen partial pressures within safe limits. c. Varies breathing bag volume to optimize for depth and diver work rates. d. All of the above.

5. Rebreather Certifications available include: a. Semi-Closed Rebreather Diver. b. Closed-Circuit Rebreather Diving. c. Advanced CCR Diver. d. All of the above.

6. The primary purpose of the type rating is to: a. Define the rebreather a diver has been trained to use. b. Set the depth limits that a diver has been certified to dive.

c. Specify which gases a diver is qualified to use. d. None of the above.

History of Rebreathers Overview Rebreathers are neither new nor uniform in design. In this chapter you will review the developing technology of rebreathers over more than 2,000 years. You will be introduced to the various types of rebreathers in the context of their historical development, but a structured overview of basic designs will be presented in the next chapter. Finally, you will be told briefly how recent rebreather use has advanced the pursuit of scientific and recreational exploration and research.

Objectives After reading this chapter, you will be able to: 1. Describe four historical events in rebreather development. 2. Explain the impetus behind rebreather development during the twentieth century. 3. Explain what types of dives are currently being done on rebreathers that could not be accomplished using open-circuit scuba.

Early Concepts

Humans have tried to imitate fish in water for all of recorded history. Representations of efforts appear in mosaics, bas-reliefs, and early writings of diverse civilizations. Embossments in the British

Museum show men under water with pigskin bladders connected to their mouths as early as 900 BC. Historians debate whether they represent water wings or very early attempts at scuba. In the essay "Problemata," Aristotle discussed pressure problems faced by free divers circa 360 BC. He also referred to his erstwhile student, Alexander the Great, supposedly descending in a glass diving bell to view divers at work during the siege of Tyre in 332 BC apocryphal, no doubt, but noteworthy as a diving idea from ancient history that became a reality in modern technology using diving bells and other submersibles.

Figure 2.1: Bas relief of diver using pigskin bladder. Courtesy of U.S. Navy

For the next two millennia, innovators experimented with tethered diving dress, armored diving suits, and diving bells. But the first recorded discussion of a self-contained underwater breathing apparatus ("scuba") for a free swimming diver does not appear until 1680. The Italian mathematician and physicist, Giovanni Alfonso Borelli (1608-1679) is credited with having proposed a rebreather consisting of a two-foot (60 cm) diameter metal helmet with a glass faceplate, sealed around the diver's neck and laced to a watertight suit of goatskin. A curved metal tube ran from the diver's mouthpiece

over his right shoulder to a leather bag, then from the bag into the rear of the helmet. Borelli reasoned that the cooling properties of the surrounding water would cause the impurities in the exhaled breath to condense along the tube walls, and collect in the leather bag. He estimated that the unit would sustain a diver for about thirty minutes. Two hand valves at the top rear of the helmet allowed the diver to replenish the breathing supply at the surface. Borelli's design included a water displacement mechanism for ascent and descent, a mixture of lime and egg white (albumen) in the faceplate for illumination, and clawed feet that may have represented foot fins for propulsion.

Figure 2.2: Borelli rebreather design, 1680. Courtesy of Ed Buie

Rebreather technology advanced considerably in 1726 when the English botanist Stephen Hales designed a system that included a primitive chemical absorbent consisting of flannel cloth soaked in sea salt and tartar (sodium chloride and calcium phosphate). It was used strictly as a mine rescue unit and not under water, but clearly the concept was born. Users of Hales' equipment unwittingly faced the ever-present risk we know today as hypoxia. Another 50 years

would pass before Carl Scheele (1772) and Joseph Priestley (1774) independently discovered "oxygen" as a vital element in the air we breathe. Nearly a century after Borelli developed the theory of "regenerating" breathing air, the Frenchman Sieur Freminet invented and actually dived a breathing system much like the Borelli design, but with the addition of a gas reservoir and a small fan driven by a windup clockwork motor. When the system was demonstrated to the Commission d'Academie des Sciences in 1774, the clock spring broke and the diver was forced to abort. Freminet eventually died using his unit, either of carbon dioxide poising or oxygen toxicity, during a 20-minute immersion. The use of oxygen was slow in coming. Fully 60 years after its discovery, the German company Wherle devised an oxygen rebreather for use as a rescue unit in the mining industry. It consisted of a gas mask and an absorption cartridge. Many mine rescue units emerged during the 1830s and 1840s, culminating in the first truly portable rebreather rescue system, introduced by Professor Theodore Schwann at the Belgium industrial fair of 1853. The next major advance in scuba technology was the demand valve. It was invented by two Frenchmen, mining engineer Benoit Rouquayrol and Naval Lieutenant Auguste Denayrouze. Their apparatus integrated an automatic demand mechanism with an air reservoir at the surface, distinct from Freminet's popular surfacesupplied systems using bellows to pump air directly to the diver. It would be another 78 years before Émile Gagnan and Jacques-Yves Cousteau would refine a demand valve into the scuba we use today.

Figure 2.3: Lethbridge 1715 diving dress. Courtesy of U.S. Navy

In 1878, the Englishman Henry A. Fluess received a patent for a portable underwater breathing apparatus. Fluess sold his patent to and began working for Siebe, Gorman and Co. This company manufactured the closed helmet diving dress system developed by Augustus Siebe and the Deane brothers, the forerunner of present day surface-supplied systems. Their first portable apparatus consisted of a rubber waterproofed fabric face mask, a breathing bag, a copper cylinder charged to 450 psi (30 bar) with oxygen, and a CO2 absorbent canister of "tow" (coarse, broken fibers of hemp) impregnated with caustic potash - all mounted on the diver's back. Oxygen flow into the breathing loop was controlled manually by the diver operating the cylinder valve. This year, 1878, marked the arrival of the first true rebreather. The practicality of the Fluess design was demonstrated dramatically in 1880. Fluess volunteered to dive into the flooded Severn Tunnel to close a sluice door deep in the back of the construction site. The tunnel was filled with debris, which would have made tethered surface-supplied diving extremely dangerous. Fluess was a brilliant engineer and talented inventor, but his diving skills were limited and he had no experience with underwater construction. After an unsuccessful dive, he recognized his shortcomings and he

promptly trained Alexander Lambert in the operation of his rebreather equipment. Lambert was one of the most experienced and respected divers of his time, and he successfully located and closed the sluice using the Fluess rebreather. This was a seminal event introducing more than a century of pioneering rebreather accomplishments. Two Americans further improved the basic rebreather design in 1881. Achilles Khotinsky and Simon Lake (of later submarine fame) patented a rebreather consisting of a bellows counterlung, a compressed oxygen supply with a reduction valve, a canister of barium hydroxide, and an automatic oxygen addition system.

Figure 2.4: Rouquayrol/Denayrouze apparatus, 1865

Leslie Leaney

Rebreathers at War

Courtesy of

Prior to the outbreak of World War I, there was strong competition between Siebe, Gorman & Co. in Britain and Drägerwerk in Germany. In 1907, Siebe, Gorman & Co combined the use of oxylite (a potassium and sodium peroxide mixture - which produces oxygen when in contact with water), with rebreathing technology to produce a submarine rescue apparatus. In response, Dräger adapted its mine rescue unit for underwater use - prompted no doubt by the perceived

requirement for a submarine escape system. By 1914 both companies had semi-closed nitrox rebreather systems, but the advent of World War I surprisingly slowed further development.

Figure 2.5: Fluess rebreather, 1878. Courtesy of HDS, USA and Siebe, Gorman & Co.

OC technology slowly emerged after the Great War. Captain Yves Le Prieur and Maurice Fernez patented a "self contained diving apparatus" in 1926. It consisted of a l2 cf (3 L) steel cylinder filled to 1,700 psi (115 bar), a short air hose to a mouthpiece, a pressure gauge, small goggles, and a nose clip. The system was back mounted, and the diver controlled the constant flow of air by adjusting the cylinder valve to match breathing needs. By 1933, the system had been modified to a hip mounted 26 cf (6.7 L) cylinder, and full-face mask. But the constant flow feature still imposed an impractical limit on the diving duration. Because of its simplicity, Cousteau chose this system to improve.

Figure 2.6: Dr. Dräger testing rebreathers in 1911. Courtesy of Drägerwerks

Figure 2.7: Italian Chariot: two-man torpedo, WW II. Courtesy of Ed Buie

The first recorded use of the Siebe, Gorman & Co. submarine rescue device, the "Davis Escape Apparatus," was in 1931 when HMS Poseidon sank in the China Sea. Six men escaped death. That same decade, in 1937, Max Gene Nohl used a self-contained suit

and helmet with its own helium/oxygen mixture and carbon dioxide absorbent canister, setting a depth record of 420 ffw (128 mfw) in Lake Michigan. Italy was the first nation to recognize the value of rebreathers in underwater combat. The Italians had been working on the concepts of closed-circuit systems since World War I. In 1934, two engineers, Lt. Teseo Tesei and Lt. Elios Toschi, designed a primitive version of what we now call a "Swimmer Delivery Vehicle." It was an underwater torpedo that could carry a two-person team, breathing gas, and explosives that were called "pigs" by the crew. The project was discontinued by the Italian Naval Command, then resurrected in 1939. By 1940, prototype torpedoes were ready to use.

Figure 2.8: Hans Hass using a Dräger Gegenlung. Courtesy of HDS, USA, and Hans Hass

World War II provoked enormous interest in underwater breathing devices. The Italian military was quite successful destroying Allied shipping using their Pirelli diving units and modified versions of the

Davis Escape Apparatus previously sold to them by the British. The most impressive feat of the Italian underwater units was the creation of a secret attack base in the neutral harbor of Algeciras. When Italy entered the war, the tanker Olterra was scuttled in the harbor. Under the pretense of repairing her for use at war's end, the Italians were able to gain entry and "fix" her. They cut a 25 foot (7m) steel section out of the bulkhead between the bow compartment and the cargo hold, hinged the section, replaced it, and pumped the water out of the forward tanks to raise the bow. They then cut and hinged a 4 foot (1.3m) section of the hull in the bow compartment, some 6 feet (2m) below the waterline. When the ship was raised to her normal position and pumped dry, the cargo hold provided a comfortable working environment, and torpedoes could be launched through the bow compartment. This subterfuge was not discovered until the Italian Armistice occurred in September, 1943. Surprisingly, while Germany was a leader in developing underwater technology, sleds and equipment, it lagged behind both the Italians and the British in exploiting this research and development in warfare. Early in 1941, the diving pioneer Hans Hass began working with Hermann Stelzner on an improved version of the Dräger Gegenlung, and, by 1942, Hass was actively diving the system in his underwater marine studies. The unit had been modified, by replacing the constant flow valve with a pushbutton control for adding oxygen, and repositioning the counterlung as a back mount. Hass dove this oxygen unit to the maximum depth recommended by Stelzner of 66 fsw (20 msw). Despite at least two oxygen toxicity incidents, he continued to use the equipment because of the tremendous advantages it offered him in his work. German authorities sent their first group of divers to Italy for training in 1943. By the following spring, Germany could claim a small contingent of qualified underwater commandos, but the war chronicles recount only a few underwater actions by them. The attack against the twin bridges at Nijmegen in October, 1944 is probably the most frequently documented deployment. The insignificance of underwater teams in Germany's arsenal contrasts sharply with the work of the Italians, who were responsible for damaging or sinking an estimated 150,000 tons of Allied shipping,

targeting such notable vessels as HMS Valiant and HMS Queen Elizabeth.

Figure 2.9: German limpeteer diver, fully dressed. Courtesy of Ed Buie

Figure 2.10: Dunlop CDBA rebreather being used to explore caves in Britain, 1961. Courtesy of Martyn Farr

Figure 2.11: LARU (Lambertsen Amphibious Respiratory Unit), 1947. Courtesy of Christian J. Lambertsen, University of Pennsylvania

The need to clear mines and unexploded ordnance at deeper depths led to the formation in Britain of "P-Parties." As more became known about the physics and physiology of deeper diving, work began in earnest on systems capable of operating deeper than the existing oxygen rebreathers allowed. Researchers concentrated on developing reliable semi-closed circuit gear. Other diving technologies were also progressing, such as the thermal protection needed in the waters of northern Europe and, of course, the open-circuit scuba designed by Emile Gagnan and Jacques Cousteau in 1943.

Figure 2.12: Dräger LAR V oxygen CCR. Courtesy of Clark Presswood

Post War Developments

In various guises, the Clearance Divers' Breathing Apparatus (CDBA) has been the backbone of the British fleet design since WW II, both as an oxygen closed-circuit unit and as a semi-closed circuit rig. Its depth rating is 180 fsw (53 msw) with a maximum dive time of 51 minutes. In the oxygen mode, with a flow of 4 liters per minute, dive time is increased to 90 minutes. The CDBA incorporates a chest mounted counterlung with a circular scrubber mounted on the front of the lung and opening into it. With a single breathing hose to the scrubber, the diver inhales and exhales through the absorbent in a pendulum or back and forth manner. Lead balls are secured to the diver's back with a quick release mechanism. The reducer and regulator assemblies are affixed to the diver's right side, with an optional cylinder on the diver's back or stomach, depending on whether the system is configured for oxygen or semi-closed use. The Royal Marines use a special version known as the Long Endurance

Breathing Apparatus (LEBA). It operates at 82 fsw (25 msw) for up to three hours, depending on work rate.

Figure 2.13: The British Cave Diving Group using rebreathers to explore Wookey Hole in 1949. Courtesy of Martyn Farr

The development of rebreather technology in the United States paralleled much of the work in Europe. In the early 1930s Charles Momsen designed a submarine escape apparatus similar to the Davis design. Soon after the war Dr. Christian J. Lambertsen focused on the need to correlate the design of closed-circuit diving equipment with physiological requirements. The Lambertsen Amphibious Respiratory Unit (LARU - not to be confused with the Dräger LAR V) was the antecedent and underpinning of most of the closed-circuit development in the United States. Dr. Lambertsen was instrumental in developing underwater training and equipment - first as a member of the Office of Strategic Services, and then as a trainer for the U.S. Coast Guard Air/Sea Rescue Service, the U.S. Army Corps of Engineers, and the U.S. Navy Underwater Demolition

Teams. The Emerson-Lambertsen closed-circuit oxygen rebreather a refined version of the LARU - remained in military service until about 1982. Its replacement, the Dräger LAR V oxygen CCR, along with the LAR VI (the low magnetic signature version) LAR VII (a combination oxygen CCR and nitrox SCR) are in widespread use today, with an estimated four thousand units in all branches of the U.S. military. Cave divers were the most active civilian users of rebreathers in early post-war years. Attempting to push cave penetrations further than surface-supplied or "open-circuit" Le Prieur gear would allow, these divers turned to closed-circuit gear.

Figure 2.14: DESCO oxygen CCR. Courtesy of Leslie Leaney and DESCO

In Britain, Graham Balcombe pioneered the way, with help from friends at Siebe, Gorman & Co. Balcombe began working with a modified Davis Submarine Escape Apparatus (DSEA) in 1944, and successfully completed a 40-minute dive the following year. Many other cave divers followed. The Cave Diving Group formed in 1946

and developed a formal training program. Standard cave diving equipment became a closed-circuit oxygen rebreather, with postwar cave divers obtaining ready access to the DSEA, Protus, Salvus, and Amphian rebreather systems. In the late 1940s and early 1950s, cave diving with closed-circuit oxygen rebreathers flourished. The depth limitations of pure oxygen rebreathers and the introduction of the open-circuit Aqualung led to re-evaluations of cave diving equipment choices during the late 1950s. The first recorded cave dive with a SCR took place at Wookey Hole in December, 1956, but this technology was not generally available to the public. The military was reluctant to release semi-closed technology. Generally, rebreather training, the gear, its maintenance, and operation were expensive. Cave divers moved to the far more accessible and cheaper Aqualung. By 1964, rebreathers were phased out of British cave diving altogether, and were replaced by the Aqualung patented 20 years earlier by Gagnan and Cousteau. Sport diving emerged with burgeoning post-war sales of the Aqualung. Early manuals published by sport diver training agencies describe all types of scuba, including basic rebreather designs. However, by the mid-1950s OC scuba was emphasized as the preferred means of sport diving. By the end of the decade, rebreathers were actively discouraged by the training agencies, although they remained available to sport divers into the mid-1960s through military surplus outlets and manufacturers such as American Diving Equipment, U.S. Divers, Desco, and Pirelli.

Figure 2.15: Arawak or "Miser" SCR. Courtesy of Steve Reimers, Reimers Engineering

In the 1950s, Fenzy of France introduced a passive addition semi-closed circuit unit, known as the DC55. It used a "respiratory linked injection" process at a constant ratio of the pre-mixed breathing gas, keyed to the diver's respiratory minute volume (RMV). The diver's exhaled gas was cleansed and routed into dual counterlungs configured one inside the other. Exhalation inflated both counterlungs and the total counterlung volume remained constant. As the diver inhaled, a proportion of the cleansed gas was inhaled from the larger counterlung, causing the smaller lung to constrict and vent its contents. This activated a demand valve that automatically added the vented volume. The system offered the advantage of maintaining a reasonably constant oxygen fraction (FO2). As early as 1952, the U.S. Navy Experimental Diving Unit (NEDU) evaluated a "constant partial pressure apparatus" produced by the Old Dominion Research and Development Corporation. This is one of the earliest references to a closed-circuit mixed gas system. Supposedly, another unit known as the Rex was also tested before 1960. It utilized a sonic gas analyzer to measure and regulate gas

flow into the loop. Unfortunately, little information is available about either unit today.

Commercial Applications

A spate of underwater exploration occurred from the late 1950s through the early 1970s. Several attempts were made during that period to bring rebreathers out of the military sector and into the commercial and recreational sectors. Most were unsuccessful. Commercial divers were freed from a surface supply of air, saving gas costs, but they still needed umbilical support for thermal protection and power. This continued dependence on surface support negated much of the perceived benefit of rebreathers. Several were developed and used in the commercial sector, but primarily as emergency bailout systems. The most noteworthy variant of rebreather in the commercial sector was the Arawak, designed by Jerry O'Neill. Nicknamed "the Miser," it was originally offered for sale by J.H. Emerson & Co. in 1964. The system operates primarily in CCR mode, using the controlled atmosphere of a diving bell to supply gas to the diver. In its bailout mode, the Arawak operates in a semi-closed configuration. The Arawak designs eventually were sold to Westinghouse, after fatalities associated with similar systems generated an industry reluctance to adopt the systems. Work continued at Reimers Engineering, and then at Divex. Other semi-closed circuit commercial bailout systems also appeared on the market, including the BOS II by Comex-Pro and the SLS of Divex.

Figure 2.16: U.S. Navy Mk6 SCR. Courtesy of Scott Cassell

In 1969, Northrop produced the Mk6, an umbilical semi-closed system, for use by the U.S. Navy. Other umbilical semi-closed systems include the Abalone, the Mk 11, Dräger's SMS 1 and SM IIIS, and Siebe Gorman's SG 700. The Dräger SMS 1 incorporated a constant flow of oxygen, and a hydrostatically compensated diluent addition dosing system. A similar method is presently employed by the SIVA + system built by the Fullerton Sherwood division of Carleton Technologies. Like the Fenzy DC55, the ACSC and DCSC systems by Interspiro of Sweden are also linked to the diver's respiratory minute volume. These military units use a breathing reservoir consisting of an intricate combination of counterweighted bellows attached to a mechanical linkage to control dosage. The dosage subassembly works at ambient pressure, and the overpressure relief valve is mechanically controlled by the position of the breathing bellows, thereby offering greater control of buoyancy and reduced breathing resistance.

Mixed gas rebreathers emerged in the 1960s, with the development of oxygen sensing technology. Alan Krasberg was one of the first designers to build a system that included an oxygen sensor in the loop. His prototype was built with his father's gold filling as the cathode! Oxygen addition into the loop was manually controlled. A refined version employed a thermostat to switch the oxygen supply on and off. By 1964, Krasberg was able to offer a commercial version called the Scubalung, sold through J.H. Emerson & Co. It was eventually sold to Westinghouse, and offered for sale as the Krasberg Scubarig, KSR-5. In 1967, the first cryogenic rebreather was tested in shallow water off Catalina Island, California. Invented by Halbert Fischel, the prototype weighed 104 pounds (47 kg), and had an expected eighthour duration. Fischel's company, Sub-Marine Systems, Inc. sold the idea to Sterling Electronics in 1968. Skin Diver magazine ran an article on the unit in June 1969. The Electrolung, another closed-circuit mixed gas system, appeared at the end of 1968. It was designed by Walter Stark and John Kanswisher. It utilized polargraphic sensors that the diver assembled prior to the dive by adding an electrolyte (potassium hydroxide) and installing a Teflon membrane. The unit was offered to select customers, and by the end of 1969 the design was sold to Beckman Instruments Co. Beckman modified the design and offered it for sale at $2,975 in 1970. The unit was withdrawn from the market after several fatalities.

Figure 2.17: Nick Icorn with a Fenzy RMV-keyed SCR. Courtesy of Nick Icorn

Figure 2.18: Beckman Instruments Electrolung CCR, 1969.

Also in 1969, a group of former General Electric engineers formed Bio-Marine Industries. They had decided to apply their space engineering experience to underwater applications. They produced and marketed a closed-circuit mixed gas system with a galvanic oxygen sensor called the CCR 1000. Innerspace Services Corp.

then adapted the CCR 1000 and marketed flyaway (self contained portability) specialist services and equipment. Their version became known as the Porpoise Pack. The CCR 1000 endured as the basis for the design of the U.S. Navy Mk 15 and the successor in current production, the Mk 16.

Figure 2.19: Biomarine Industries CCR 1000.

Carleton Technologies, Inc., the present licensee building the U.S. Navy's Mk 16, recently received approval to sell that unit to other navies, including Britain and Australia. Carleton has a contract with the British government to build a variation of the Mk 16, which will supercede the CDBA. British interest in the technology of mixed gas rebreathers was prompted by the lure of oilfield development in the North Sea. In 1979, Normalair-Garrett designed the DD500 rebreather for divers operating from the Pisces submersibles. Although the design specifications called for a unit to sustain a diver under light work conditions at 1,650 fsw (500 msw), the manned tests were actually conducted at only 1,150 fsw (350 msw) because test divers were showing symptoms of high pressure nervous syndrome (HPNS). In 1981 moviegoers saw the DD500 in the James Bond film, "For Your Eyes Only," however the system never achieved widespread use. A change in British government regulations required that all life-support systems must be capable of providing 72 hours of independent

bailout life support. Attempts were made to shift the focus of the DD500 to a bailout system, but its size and weight became a critically limiting factor.

Figure 2.20: Military needs fueled rebreather development during most of the twentieth century. Courtesy of Elaine Jobin

Recreational Market Developments

During the 1980s and 1990s, cave divers increasingly felt the urge to go deeper, go further, and stay longer than the limits of OC scuba allowed. They began to rediscover the benefits of rebreathers for some of their more extreme pushes. Requirements for reduced decompression obligation, bubble-free operation, gas economy, extended depth and duration were generated by an era of expeditions: the blue holes of the Bahamas by Rob Palmer and Stuart Clough, Wakulla Springs and other deep caves by the WKPP,

Wakulla Springs and Huautla by Bill Stone, and the cave pushes of Oliver Isler. In 1980, Jochen Hasenmayer designed and built a closed-circuit mixed-gas rebreather known as the Speleo-Twin or STR 80. Effectively, it was two separate (i.e., redundant) rebreathers, each with a potential 24-hour duration. It was computer-monitored, with a single oxygen sensor in each system and optical and acoustical alarms. In the event of failure, the unit would operate in semi-closed mode, integrating a l00 cf (12 L) open-circuit bailout capacity. In 1981, Hasenmayer successfully used the unit to achieve a depth record of 433 ffw (143 mfw) in Fontaine de Vaucluse. He also used the STR 80 at Emergence du Ressel in the Dordogne, achieving a round trip of approximately 12,500 feet (3,800m) with only fin propulsion. Hasenmayer's bottom time for this dive was about three and a half hours, and his decompression obligation was only about four hours - significantly less than the 15 hours open-circuit equipment would have required.

Figure 2.21: The Halcyon RMV-Keyed SCR has been used to explore the Walkulla cave system in Florida at depths of 300 ffw

(90 mfw).

Courtesy of WKPP, George Irvine

In 1985, Bill Stone designed and built his CisLunar Mark I, another redundant, closed-circuit, mixed-gas rebreather. A prototype was ready for testing by 1986, and, using it, Stone remained under water for 24 hours. Over the next eight years, the successor modifications showed their mettle with penetrations 1,300+ feet (400m) into the San Agustin sump of Huautla in Mexico. In 1990, Stone patented the CisLunar. After many modifications, it became the basic diving tool of the U.S. Deep Cave Team for the Wakulla 2 project in 1998-99, generating three-dimensional cave maps corresponding with the surface topography of Wakulla Springs, Florida.

Figure 2.22: On December 3-4, 1987, Bill Stone successfully used the CisLunar MK I CCR to complete a 24-hour dive. Courtesy of William Stone

Alain Ronjat and Olivier Isler designed and built a redundant semi-closed system, the RI 2000, which Isler dived in the La Doux De Coly system in 1989. The following year, Isler used the RI 2000 to exceed Hasenmayer's push in the Emergence de Ressel.

Figure 2.23: The Dräger Dolphin SCR was designed for the active recreational diver. Courtesy of Micropore

Rebreather introduction into the recent mainstream recreational market really began with Dräger's introduction of the Atlantis I in 1995. This was the first unit to gain significant market acceptance. This semi-closed rebreather was aimed at the average advanced diver, with mainstream training agencies offering specialty classes in its use. Dräger introduced a newer version of the rebreather, the Dolphin I (Figure 2.23), two years later. Finally, they introduced a simplified version of this nitrox SCR, the DrägerRay. This was

designed for the diver looking for simplicity in rebreather setup and use. It is estimated that more than 7,000 of the three models were sold. Ambient Pressure Diving succeeded in similar market acceptance with their Inspiration CCR (Figure 2.24). This mixed gas CCR was first shown in 1997, and has since been modified several times. In 2005 they introduced the Evolution, a smaller, lighter CCR with more advanced electronics. To date, their estimated sales exceeds 6,000 units, by far the most CCRs sold by any manufacturer into the recreational market. While these two companies have respectively set the standard for SCR and CCR rebreather use, they are not the only manufacturers who have developed units or expressed their intent to market rebreathers. Since 1990 over 50 units have been designed for the sport, technical, and scientific diving markets (Table 2.1). While many are no longer in production, new units are introduced every year.

Figure 2.24: AP Diving introduced the Inspiration CCR in 1997, becoming the first manufacturer to sell more than 1,000 CCRs into the recreational diving marketplace. Courtesy of Elaine Jobin

This is an exciting time in the history of rebreather development. It is analogous to the introduction of automobiles in the late 1800s and early 1900s, when scores of companies and individuals had vehicles they believed to be the "best" car. In the United States, these many firms eventually were winnowed down to just three or four major manufacturers. In the scuba industry, it is likely that the eventual market dominators have not even introduced the rebreathers that will become the standard of the future.

Interest clearly indicates that rebreather systems have "arrived," and, although they require more work and commitment to maintain and operate than open-circuit scuba, their benefits and their reliability make them an attractive alternative - and for some the only - way to enjoy the "Silent World." Table 2.1: Rebreathers introduced into the sport diving market since 1990. Abyss "Vampry" RB AP Diving "Evo Plus" CCR AP Diving "Evolution" CCR *AP

Diving "Inspiration" CCR AP Diving "Inspiration--Vision" CCR B&E Manufacturing "Nautilus" CCR *Biomarine

Industries "BMR 500" CCR

*BMD

"SCR-4" Cameleon "Adaptive" CCR *Carmellan

Research "Kraken"

*CisLunar

"Mk IV" CCR

*CisLunar

"Mk V" CCR

*Cochran

Consulting "Cochran Rebreather" CCR Dive Rite "O2ptima" CCR *Dräger

"Atlantis I"SCR Dräger "Dolphin I"SCR Dräger "DrägerRay"SCR *Grand

Bleu "Fieno"SCR Grey Wolf Innovations "Continua" CCR *Halcyon

"Halcyon II"SCR Halcyon "RB-80"SCR

HB Technologies "Voyager" CCR Hollis "PRISM II" CCR HSE "Neptune Explorer Sidemount" CCR Hysrospace Engineering "Neptune Explorer" CCR Infinito "EANT 444 FC" CCR Infinito "EANT 444 SC/O2"SCR and O2 CCR Infinito "EANT 444 XC" CCR Infinito "EANT 888"SCR/CCR Innerspace Systems Corporation "Megalodon" CCR Innerspace Systems Corporation "Pathfinder" CCR Innerspace Systems Corporation "Tetradon" CCR *Japan

Oxygen Co. Ltd "EOBA" Jetsam Technologies "KISS" CCR Jetsam Technologies "Sport KISS" CCR Jetsam Technologies "Travel KISS" CCR JJ-Technique "JJ-CCR" CCR Juergensen Marine "Hammerhead" CCR LRT "Frog"SCR LRT "Gator"SCR LRT "Probe" CCR Mentes "IQ Sub" CCR Ocean Management Systems "Tesseract" CCR *Oceanic

"Phibian" CCR

*Olympic

Submarine Technologies "CCR2000" CCR Open Safety "Apocalypse IV" CCR Oxpro "Explorer" O2 CCR Poseidon "Discovery VI" CCR Radikal/Shape "Radikal" CCR Rebreather Lab "Pelagian" CCR

Rebreathers Australia "Stingray" CCR Rebreathers Australia "Abyss" CCR rEvo Rebreathers "rEvo" CCR *San

O Sub "Azimuth"SCR

*San

O Sub "Nemesis" CCR

*SeaStealth

CCR Seaway "Cora II" CCR STDE "ED04" CCR *Steam

Machines "PRISM Topaz" CCR

*Steam

Machines "SM1600" CCR Submatix "CCR 100 SMS" CCR Submatix "SCR 100 ST"SCR Swiss CCR *TaucherBiebl

"Biebl" CCR The Rebreather Company "Odessey" CCR Titan Dive Gear "Titan" CCR *Undersea

Technologies "UT-160" CCR

*Undersea

Technologies "UT-240" CCR VR Technologies "Ouroborous" CCR VR Technologies "Sentinal" CCR Wassersport "Tourill Mk 1.5"SCR *No

longer available

1. List four historical events in rebreather development: a. __________________________________________. b. __________________________________________. c. __________________________________________. d. __________________________________________.

2. Early (pre-1870) rebreather development occurred primarily in which industry? a. Mining. b. Commercial diving. c. Military diving. d. Aerospace.

3. Early and mid-twentieth century rebreather development occurred primarily in which industry? a. Mining. b. Commercial diving. c. Military diving.

d. Aerospace.

4. Rebreathers have recently been recreational/scientific communities to:

used

in

the

a. Allow deeper or more extended dives in which gas economy is a critical factor. b. Sink foreign naval and shipping vessels. c. Increase interest in major feature films. d. All of the above.

Types of Rebreathers Overview

We have seen that many types of rebreathers have been designed. Some designs are outdated. Some permit or preclude some types of diving. Some designs were developed for recreational diving, others for the military, commercial, or scientific diving sectors. This chapter will focus on those types of units that are commonly encountered in recreational or scientific diving. You will learn the primary design characteristics of various types of rebreathers, and the strengths and weaknesses of semi-closed circuit rebreathers, oxygen rebreathers, and mixedgas closed-circuit units. You will read about various types of rebreather diving systems now available, and you will learn selection criteria to help you determine which rebreather best suits your needs.

Objectives After reading this chapter, you will be able to: 1. Draw generalized schematic diagrams of oxygen CCR, SCR, and mixed-gas CCR rebreathers, noting their differences. 2. Describe the four different SCR designs, and how each operates. 3. Define a "breathing loop." 4. List four tests of rebreather performance, and explain the importance of each. 5. Define three attributes to consider before purchasing any rebreather.

What is a Rebreather?

An Overview of the Types and Classifications The equipment we think of as "normal" scuba has a cylinder of compressed breathing gas. The gas is delivered to the diver through a regulator that compensates for ambient pressure. After the diver has inhaled the gas once, it is exhaled into the water. This is termed "OC" or "open-circuit," because after the gas leaves the diver it is no longer part of the breathing cycle. OC diving gear is simple and inexpensive, but it is not very efficient, since most of the gas is wasted. Inventive divers over the years have used a variety of methods to try to capture and reuse the exhausted breathing gas, to close the breathing circuit, in their quest to develop more efficiency. Let's look at the design of these systems and their typical categorization. The term "rebreather" aapplies to any breathing system that extends gas economy by recycling a diver's breathing gas. As "closed-circuit" systems, all rebreathers have components in common. In order to be a rebreather, a breathing system will incorporate some sort of variable volume container to capture the diver's exhaled breathing gas. This usually takes the form of one or more breathing bags or counterlungs but may be as simple as a set of corrugated hoses that expand or collapse as the diver breathes.

The second key component of any rebreather is some means to remove expired carbon dioxide from the breathing gas as it is recycled. Many techniques, from cryogenics to chemicalsoaked rags, have been applied to the task. The resulting device, whatever its configuration, is typically called a scrubber. Most scrubbers in use today pass the breathing gas through a quantity of disposable chemical carbon dioxide absorbent - most typically sodalime. We will address scrubber design in more detail in Chapter 7.

Figure 3.1: A simple breathing loop.

The portions of a rebreather that incorporate these two elements are called the breathing loop. The breathing loop is the path taken by the breathing gas through an underwater breathing apparatus. Mechanically, it includes the mouthpiece, breathing hoses, counterlung, and scrubber. The loop is generally thought to include the diver, because the gas is in the diver's lungs during part of the process. While all rebreathers incorporate these two features, (variable volume container and scrubber) they vary greatly in specific design, usually being grouped into three categories. We will conform to that convention. Each category has advantages and disadvantages, and each requires different training. These three major classes of rebreathers are oxygen closed-circuit, semi-closed circuit, and mixed-gas closed-circuit.

Oxygen Closed-Circuit Rebreathers

Often simply referred to as oxygen rebreathers, oxygen closed-circuit rebreathers are the simplest and least expensive rebreather design. As the name implies, the breathing gas is 100 percent oxygen. Since there is no inert gas in the breathing loop, the diver incurs no decompression obligation. Properly used, oxygen rebreathers are really the only bubble-free diving systems. In such a system, oxygen is added through a valve designed to keep the volume of gas in the breathing loop constant. As that volume decreases due to metabolic consumption of the oxygen or descent pressure, oxygen is added to compensate. Since most oxygen CCRs have no built-in means to vent overpressure gas, oxygen closed-circuit rebreathers are the only "fully" closed-circuit rebreathers. If the diver maintains a given depth, or ascends slowly enough to make up for gas expansion during ascent by metabolic consumption of the oxygen, an oxygen CCR can be operated with absolutely no bubbles. It is important to note that an oxygen

closed-circuit rebreather diver who ascends faster than this metabolically determined ascent rate must vent gas out of their mask or mouth to avoid pulmonary rupture.

Figure 3.2: Oxygen closed-circuit rebreather.

Figure 3.3: The Draäger LAR V oxygen CCR is a system commonly used by military divers Courtesy of Dräger

Oxygen CCRs have another limitation. Because the diver is breathing essentially 100% oxygen, the system is limited to depths shallower than about 20 fsw (6 msw). Using an oxygen rebreather deeper than this depth greatly increases the risk of CNS oxygen toxicity as the PO2 (oxygen partial pressure) exceeds 1.6 atm. Most recreational training agencies recommend a limit of 1.4 atm, a depth of 13 fsw (4 msw). Oxygen closed-circuit rebreathers are used frequently by military combat swimmers and for covert shallow water operations. They are also commonly used in surface applications by

firefighters, as mine rescue apparatus, and as medical devices. Because of their limitations, recreational divers very rarely use them, although they are occasionally used by scientists in some research applications. The use of oxygen closed-circuit rebreathers requires specific training.

Semi-Closed Circuit Rebreathers

To dive deeper, the oxygen in the breathing mix must be diluted with some inert gas. Semiclosed circuit rebreathers (SCR) typically utilize an enriched air nitrox gas (EANx) as the base gas. Enriched air nitrox is an oxygen-nitrogen mixture containing a higher oxygen percentage than the nominal 21% in air. As we can see in the breathing loop diagram (Fig. 3.4), a semi-closed circuit rebreather is similar to the oxygen CCR with alterations to accommodate an inert gas mix. The first noticeable change is the addition of an overpressure valve to vent gas to keep the breathing loop at ambient pressure. The second important feature is a metering valve that handles the injection of breathing gas. There are several philosophies regarding the method used to inject the breathing gas in a semi-closed circuit rebreather, but the primary design goal of all of them is to inject at least enough breathing gas to resupply the diver's consumed oxygen. The semi-closed circuit rebreather classification can be broken down further into four primary sub-classifications: 1. Constant Mass Flow 2. Respiratory Minute Volume Keyed 3. Constant Ratio 4. Electronically Controlled

Constant Mass Flow (Active SCR)

The constant mass flow semi-closed circuit design is the simplest type of recreational rebreather. Such systems meter a constant flow of fresh EANx into the breathing loop with the flow rate based on the constant FO2 (fraction of oxygen) gas mix. A corresponding amount of gas is vented from the loop by the overpressure valve. Once these systems are turned on, gas injection is continuous. It makes no difference if the diver is breathing on it or not. For this reason, constant mass flow units are also called Active SCRs. A critical component of a constant mass flow system is the mass flow controller, since it must precisely regulate the flow of gas by mass, regardless of depth. We'll look at the constant mass flow valve in more detail in Chapter 6.

Figure 3.4: Mass flow controlled semi-closed circuit rebreather.

Figure 3.5: One rebreather gas cylinder can replace many open-circuit gas cylinders.

Figure 3.6: The DrägerRay (left) and Dolphin constant mass flow SCRs. Courtesy of Dräger

Respiratory Minute Volume Keyed (Passive SCR, QuasiConstant FO2

A slightly more complex method of gas injection is termed "respiratory minute volume keyed" (RMV-keyed). This method attempts to more closely match injected gas volumes to the diver's metabolic consumption by venting a portion of the breathing loop gas on each breath. To keep the breathing loop volume constant, fresh EANx is injected by a valve keyed mechanically to the diver's breathing rate. These systems maintain a reasonably constant fraction of oxygen in the breathing loop. RMV-keyed systems are also called Passive SCRs, since the effort of the diver breathing is required to initiate gas injection. A variety of RMV-keyed systems have been designed over the years. Most are somewhat more efficient than constant mass flow systems, but all are more complicated to maintain and to dive.

Figure 3.7: RMV Keyed Counterlung. During the exhalation cycle (upper), both of the counterlungs (discharge and outer) are filled. During the inhalation cycle (lower), the diver inhales gas from the outer counterlung simultaneously venting from the system the gas in the discharge counterlung.

Figure 3.8: Halcyon RMV Keyed Bellows Counterlung. Courtesy of George Irvine

Constant Ratio (Quasi-Constant PO2)

A third classification of semi-closed design differs from the others in that it has two gas cylinders – one of oxygen and one of another gas used to dilute the oxygen, called diluent. The diluent gas, which is usually air, is added to the breathing loop through a hydrostatically compensated valve. Diluent addition varies with depth. Oxygen is usually added via a traditional mass flow controller. This system can maintain a nearly constant partial pressure of oxygen in the breathing loop over varying depths, but it does not compensate for varying diver consumption rates. The system requires mass flow metering based on worst-case workload estimates, which results in continuous exhaust from the system overpressure valve. The SIVA Plus military rebreather uses this design approach. At this writing, no constant ratio rebreathers are available to the recreational diving community.

Figure 3.9: SIVA Plus Constant Ratio SCR. Courtesy of Fullerton, Sherwood Engineering, Ltd.

Electronically Controlled

This sub-class is more a mode of operation than a design classification. Some mixed-gas closed-circuit rebreathers are designed so that they can be operated in semi-closed circuit mode. This requires an adjustable over-pressurization valve and the use of EANx or other mixed gas in place of pure oxygen. Because this mode of diving works with mixed gas CCRs, it is usually only addressed in CCR courses.

Longer, Quieter Dives

While all semi-closed circuit rebreather systems exhaust some bubbles, they provide a much quieter diving experience and typically as much as a fourfold increase in gas economy contrasted to open circuit. They allow diving as deep as 130 fsw (40 msw), and are simpler and less expensive than their mixed-gas closed-circuit brethren. In the mid- to late-1990s, semiclosed circuit rebreathers were the type most widely used by recreational divers. However, since that time mixed-gas closed-circuit rebreathers have increasingly become the design of choice.

Mixed-Gas Closed-Circuit Rebreathers

Mixed-gas closed-circuit rebreathers (CCR) are designed to improve on the efficiency of semi-closed circuit designs by injecting fresh oxygen only as it is needed to compensate for the actual oxygen consumption of the diver. They accomplish this by measuring the actual partial pressure of oxygen in the breathing loop and injecting more oxygen to keep that partial pressure constant. For this reason, mixed-gas closed-circuit rebreathers are also correctly termed constant partial pressure rebreathers. Some constant-ratio-based, semi-closedcircuit rebreathers may approach constant partial pressure operation, but, because they vent gas from the breathing loop, they are classified as semi-closed circuit designs. Since they must measure breathing loop oxygen partial pressure, all mixed-gas closedcircuit rebreathers designed to date use some sort of oxygen sensor(s). There are two basic design philosophies of mixed-gas CCRs, manually controlled systems (mCCRs) and electronically controlled systems (eCCRs). The diver controls some or all of the gas injection in mCCRs, while in eCCRs electronics control oxygen addition. Purely manually controlled systems have been built, but are generally not used in recreational diving.

Figure 3.10: Manually controlled CCRs are the simplest type of mixed gas CCRs

Figure 3.11 Manually Controlled Mixed-Gas Closed-Circuit Rebreather (mCCR)

The primary design goal of a mixed-gas closed-circuit system is to deliver to the diver a gas mix that contains a constant partial pressure of oxygen. This equates to delivering to the diver the best possible gas mix throughout the dive. A look at the breathing loop diagram of the mixed-gas closed-circuit systems (Figures 3.11 and 3.12) will also reveal several other features required by this design. The most obvious difference between these designs, most semi-closed circuit designs, and all oxygen closed-circuit designs is the addition of a second compressed gas cylinder. A mixedgas closed-circuit system uses two cylinders. One usually contains pure oxygen. A second cylinder carries some other gas mixture used to dilute the oxygen in the breathing loop so the oxygen partial pressure can be kept constant with increasing depth, preventing oxygen toxicity. This second gas mix is termed diluent and at recreational diving depths is most often air. Heliox, trimix, and other more exotic gas mixes are used as diluents for deep diving. Any diluent gas mix should be capable of supporting life if used alone. Nearly all mCCRs constantly add oxygen to the breathing loop. The design objective is to add sufficient oxygen to replace what a typical diver would metabolize while at rest. This gas addition takes place through a metering valve or mass flow controller. When the diver is working, he must manually add the additional oxygen needed to make up for the oxygen being consumed. This is done by activating an oxygen addition device (usually a button or lever) whenever the partial pressure of oxygen in the breathing loop falls below the desired level.

Figure 3.12 Electronically Controlled Mixed-Gas Closed-Circuit Rebreather (eCCR)

Reacting to sensitive oxygen partial pressure sensors, the electronics in eCCRs control the addition of oxygen through an electrically operated valve (Figure 3.12). This keeps the oxygen partial pressure reasonably constant with varying diver metabolic consumption and depth. With an eCCR, however, the diver must regularly monitor the oxygen level to ensure that the electronics are functioning properly. Most mixed-gas CCRs use three redundant oxygen sensors, although some are designed to rely primarily on a single sensor that is continually checked for proper output against a known gas (more on this in Chapter 7). The readings from these sensors are used to control oxygen addition. Diluent is added, in most cases automatically, to keep the volume of the breathing loop constant while descending. Some eCCRs provide manual addition valves to allow the diver to add diluent and/or oxygen in emergencies, or to operate the unit manually independent of the electronics. Most importantly, all mixed-gas CCRs display to the diver the actual partial pressure of oxygen in the breathing loop. Mixed-gas closed-circuit rebreather systems are by far the most complex and expensive of the three rebreather classifications. They require a higher level of attention before, during and after the dive, and require more extensive training than semi-closed circuit rebreathers. In exchange, mixed-gas closed-circuit rebreathers offer minimum bubble operation, very good gas economy, reduced decompression obligation, and extended depth capability. Table 3.1: Summary of Rebreather Parameters Classification Oxygen Oxygen Partial Percentage Pressure Oxygen Variable Constant ClosedCircuit Semi-Closed Circuit (SCR) Constant Variable Variable

Complexity Depth Gas Bubbles Deco Range Limit Economy Obligation (fsw/msw) (vs. OC) Simple 20/6 20x+ None None

Simple

130/40

3x-4x

Some

Same

Mass Flow RMV Keyed Constant Ratio Mixed-Gas (CCR)

Variable

Quasiconstant Variable

Quasiconstant Constant Variable

Complex

130+/40+ 4x+

Some

Same

Complex

130+/40+ 4x+

Some

Same

Complex

130+/40+ 10x+

Ascent Only

Much less

With rebreathers, "bigger" does not necessarily mean "better." As with any equipment, the diver must determine the desired objective and select gear accordingly. This is especially true with rebreathers, where the difference between units may involve huge differences in training time and purchase price. For many types of diving, open-circuit scuba may still be the system of choice, but increasingly, nothing beats the right rebreather.

Choosing a Rebreather – Criteria

Investing in a rebreather is a serious decision. When you consider that units may cost from less than $2,000 to more than $40,000, you want to be sure that the system you purchase will meet your needs. You also want to know that it will operate as promised, and that it will provide many years of service. Before purchasing a unit, there are many factors to consider. Where do you start? Rebreathers are nothing more than tools to allow you to remain underwater for an extended period. Other alternative tools that accomplish much the same goal include the snorkel, opencircuit scuba, surface-supplied air, underwater habitats, submersibles, and submarines. So how do you choose? Basic selection depends on your objectives. If you wish to remain under water for several days, then a habitat or submarine may be the answer. A more typical objective for a recreational diver might be to explore a wreck. Let's assume Mary wants to explore a wreck at 18 fsw (5 msw), and her task does not involve penetration into silt-laden passages. OC scuba would be her simplest solution. Oxygen rebreathers, SCRs and CCRs would also work, but the cost would be greater and these alternatives would entail more training and effort to prepare for the dive.

Figure 3.13: Work of breathing test machine. Courtesy of Micropore

Figure 3.14: Typical work of breathing test results.

If she decides to penetrate into the silty wreck, then a dive unit that reduces bubble exhaust would be preferred. All of the rebreathers would be a better option than OC scuba, based on this parameter. Suppose that the wreck is at 45 fsw (14 msw). This would limit her options by removing the oxygen rebreather from consideration. A dive at 225 fsw (69 msw), or requiring a bottom time of 400 minutes, would eliminate most SCRs from the list of options. As you can see, selection of a particular system varies based on environmental constraints as well. Once you have defined the type of unit that would best fit your needs, based on objectives, dive environment, cost constraints, and training requirements, you will still need to select a particular model or manufacturer. There are several things to look for when doing this, including testing, performance standards, certification, and future availability of service and spares for the unit. Test data provide the most important criteria describing the performance of any diving system. For example, you may have seen tests comparing regulator function in diving magazines, or published by the U. S. Navy. With rebreathers, several types of tests commonly define performance. Work of breathing tests determine how difficult respiration is with a unit. In a rebreather, unlike OC systems, the diver powers the movement of gas through the system. The effort (work) required to do this can have a significant effect on the diver's performance during the

dive. During a work of breathing test, a series of graphs is developed that depicts the effort necessary to inhale and exhale through the breathing loop. This testing is generally performed at a series of depths, a range of breathing rates, and often with the unit in a variety of orientations. This is necessary because units that perform well in shallow depths may not perform at all in deep water, and changes in your position will definitely affect some units' operation. All else being equal, the lower the work of breathing, the better the unit. Canister (scrubber) duration is another important specification to consider in your rebreather selection. The scrubber canister is filled with an absorbent that chemically removes the CO2 (carbon dioxide) from the recirculating gas. When the scrubber fails to remove all CO2, breakthrough is said to occur. At breakthrough, dangerously high levels of carbon dioxide can build up in the breathing loop and be passed to the diver, so use of the rebreather must stop until the scrubber is replenished. Scrubber duration tests are performed at various diver work rates, breathing rates, and ambient temperatures.

Figure 3.15: Typical CO2 breakthrough test results

Figure 3.16: Rebreather computer controls and algorithms should be independently tested.

While it might seem that longer lasting scrubbers are better, there are more important considerations. The scrubber should be designed to meet the needs of the type of diving activity planned. It makes little sense to haul around an 80lb (36 kg) rebreather with a scrubber capable of supporting you through a 12 hour arctic dive if you are just doing an afternoon hop

down to check out the reef at 60 fsw (18 msw). You will have a much better time using a small, lightweight unit with a two or three hour scrubber. The chemicals used to absorb carbon dioxide in most rebreathers produce an exothermic reaction - meaning they generate heat. This allows us to breathe warm, moist breathing gas when using a rebreather. The reaction also requires a certain level of heat to operate. If the "bed" of absorbent chemical falls below a certain critical temperature, the absorbent reaction becomes inefficient and the scrubber duration - the working time before breakthrough decreases markedly. Some rebreather makers insulate their scrubber canisters and so gain much better cold water performance. When reviewing scrubber duration results, look at cold water performance if you plan on using your rebreather in cold water. A stable oxygen level is also an important design criterion - especially for a mixed-gas closed-circuit system. All rebreather designs allow for some fluctuation in the PO2 in the breathing loop. Gas control tests measure the frequency and amplitude of these variations. Variables analyzed in a typical test include respiration rate changes, depth variations, and manual versus automatic gas addition. A system maintaining stable PO2 levels is generally preferable to one that allows wide variations. Some rebreathers may include computer controls or algorithms to calculate inert gas loading for the diver. This makes determination of inert gas loading easy, as the diver does not need to use tables, calculate equivalent air depths, etc. However, any such model must be validated, and the software tested for its robustness and visual display ergonomics. This is the purpose of computer/algorithm testing. A word should be said here about constant partial pressure decompression algorithms. There is currently a relatively large statistical base of OC dive experience from which to draw data for OC dive tables and subsequent computer algorithms. As a result, we can have fairly high confidence levels in the ability of current dive computer algorithms to accurately predict decompression obligation on OC dives. As of this writing, no large statistical base exists for closed-circuit diving. If you choose to use a constant partial pressure rebreather because of its advantage in reduced decompression obligation, keep in mind that no well-proven constant partial pressure decompression algorithms yet exist. There is not yet a significant base of diving experience at elevated constant partial pressures While the rule of thumb with ALL decompression computer use is BE CONSERVATIVE, this is especially true when using constant PO2 dive computers or dive tables. Integral to any dive computer system is the transducer, which provides information to the processor. A transducer is nothing more than an electronic means of measuring pressure. Multiple transducers may be used in a rebreather, each with its own range of operation. One or more transducers may measure high pressure (such as cylinder pressure), and another measures low pressure (for use as a depth gauge). Redundant transducers may be used to provide backup or verification capability. Transducer testing must be done to ensure that the transducer operates as designed and within accuracy parameters.

Figure 3.17: DCIEM test facility.

Courtesy of Fullerton, Sherwood Engineering, Ltd.

Some other tests include durability testing, ergonomics evaluation, and battery life evaluation. Durability testing is a means of evaluating overall robustness of the rebreather under typical operational conditions. Part of such testing might involve dropping the unit from a fixed height onto a hard surface, like a concrete floor, to see if the materials shatter, crack, or dent. Ergonomics evaluation is a series of tests to see how easy it is to utilize controls and read displays in a variety of conditions, such as limited visibility or while wearing heavy mittens. Battery life evaluation tests how long batteries will continue to operate the rebreather in different environmental and operational conditions. Finally, any rebreather system you choose should be tested for reliability. Often, electronic components are tested separately from mechanical systems, as the analyses performed differ significantly. The analyses yield a mean time between failures (MTBF)– an indication of how long the system may be expected to operate without a major failure. In many cases, rebreather manufacturers conduct these tests "in house," having their own staff perform them. In these cases, the manner in which the tests are performed may vary, yielding results that are difficult to compare among manufacturers. In some cases, because the manufacturers have a vested interest in showing that their units work well, test protocols or results may be inadvertently skewed. This occasionally occurs as the evaluators unconsciously turn a blind eye to results that might indicate a possible problem. Because of this, independent testing by a noninvolved party is considered best. Agencies such as the U. S. Navy Experimental Diving Unit (NEDU) or Canada's Defense and Civil Institute of Environmental Medicine (DCIEM) have experience in conducting tests of this nature. While most of their experience has been gained from the need to see that their organizational requirements are met, they have also done independent testing for rebreather manufacturers as well. Table 3.2: European Committee for Standardization (Comité Européen de Normalisation, CEN) Member Countries Austria Belgium

Hungary Iceland

Norway Portugal

Czech Republic Ireland Slovakia Denmark Italy Spain Finland Luxembourg Sweden France Malta Switzerland Germany Netherlands United Kingdom Greece Table 3.3: European Standards Pertaining to Rebreathers. EN132: Respiratory protective devices—Definition of terms and pictograms EN134: Respiratory protective devices—Nomenclature of components EN144-1: Respiratory protective devices—Gas cylinder valves—Part 1: Thread connections for insert connector EN144-3: Respiratory protective devices—Gas cylinder valves— Part 3: Outlet connections for diving gases nitrox and oxygen EN250: Respiratory equipment—Open circuit self contained compressed air diving apparatus—Requirements, testing and marking EN12021: Respiratory protective devices—Compressed air for breathing apparatus EN14143: Respiratory equipment—Self-contained re-breathing diving apparatus EN61000-6-1: Electromagnetic compatibility—Part 6-1: Generic standards, Immunity for residential, commercial and light industry environments ISO/IEC12207: Information technology—Software life cycle process IEC61508: (all Functional safety of electrical/electronic/programmable electronic safetyparts) related systems ICE60300-3-6: Dependability management—Part 3: Application guide—Section 6: Software aspects of dependability Independent testing only portrays part of the picture. In order to place test results into perspective, a standard for each of the key functional areas must be defined. Test results may then be compared to this minimal performance standard to ensure that the system operates at a reasonable level. At this time, the various rebreather manufacturers have not yet agreed upon uniform industry-wide performance standards or testing protocols. However, in recent years there has been an accelerating effort to codify these. Most European countries belong to a cooperative group called the European Committee for Standardization (CEN), Table 3.2. They set equipment standards to which all member countries adhere. Several standards apply to rebreathers. Specifically, European Standard (Norme Européenne) EN14143 pertains to rebreathers. This standard defines rebreather performance standards, for individual components as well as assembled rebreather systems. It also sets which tests must be conducted, as well as the variable range of those tests. EN14143 also cites a number of other European standards (Table 3.3), notably EN250 and IEC61508, which are incorporated by reference. EN14143 is still in development. This is acknowledged in the Introduction of that standard, which states, "The production of the standard has raised new questions regarding the interpretation of the physiological and equipment acceptance limits for the diving application which have not been fully answered. However, this standard has been published to provide a level of safety for re-breathing diving apparatus."

In the United States, the issue of testing and design parameters for rebreathers has traditionally been done by the U.S. Navy at NEDU. However, these standards were written to evaluate rebreathers for their military mission requirements, which differ from those of the sport and scientific communities. Recognizing this, in 2004 the National Oceanographic and Atmospheric Administration (NOAA) published a set of proposed manufacturing and performance standards for rebreathers that would be used by their divers or sponsored divers. In their proposed requirements, they specifically stated that manufacturers must conform to an ISO9002 quality control program, as well as providing a Failure Mode Effect and Criticality Analysis (FMECA) for all components and software. They further established a variety of manufacturing and performance criteria, some of which parallel those of the U.S. Navy and EN, some of which do not. Concurrently, OSHA (Occupational Safety and Health Administration) set forth standards for the use of rebreathers in the workplace. This was part of their effort to expand the recreational scuba exemption to allow diving with nitrox. One of the most interesting points on which they are unique is on the concept of carbon dioxide monitoring. In the draft standard they require a CO2 monitoring capability. Recognizing that no manufacturers had that technical capability, they listed the requirement to provide an impetus to achieve it. It is apparent that interest in performance and testing standards for rebreathers is quite high. Governmental diving groups, regulatory agencies, manufacturers, and end user groups are all working to define what these standards should be. While we have some standards in place now, they often conflict, and are recognized to be in a state of flux. We can expect significant evolution in this area. In some areas, minimal production standards have been set for rebreathers. In the United States, the American National Standards Institute (ANSI) theoretically performs this function, but they have yet to address rebreathers. In Europe, the program is Conformite Europeenne (CE). Upon determination of acceptable criteria, and proof by the manufacturer that they have met those criteria, a CE mark may be applied to the unit. This certifies that the unit has met those minimum standards as established by CE. However, since the standards for applying the CE mark may not include all of the above-mentioned testing, results of more complete testing should also be reviewed as part of your evaluation process. Once you have the rebreather, you need to be able to maintain it in working order. In many respects, the civilian rebreather market is in its infancy. Manufacturers are advertising units before they are built and ready to sell. Some manufacturers have even collected deposits on units that have not yet been constructed, tested, or distributed, trusting that they will be able to bring a unit to market in the near future. It is reasonable to expect that many of these manufacturers may not be in business in the future. How would you like to be the proud owner of a new, expensive automobile, only to have the company go out of business in six months? Just think - no qualified service technicians, no warranty support, and no spare parts available to repair damaged or worn components. The state of the rebreather market today is definitely "Caveat emptor," or "Let the buyer beware!" Investigate the stability of the manufacturer before making a purchase decision. A company with a long track record of activity in the rebreather market, a sizable financial base, and commitment to supporting their products is probably more likely to be around when you need them.

Figure 3.18: CE Mark certifies that the equipment has been tested and has met European standards.

In order to reduce the risks of purchasing a rebreather, many divers opt to rent or lease them on a short-term basis, or join a "time-sharing" program such as that of The Rebreather Club®. This puts most of the responsibility for maintenance with the rental agency. It also may be a far more cost-effective means of using a rebreather, since many of us do not have the need or luxury to go diving every day. In addition, it provides the opportunity to try different units prior to making a purchasing decision, offering another step in the decision-making process. This discussion merely provides a starting point for your evaluation. You will not be able to make an informed decision without researching many different data sources. Take your time, talk to people, look for independent information sources, and, if possible, try before you buy. If you do these things, you will find that you have a much better chance of being happy with your acquisition many years from now.

Figure 3.19: Matching the proper rebreather to your underwater interests will add to your diving enjoyment. Courtesy of Andrew Sallmon, Sealt.com.

1. What are the basic classifications of rebreathers? a. Semi-closed and fully closed circuit. b. Open-circuit and closed circuit. c. Oxygen and nitrogen. d. A & B.

2. Draw a simple schematic of a mass flow controlled SCR and label the components.

3. Which of the following are types of semi-closed rebreathers? a. Mass flow controlled. b. RMV Keyed. c. Oxygen. d. A & B.

4. Describe how a mass flow controlled SCR operates:

______________________________ ______________________________ ______________________________ 5. Which of the following components comprise the breathing loop? a. Counterlung, buoyancy compensator, mouthpiece and scrubber. b. Mouthpiece, hoses, buoyancy compensator, scrubber and water trap. c. Counterlung, breathing hoses, mouthpiece and scrubber. d. Scrubber, mouthpiece, counterlung, gas supply cylinder, and breathing hoses. 6. Which of the following test result sets should one look at before purchasing a rebreather? a. Scrubber duration. b. Work of breathing. c. Gas control. d. All of the above. 7. Rebreather performance tests are best conducted by: a. The rebreather manufacturer. b. An independent testing house. c. The end user.

d. The Rebreather Advisory Council.

Diving Physics Overview

As we have seen, rebreathers are, in effect, nitrox mixing machines. However, instead of the mix being fixed and wholly dependent on the initial blending, the oxygen fraction may vary with depth, diver control, and work effort. It is because of this that one common prerequisite for the rebreather course is to be a basic Nitrox Diver. The material that is taught in a basic nitrox course is absolutely essential to understanding rebreather use. Some agencies now allow rebreather training without prior OC or nitrox diving experience. This chapter is a review for those who may have taken prior nitrox courses, or as an introduction to nitrox diving concepts for the new user.

Objectives After reading this chapter, you will be able to: 1. Identify and define the various abbreviations associated with pressure, volume, and nitrox gas mixes. 2. Determine absolute pressure at depth. 3. Quantify volume changes related to pressure changes in flexible containers. 4. Calculate the partial pressure of any breathing gas constituent at any depth. 5. Calculate the maximum operating depth for a mix given its FO2. 6. Determine equivalent air depths. 7. Compute breathing gas surface air consumption rates, and use these in dive planning. 8. Compute gas volume available in cylinders and flasks of varying sizes and pressures.

Pressure and Depth

As you were told in your first scuba course, the surface of the Earth is covered by a blanket of air. While air does not weigh very much in small amounts (about 0.0807 pounds per cubic foot, or 1.29 kg/m3), the 60-mile-plus (100 km) column of air resting

upon us at sea level has significant weight. This weight is transmitted as a force upon the surface of every object on the planet. At sea level the force exerted by this invisible column of air is 14.7 pounds per square inch (14.7 psi; or 1.01 bar). This pressure is known as one atmosphere (1 atm). As we slip beneath the ocean's surface, the water above us begins to add its weight to that of the air above it. Water is much denser than air. Fresh water, at 62.4 pounds per cubic foot (1 g/cm3), is about 800 times denser than air. Ocean water, with dissolved salts adding to its weight, is even heavier at an average of 64 pounds per cubic foot (1.03 g/cm3). Because it is so much denser, it does not take 60 miles (100 km) of water to reach an additional pressure of one atmosphere. For every foot you descend in fresh water, the pressure increases by 0.432 psi. In salt water, it increases by 0.445 psi per foot of depth (0.1 bar/meter). This means that every 33 feet (10 m) of salt water (fsw or msw), adds the equivalent of 1 atm of pressure. You can calculate that for yourself: 33 fsw x 0.445 psi/ft = 14.7 psi, or 1 atm. In fresh water, pressure increases by 1 atm for every 34 feet of fresh water (ffw) depth increase (10.4 mfw).

Figure 4.1: Divers and the equipment they wear are subject to fluctuations in pressure as they decend and ascend in the water column. Courtesy of Elaine Jobin

When we are under water, the total pressure exerted on us is a combination of water and air pressure. The portion of the total contributed by the weight of the water is also known as gauge pressure, and is read as depth. The pressures from air and water may be figured independently, and then added together to get the total, or ambient, pressure. If we are diving at sea level, then the pressure is that of the water above us, plus one atm for the air pressure above the water. Because this is measured against a vacuum, it is also referred to as absolute pressure. As an example, if we were 33 feet (10 msw) deep in the ocean, the absolute pressure would

be two atm, one atm for the water weight and a second atm for the Earth's atmosphere. This is also referred to as 2 ata, or atmospheres absolute. As we can see, there are many different ways of presenting pressure. We can use psi, ffw, fsw, atm, or ata. These are all Imperial units of measure. With the Metric system, one atm is also represented by 1.033 kilograms per square centimeter (kg/cm2), or 1.01 bar. Table 4.1 gives a comparative look at these units. In diving, especially in nitrox diving, we most commonly use atm when looking at gas pressures, because that is how we measure exposure to oxygen. We use ata or fsw/msw when we are talking about pressure due to depth while diving in the ocean. (Of course, diving in fresh water we use ata or ffw/mfw.) Table 4.1: Pressure/Depth Equivalents GAUGE PRESSURES ABSOLUTE PRESSURES atm psig fsw msw ffw bar ata kg/cm2 psia 0 0 0 0 0 0 1 1.033 14.7 1 14.7 33 10 34 1.01 2 2.066 29.4 2 29.4 66 20 68 2.03 3 3.100 44.1 3 44.1 99 30 102 3.04 4 4.133 58.8 4 58.8 132 40 136 4.05 5 5.166 73.5 5 73.5 165 50 170 5.07 6 6.199 88.2 In nitrox diving, we must be able to measure the absolute pressure at any depth. To do so, we can use the following equation for ocean diving:

Where: Data D

Depth(ata) = Pressure at depth (ata) Depth (fsw/msw)

If we were diving in fresh water at sea level, the equation would be:

Where: Data D

Depth (ata) = Pressure at depth (ata) Depth (ffw/mfw)

Example 4.1 Determine the absolute pressure on Marvin if he is diving in the ocean at a

depth of 115 feet (35 meters).

Example 4.2 Determine the absolute pressure on Mary if she is diving in a sea level lake at a depth of 72 feet (22 mfw).

The surfaces of bodies of fresh water are usually not at sea level. Many lakes, rivers, and quarries are at altitudes of thousands of feet. The column of air at these locations is shorter, and is missing the densest section of that column. Because of this, the column of air weighs less. Equation 4.1 is thus modified to:

Where: Data D P

Depth (ata) = Pressure at depth (ata) Depth (ffw or mfw) Atmospheric pressure at altitude (atm or bar) (see Table 8.1, page 237)

As an example, at an altitude of about 18,000 feet (5,500 meters) above sea level, the column of air weighs about half as much as at sea level, or 7.35 psi (0.5 bar), or 0.5 atm. For a dive to 50 ffw (15 mfw), we would find that: Example 4.3

In the rest of this text, all problems and examples assume the diver to be at sea level. If you will be diving at other altitudes with scuba, nitrox or rebreathers, consult the appropriate altitude diving information in any specialized or advanced level textbook, like Bruce Wienke's Diving Above Sea Level.

Figure 4.2: Diving Above Sea Level contains more extensive coverage of dive physics and altitude diving.

Pressure and Volume

Gases are very compressible. As they are subjected to increasing pressures, the molecules are squeezed together. If a gas is held in a flexible container, the size, or volume, of that container will change radically at the pressures encountered in recreational diving. This is very different from most solids and liquids, wherein the

molecules are held in a solid lattice or relatively collapsed structure. With little free space between the molecules, they are not compressed at the relatively low pressures we encounter in diving.

Figure 4.3: Variations in lift bag volumes under water are explained by Boyle's Law. Courtesy of Elaine Jobin

Boyle's Law states that the volume of a quantity of gas varies inversely with its pressure, so long as its temperature stays the same. This relationship between pressure and volume allows us to determine the volume of gas at different depths in such spaces as our lungs, BCs (buoyancy control devices), dry suits (air volume only), and rebreather counterlungs (don't worry if you do not remember what these are—they will be explained in detail). These are examples of flexible containers associated with diving. Boyle's Law does not explain volume changes in spaces such as our ears, sinuses or scuba cylinders, which are rigid or semi-rigid containers. Boyle's Law may be represented as:

Where: P1 P2 V1 V2

Pressure at the initial depth in ata or bar Pressure at the final depth in ata or bar Volume at the initial depth in cubic feet (cf) or liters (L) Volume at the final depth in cubic feet (cf) or liters (L)

This equation may be rearranged many ways to find any one missing variable of the four. Just isolate the missing variable. For example, if we need to find the final volume of a container where we know the initial size and pressure (or depth) and the final pressure (or depth), then the equation would look like:

Example 4.4 If Marvin partially fills a 100-pound (1.6 cf; or 45.5 kg/45.5 L) lift bag with 25 pounds (11 kg) of air at a depth of 85 fsw (26 msw), what will the final volume and lift be when it reaches the surface?

Breathing Gas Composition

At the surface we breathe air, which is composed primarily of two gases, nitrogen (79%) and oxygen (21%). It actually contains various trace gases making up about 1% of the volume, but we can ignore that for our purposes. In diving, we refer to the composition of a gas mix as made up of fractions, or the percentage of each gas component represented in decimal form. Thus, the fraction of nitrogen in air is 0.79, and of oxygen is 0.21. When you add the fractions of all of the component gases in a

mix together, the total is always 1.0. Most OC scuba diving is done with air. Because the gas composition does not change during the dive, this is referred to as a constant fraction breathing mix. If we fill the scuba cylinders with enriched air nitrox (EANx), the fraction of oxygen will be higher than with air. Common nitrox mixtures include NOAA Nitrox I, containing 32% oxygen (EAN32) and NOAA Nitrox II, containing 36% oxygen (EAN36). If we use these gases in open circuit diving, we are still diving a constant fraction breathing mix, as the fractions of those gases remain fixed throughout the dive. This is not true with rebreathers. Dalton's Law states that the pressure effects of a gas in a gas mixture is the same as if that gas were separated out by itself.

In diving, we are concerned with the effects the various components of our breathing gas have on our body. We measure this by looking at the partial pressure of each gas. Dalton's Law states that the pressure exerted by any gas component in a mixture of gases is equal to the fraction of that gas in the mix times the absolute pressure of the total gas. In diving, the absolute pressure of the gas is the same as the absolute pressure at depth, as expressed in Equations 4.1 and 4.2, page 69. If we first express Dalton's Law, and then look at the pressure at depth in the ocean, we find that for ocean diving Dalton's Law may be shown as:

Then substituting Equation 4.1:

Where: Pg Fg Data

Partial pressure of gas at depth (atm) Fraction of gas in breathing mix Depth (ata)

One factor we must consider is the partial pressure of oxygen (PO2). Oxygen becomes toxic at partial pressures exceeding 0.5 atm. The time it takes to affect us is directly related to the pressure, i.e., at higher pressures we experience problems

sooner. For this reason, we must be able to calculate the PO2 at the depth we are diving breathing whatever gas mixture we are using. Example 4.5 Determine the partial pressure of oxygen for a diver breathing EAN40 at a depth of 82 fsw (25 msw).

Maximum Operating Depth

The maximum depth at which we can use any breathing gas is set by the fraction of oxygen in the gas and the maximum allowable PO2, to avoid CNS toxicity. Generally, it is recommended that a diver not be exposed to PO2 greater than 1.4 atm during a dive. So, part of our regular pre-dive planning when using nitrox mixtures on OC scuba or rebreathers is to determine our maximum operating depth (MOD). Maximum operating depths may be calculated using the following equations for ocean diving:

Figure 4.4: MOD calculations must be done during your predive planning

Where: MOD maxPO2 FO2

Maximum Operating Depth (fsw or msw) Maximum allowable oxygen partial pressure (atm) Fraction of oxygen in breathing gas

Example 4.6 Determine Mary's maximum operating depth if she is breathing EAN60, using a maximum allowable oxygen partial pressure of 1.4 atm, in fsw and msw. maxPO2 = 1.4 atm FO2 = 0.60

The Case of the Wrong Mix

An experienced SCR diver was on an expedition with a group of rebreather divers in a remote location. This diver made a "fun" dive after the expedition dives had been completed. As this was to be only a simple, shallow dive, he donned the unit and jumped in alone. Two hours later, he had not returned to the boat. Several of the expedition team entered the water to begin a search. They found the missing diver dead on the

bottom. His depth gauge read 92 fsw (28 msw). They recovered the body and analyzed the gas in his mass flow controlled SCR. Upon analysis, they found the cylinder contained EAN80, which has a MOD of 25 fsw (7 msw). Apparently the deceased diver had mixed a cylinder of gas he believed was suitable for his dive, but did not subsequently analyze it. Alternatively, he may not have performed any of the calculations necessary to determine MOD and other parameters prior to diving. This underscores the need to always know what gas you are diving. It also points out the importance of completing all steps of a dive plan, including calculation of constraints for the gas in use.

Equivalent Air Depth

Depth is one of the most important variables in diving. In addition to its importance for determining oxygen exposure, it is one of the two major variables controlling absorption of inert gas in the body. As you undoubtedly recall from your scuba diver class (and many years of watching Sea Hunt), if you dive too deep for too long a time, or come up too quickly, you can get "the bends," or decompression illness. When diving air using open circuit scuba, the concept of depth is easily understood. A diver merely reads it from the depth gauge or dive computer in use. Because all dive tables were originally developed based on a person breathing air and diving in the ocean, no special tools or conversions were needed to use the existing dive tables. However, once we change the breathing mixture to something other than air, one of the basic premises upon which the dive tables were designed is no longer met. As you learned in your nitrox training, as soon as you slip below the surface breathing nitrox, "depth" changes from a simple concept to one more elusive. In EANx diving, the fraction of nitrogen is reduced from that in air. Therefore, less nitrogen is absorbed into the tissues, allowing us to remain longer at the same true depth. This is one of the major benefits of using EANx. But, in order to use air-based dive tables when breathing EANx, the true depth must be converted to one that can be used with the air tables. Equivalent air depth (EAD) is the depth at which a diver who is actually breathing nitrox would be on-gassing nitrogen at the same rate had he been breathing air instead. If breathing EANx, the EAD is always shallower than true depth. If breathing a nitrox with an oxygen fraction less than that of air (or a nitrogen fraction > 0.79), the EAD is always deeper than true depth. Every nitrox mix other than air requires the diver to convert true depth to EAD, unless a special dive table is available for the nitrox in use. Many training agencies have special tables for EAN32 and EAN36, but the use of any other nitrox breathing gas necessitates manual computation of its EAD. When you use rebreathers, the nitrox involved generally varies from the two special cases noted above. Therefore, you must be able to manually determine the EAD. Use the following equations to do so:

Where: EAD FO2 (1-FO2) D

Equivalent Air Depth (fsw or msw) Fraction of oxygen in nitrox breathing gas FN2 Fraction of nitrogen in nitrox breathing gas Depth of dive (fsw or msw)

Example 4.7 Determine Marvin's equivalent air depth if he uses EAN42 at a depth of 75 fsw (23 msw).

Thus, this diver would read the 50 fsw (15 msw) line from a set of air tables for this dive.

Gas Consumption

In OC scuba training, we learned that surface air consumption (SAC) rate depends on many factors. Some of these are our comfort in the water, diving experience, depth, exertion, the temperature, and the drag the gear we carry creates. In our nitrox diving class, we found that even though the gas contained more oxygen than air, our

SAC rate remained the same. This was because the trigger that causes us to breathe is the carbon dioxide (CO2) produced as a byproduct of our oxygen metabolism, not the reduction in the amount of oxygen available to us in our breathing gas. Breathing air or nitrox, we exhale most of the oxygen we inhale. Gas consumption in rebreathers is dealt with differently than in OC diving. The CO2 is removed from our exhalations by the rebreather and the remaining oxygen is recirculated. However, being able to determine our SAC using OC scuba is still important. As part of our emergency procedures, we need an OC breathing gas reserve sufficient to reach the surface, should we have problems with the rebreather. This can be part of the rebreather gas supply, to be used in OC mode. Alternately, the required amount of breathing gas could be carried in a bailout bottle with its own regulator. The reserve we need to leave in the rebreather gas supply, or the size of the bailout bottle, is determined by using our SAC rate. (SAC rate will be used throughout this text, even though it is a misnomer. Once we begin breathing nitrox or other gas blends different from air, we really should refer to our surface (gas) consumption rate, or "SCR." Since SCR is used to denote "semi-closed circuit rebreather" in this text, SAC is used instead.) Before we can predict how much gas volume to reserve for emergencies, we must first determine our SAC. This can be done by dedicating part of an OC scuba dive to this task, or by looking back in your dive logs to get information from actual dives. The best way is to use information from several of your past dives, to get an average SAC rate. Optimally, you should use dives in which you remained at the same depth throughout the dive, except for descent and ascent. The data you must have to calculate your SAC rate is: beginning and ending cylinder pressures, cylinder size and rated capacity pressure, bottom time, and average depth. If working in metric, the water capacity of the cylinder is needed. Then use the following formula for ocean diving:

Where: SAC Pi Pf Pcyl Vcyl WCcyl T

Surface Air Consumption (standard cubic feet or liters per minute, scfm or lpm) Starting pressure in cylinder (psi or bar) Ending pressure in cylinder (psi or bar) Pressure of cylinder at rated capacity (psi) Volume of cylinder at rated capacity (cf) Water capacity (actual volume) of cylinder (L) Time of dive (min)

D

Average depth of dive (fsw or msw)

Example 4.8 What is Marvin's SAC rate if he started with 3000psi (200bar) in an Aluminum 100 cf (13.3 L) cylinder, and finished with 1500psi (100bar), after having been taking photographs at a depth of 33 fsw (10 msw) for 35 minutes? D = 33 fsw (10 msw) Pi = 3000psi (200bar) Pcyl = 3000psi WCcyl = 13.3 L T = 35min Pf = 1500psi (100bar) Vcyl = 100 cf

How will this SAC rate help us plan a dive? If we use a rebreather on a dive we would normally carry a bailout bottle as a redundant air source to be used if the rebreather fails. Optimally, the bailout bottle would give us breathing gas for five minutes at the maximum planned depth, plus sufficient gas to ascend to the surface. Finally, we should have enough extra gas to perform a safety stop at 15 fsw (4.6 msw) for five minutes for an additional safety margin. To calculate the amount of gas needed to accomplish these goals, we would use the following equation:

Where: Vgas SAC Dmax

Volume of gas needed for the emergency reserve (cf or L) Surface Air Consumption (cubic feet or liters per minute, scfm or lpm) Maximum depth of dive (fsw or msw)

Figure 4.5 is a graphical representation of the volume of bailout gas you would require at different depths based on your SAC. It is based on calculations from Equation 4.9, and on 10 fsw (3 msw) and 0.1 scfm (3 lpm) SAC changes. To read it, first look on the x-axis (the horizontal axis) to find your maximum depth. Then move straight up until it intersects the line representing your SAC. From that point, move horizontally to the left to the y-axis (the vertical one). You can read the required amount of bailout gas there in cubic feet or liters.

Figure 4.5: Bailout volumes needed at different depths and air consumption rates.

You may not have Figure 4.5 available, nor wish to calculate Equation 4.9 on every dive. To avoid that, you could be conservative, and plan for the deepest depth you are qualified to dive on your rebreather for every dive. The worst case ascent would be from the maximum recreational rebreather scuba depth of 130 fsw (40 msw). Assuming an ascent rate of 30 fpm (9 mpm), we find that the above equation can be simplified to:

Where: Vgas SAC

Volume of gas needed for the emergency reserve (cf or L) Surface Air Consumption (standard cubic feet or liters per minute, scfm or lpm)

As you work this out, note that all fractions have been rounded up to increase the safety margin. That makes this fairly easy to remember: 45 times your SAC rate is the amount of reserve gas you need for any rebreather dive within entry level depths. Of course, the first method, using Equation 4.9 (page 80), is the most precise, and using Equation 4.10 (page 81) is the least precise, but what is the variation? Let's look at an example:

Example 4.9 How much gas should Marvin carry in a bailout bottle if he is planning a dive to 110 fsw (33.5 msw) using a SCR and his SAC rate is 0.59 scfm (16.7 lpm)? Try all three techniques to determine it. Dmax = 110 fsw (33.5 msw)

SAC = 0.59 scfm (16.7 lpm)

Technique #1

Technique #2 Enter at 110 fsw (33 msw) on the x-axis, move up to the 0.6 scfm (17 lpm) SAC rate line, then horizontally left to find a bailout gas volume of about 23 cf (650 L). See Figure 4.6, page 83. Technique #3

The shallower the dive, the more conservative Equation 4.10 becomes. Remember that in an actual emergency, your SAC rate will increase due to stress. To compensate in this equation, you may want to use a higher SAC rate than normal for planning purposes.

Figure 4.6: Solution for Example 4.9b

Cylinder Gas Volumes

We have one last area to review. In the example above, we had to determine the amount of gas sufficient to meet reserve gas needs. But how do we know if a gas cylinder has this amount of gas? Obviously, if we have a cylinder filled exactly to its rated capacity, we know how much gas we have. But what if the cylinder is not full? How do we determine if there is gas volume sufficient to meet our needs? Gas volume in a diving cylinder is directly proportional to the pressure the cylinder contains. If we know the pressure in the cylinder, its rated capacity, and its rated volume, then we can find its current gas volume. Remember that not all cylinders are "full" at their working pressure. Steel cylinders hold their rated capacity at their working pressure plus 10% overfill. For example, a steel 71.2 cf cylinder holds 71.2 cf of gas not at its working pressure of 2250 psi, but at 2475 psi (2250 psi +10 percent, or an additional 225 psi). To find the rated capacity pressure and volume for your cylinder, check Table 4.2. If you cannot find your cylinder on the table, check with the dive store from which you purchased it, or its manufacturer. Table 4.2: Manufacturers Cylinder Data Manufacturer

Material Service Rated Released Pressure Pressure Volume (psi) (bar) (psi) (bar) (cf) (L)

Water P–V Capacity Conversion (L) (psi/cf)

Catalina Faber Catalina Luxfer Catalina Catalina Worthinton Catalina Luxfer Faber Catalina Luxfer Faber/Technisub Catalina Heiser OMS Luxfer Catalina Catalina Luxfer Pressed Steel OMS Catalina Faber/Technisub Pressed Steel Faber/Scubapro Faber/Scubapro Catalina Catalina Luxfer Luxfer Pressed Steel Taylor-Wharton OMS Faber/Technisub Faber/Scubapro OMS

Aluminum Steel Aluminum Aluminum Aluminum Aluminum Steel Aluminum Aluminum Steel Aluminum Aluminum Steel Aluminum Steel Steel Aluminum Aluminum Aluminum Aluminum Steel Steel Aluminum Steel Steel Steel Steel Aluminum Aluminum Aluminum Aluminum Steel Steel Steel Steel Steel Steel

3000 2400 3000 3000 2015 3000 2400 3000 3000 2400 3000 3000 2850 3000 3000 2400 3000 3000 3300 3000 3500 2400 3000 2850 2250 3000 2400 3300 3000 3000 3000 3500 2400 2400 2850 2400 2400

207 165 207 207 139 207 165 207 207 165 207 207 200 207 207 165 207 207 228 207 241 165 207 200 155 207 165 228 207 207 207 241 165 165 200 165 165

3000 2640 3000 3000 2015 3000 2640 3000 3000 2640 3000 3000 2900 3000 3000 2640 3000 3000 3300 3000 3500 2640 3000 2900 2475 3300 2640 3300 3000 3000 3000 3500 2640 2640 2900 2640 2640

207 182 207 207 139 207 182 207 207 182 207 207 200 207 207 182 207 207 228 207 241 182 207 200 171 228 182 228 207 207 207 241 182 182 200 182 182

6 13 13 13.2 15 30 27 19 19.9 19 17 30 35.3 40 43.8 46 48.4 53 60 63 65 66 67 70.6 71.2 71.4 75.8 77.4 77.4 77.4 78.2 80 80 85 88.3 95.1 98

170 368 368 374 425 850 764 538 564 538 481 850 1000 1133 1241 1303 1371 1501 1699 1784 1841 1869 1897 2000 2016 2022 2146 2192 2192 2192 2214 2265 2265 2407 2500 2693 2775

0.9 2 1.9 1.8 3 4.3 4.3 2.7 2.7 3 2.4 4.1 5 5.7 6 7.1 6.6 7.6 7.9 8.5 7.6 10.2 9.6 10 11.7 8.8 11.7 10.2 11.1 10.5 10.6 9.3 12.3 13.1 12.5 14.6 15.1

500 203 231 227 134 100 98 158 151 139 176 100 82 75 68 57 62 57 55 48 54 40 45 41 35 46 35 43 39 39 38 44 33 31 33 28 27

Catalina Pressed Steel Pressed Steel Faber/Technisub OMS Pressed Steel Pressed Steel Heiser/Beauchat OMS OMS Heiser/Beauchat Faber/Technisub Heiser/Beauchat

Aluminum Steel Steel Steel Steel Steel Steel Steel Steel Steel Steel Steel Steel

3300 3500 2400 2850 2400 3500 2400 3190 2400 2400 3190 3150 4410

228 241 165 200 165 241 165 220 165 165 220 200 304

3300 3500 2640 2900 2640 3500 2640 3190 2640 2640 3190 3190 4400

228 241 182 200 182 241 182 220 182 182 220 220 303

100 100.1 104 105.9 112 120 120 120 125 131 140 139.8 190

2832 2835 2945 3000 3171 3398 3398 3299 3540 3710 3960 3960 5612

13.3 11.6 16 15 17.3 13.9 18.5 15 19.3 20.2 18 18 18.5

33 35 25 27 24 29 22 27 21 20 23 23 23

Using the metric system, calculating volumes differs slightly. Cylinder volumes are provided in terms of water capacity, or the actual internal volume of the cylinder. This information is generally provided in liters in the information stamped on the cylinder shoulder. The other information you need is the current cylinder gauge pressure in bars. Once we know these cylinder gauge parameters, we can calculate available volume using Equation 4.11:

Where: Vcon

Pcon Pcyl Vcyl WCcyl

Volume contained in the cylinder (cf or L) Pressure contained in the cylinder (psi or bar) Pressure of cylinder at rated capacity (psi) Volume of cylinder at rated capacity (cf) Water capacity (actual volume) of cylinder (L)

Example 4.10 How much gas is in Marvin's 40 cf (5.75 L) cylinder filled to 2100 psi (143 bar) if its rated capacity is at 3000 psi? Vcyl = 40 cf WCcyl = 5.75 L

Pcon = 2100 psi (143 bar) Pcyl = 3000 psi

The chart in Figure 4.7 page 86, affords another method for determining tank volumes at different pressures. First locate the line on the graph that represents your cylinder. Then find the current cylinder pressure on the horizontal axis at the bottom. From the pressure, look straight up until you intersect your cylinder line. Finally, go straight to the left until you hit the vertical axis where the volume of gas in your cylinder can be read.

Figure 4.7: Released gas volumes for popular scuba cylinders

Summary

Dive physics may be challenging for some, but mastery is necessary to understand the limitations and benefits of rebreather scuba. Take the time to work the problem set

below (either the Imperial or the Metric). If you have difficulty with any type of problem presented, review that section of this chapter again, or ask your instructor for assistance. Building a strong knowledge foundation now will make learning the theoretical and practical concepts of rebreather use much easier.

1. What is the ambient pressure at 85 fsw (26 msw)? a. 2.6 ata. b. 3.5 ata. c. 3.6 ata. d. 37.8 ata.

2. If a 0.5 cf (14 L) counterlung is full at the surface, what will its volume be a depth of 43 fsw (13 msw) if no gas is added to or removed from it? a. 0.01 cf (0.33 L). b. 0.22 cf (6.1 L). c. 0.38 cf (10.8 L). d. 1.15 cf (32.2 L).

3. If Marvin fills a 100-pound (1.6 cf or 45.5 kg) lift bag completely at a depth of 76 fsw (23 msw) and then releases it, what will its volume be upon reaching the surface? a. 0.5 cf (13.8 L). b. 0.7 cf (19.8 L).

c. 1.6 cf (45.5 L). d. 5.3 cf (150.3 L).

4. What is the partial pressure of oxygen (PO2) Mary is breathing at a depth of 55 fsw (16.7 msw) if she is breathing EAN44? a. 0.73 atm. b. 1.17 atm. c. 1.67 atm. d. 2.67 atm.

5. What is the partial pressure of nitrogen (PN2) in Marvin's breathing gas if he is using EAN32 at a depth of 69 fsw (21 msw)? a. 0.67 atm. b. 1.0 atm. c. 1.42 atm. d. 2.1 atm.

6. The maximum operating depth of recommended limits for PO2 exposure is? a. 72 fsw (21.8 msw).

EAN44

using

b. 87 fsw (26.4 msw). c. 105 fsw (31.8 msw). d. 120 fsw (36.4 msw).

7. If Marvin uses EAN61 to dive to a depth of 40 fsw (12.2 msw), his equivalent air depth is? a. 3 fsw (1 msw). b. 23 fsw (7 msw). c. 36 fsw (11 msw). d. 56 fsw (17 msw).

8. Determine your SAC rate after spending 37 minutes at 43 fsw (13.1 msw) leaves you 500 psi (34 bar) in your aluminum 80 cf (11 L) cylinder, which was filled to its rated capacity of 3000 psi (204 bar) at the start of the dive? a. 0.5 scfm (14 lpm). b. 0.6 scfm (17 lpm). c. 0.7 scfm (20 lpm). d. 0.8 scfm (22 lpm).

9. How much bailout gas should Mary carry if she is using a CCR at a depth of 50 fsw (15 msw), and her SAC rate is 0.6 scfm (17 lpm)?

a. 11 cf (311 L). b. 14 cf (386 L). c. 27 cf (765 L). d. This cannot be determined from the information provided.

10. If you require 25 cf (710 L) of gas for your bailout supply, your 30 cf (3.5 L) cylinder (rated pressure of 3500 psi, or 240 bar) must have a minimum pressure of? a. 2917 psi (200 bar). b. 2500 psi (170 bar). c. 2062 psi (140 bar). d. SAC rate and maximum depth must be provided before this may be determined.

Physiology Overview Man is not adapted for life underwater. We not only need special equipment to remain beneath the surface, but the longer one stays, the greater the chance that the body will be adversely affected. There are multitudes of physiological concerns, many of which you have been told about in your scuba courses. Using rebreathers changes the way we look at some physiological risks. Some concerns are new, specific to rebreather use. This chapter reviews the major physiological considerations we face as rebreather divers, concentrating on problem prevention.

Objectives After reading this chapter, you will be able to: 1. List four variables that affect the amount of oxygen metabolized by a diver. 2. Draw an oxygen partial pressure scale, labeling important points. 3. State a maximum PO2 exposure limit within which you will stay, and explain why you selected that limit. 4. Give definitions, symptoms, treatment and prevention for: hypoxia, CNS and pulmonary oxygen toxicity, hyperoxic myopia, hypercapnia, nitrogen narcosis, squeeze, AGE, DCS, and middle ear oxygen absorption syndrome.

5. Describe the risks associated with rebreathers due to disease transmission and allergic reactions. 6. Explain the importance of self-discipline in rebreather diving.

Oxygen Metabolism

We need oxygen to live. We metabolize oxygen to produce energy. If we do not have enough oxygen in the gas we breathe, we die. When we exhale, we exhale carbon dioxide (CO2) produced as a by-product of our metabolism. We also exhale the majority of the oxygen we inhaled. This is why rebreathers work. The amount of oxygen we actually consume varies depending on many factors. The most important factor is the level of exertion. The more you exercise, the more oxygen you metabolize. Typical oxygen consumption rates for various activities for an average-sized male (154 lb/70 kg) are shown in Table 5.1. Table 5.1: Oxygen Metabolism Activity

Oxygen Consumed (liters/min) Sleeping 1/4 Sitting 1/2 Light Activity (easy diving) 1 Moderate Activity (purposeful swim) 2 Heavy Activity (swimming against current) 3 Very Hard Exertion (possible for only short to5 time intervals) Other factors that impact our oxygen consumption rate include body size, thermal stress (hot or cold), basal metabolism rate, physical condition, buoyancy control, and trim in the water. It is not

uncommon for the average diver to consume as much as 3 liters per minute (lpm) of oxygen during parts of a typical scuba dive. One of the most interesting facts about oxygen metabolism is that it is independent of depth. For a given level of activity, all other factors being equal, a person consumes the same number of oxygen molecules at 20 fsw (6 msw) as at 200 fsw (60 msw). This is not what we were taught as OC (open-circuit) scuba divers, where gas consumption rate increases with depth. SCRs (semi-closed circuit rebreathers) are more efficient because they utilize some of that wasted gas. CCRs (closed-circuit rebreather) systems are even more efficient. How much more? Let's take the best-case scenario (for rebreathers). When we breathe we inhale and exhale an approximately constant volume with each breath (the volume of our lungs). If we consider this to be a fixed volume, then we can consider oxygen availability and use as proportional to the partial pressure of oxygen at the surface, where we inhale 0.21 atm oxygen, and exhale approximately 0.17 atm. We consume about 0.04 atm oxygen. This oxygen extraction is about 19% efficient. At a depth of 100 fsw (30 msw, or 4 ata), breathing air on a standard OC scuba system we would be breathing 0.84 atm oxygen, and exhaling 0.80 atm. The extraction efficiency drops to about 5%. The deeper we travel, the more efficiency drops. At a depth of 218 fsw (67 msw, or 7.7 ata), we inhale 1.6 atm and exhale 1.56 atm oxygen. Extraction efficiency has decreased to 2.5%. Rebreathers allow us to utilize the exhaled gas. The amount of the exhaled gas reutilized varies depending on the design, but for mass flow SCRs on average the gas is recycled about four times. Thus, the average SCR is four times as efficient, representing a 400% increase. This is true regardless of depth. This is a major advantage. But even with this increase in efficiency, much oxygen and inert gas is wasted by being expelled to the ambient environment as bubbles. Overall, at 218 fsw (67 msw), efficiency increases from 2.5% to only 10% — meaning 90% of the oxygen and an even greater percentage of the inert gas is still being wasted!

Figure 5.1: Theoretical efficiency increases in oxygen consumption CCRs vs OC scuba. Example: At 225 fsw (67.5 msw) CCRs are theoretically 40 times more efficient than OC scuba.

This is the advantage of CCR systems. Theoretically, they allow you to utilize 100% of the oxygen and inert gas you carry. This is not true, but we will ignore shortfalls for the moment. This would make CCR units up to ten times more efficient than SCRs, and up to 40 times as efficient as OC scuba (See Figure 5.1, page 94) For many people, this increase is well worth the effort and expense involved in training and equipment. These efficiency increases do not come without potential problems. Unlike OC scuba, the gas makeup varies throughout the dive. This may lead to problems with too much or too little oxygen, or too much carbon dioxide. These risks are addressed in this chapter.

Oxygen Exposure

We need oxygen to live, but if we have too much oxygen we die. Oxygen, in too great a concentration, is as toxic as some poisons. While it is extremely unlikely that we could expose ourselves to oxygen levels sufficient to cause toxicity on the surface in a short time, it is easy to do under water. It is far easier to do with rebreathers than breathing air with OC scuba. In fact, oxygen toxicity is one of the primary hazards of rebreather use. When diving rebreathers, one of our tasks is to ensure we remain within the range of safe oxygen exposure. The human body reacts to oxygen based on the partial pressure, not its fraction or percentage. As you recall, oxygen partial pressure is measured in atmospheres, which varies with depth and breathing gas content. Let's consider a continuum depicting a range of oxygen partial pressures (Figure 5.2). A normoxic condition is one in which the PO2 is 0.21 atm. This is our normal breathing status, breathing air at sea level. This is the environment to which our bodies have adapted through thousands of generations of evolution. We can live at this level of oxygen for decades with no (or minimal, if you believe in oxygen free-radical theory) ill effects. Any gas blend that contains a fraction of oxygen (FO2) of 0.21 is said to be a normoxic blend. Scuba cylinders filled with air meet this condition, but nitrox cylinders do not.

Figure 5.2: Oxygen Partial Pressure Scale (atm).

The left side of the Fig 5.2 scale is the hypoxic range. Hypoxic gas mixes contain a FO2 of less than 0.21, and a hypoxic condition exists if the PO2 is less than 0.21 atm. If the PO2 falls below 0.16 atm, most people will notice mild impairment in their ability to exercise. This is the lowest PO2 that is allowable for scuba diving on either rebreathers or OC. At an altitude of about 7,000 feet (2,130 meters) the PO2 of air is about 0.16 atm. If you have been at that altitude, you may have noticed that it was more difficult to climb stairs, or run, because of the reduced oxygen level. Discomfort and impairment increases significantly for most people at a PO2 of 0.12-0.16 atm. At about 0.10 atm, some people may lose consciousness. Remaining at levels of 0.08 atm or less may lead to death. Of course, people who spend long periods at reduced PO2 may be better able to cope with hypoxic breathing gas. The best known examples are mountaineers, who spend months acclimating to high altitude and then scale a high peak where the ambient PO2 is 0.06-0.07 atm. The hyperoxic range is on the right portion of Fig 5.2. Hyperoxic means that there is more oxygen than normal, with a hyperoxic mix containing more than 21% oxygen, and hyperoxic conditions existing with PO2 greater than 0.21 atm. The human body can tolerate hyperoxic exposures of up to 0.5 atm for indefinite periods. Once you exceed 0.5 atm, exposure times become limited by whole body oxygen toxicity. This level can be reached on the surface if you breathe a gas with at least 50% oxygen. However, the time to onset of symptoms at this level is generally several days to weeks. A more acute form of oxygen poisoning can occur at higher partial pressures. Generally, this is not a problem until the PO2 is at least 1.3 atm. As the PO2 increases above this level, the potential for this type of oxygen toxicity also increases. At a PO2 greater than 1.6 atm, the risks become unacceptably high. Most recreational training

agencies recommend an upper PO2 limit of 1.3–1.4 atm for all portions of a dive except for any decompression stops, which may be conducted with a PO2 up to 1.6 atm. This will be addressed in greater detail.

Hypoxia

The medical condition in which one is impaired by lack of oxygen is known as hypoxia. Typically, most people begin to experience symptoms of hypoxia at a PO2 of less than 0.16 atm. Hypoxia is particularly insidious because you are unaware that you are becoming a victim, and may even feel euphoric. Symptoms include inability to focus, solve simple or complex problems, and a general loss of awareness. Further oxygen reduction ultimately results in unconsciousness and, if progressive, death due to drowning or asphyxiation caused by lack of oxygen. You may have absolutely no awareness or ability to develop awareness that hypoxia is occurring. Divers who have experienced hypoxia and then brought back to a normal PO2 have no recollection that they were impaired. Yet they had little or no awareness of their surroundings, were unable to communicate, read gauges or other instruments, and in some cases actually blacked out!

Figure 5.3: Special displays allow you to monitor the PO2 in your rebreather during your dive.

We can only monitor PO2 levels with instrumentation. Many rebreather manufacturers incorporate meters that sense and display the oxygen level in the breathing loop. By reading these meters, we can determine the oxygen content of the gas we are breathing. The design and function of these meters is discussed in further detail in Chapter 7. Unfortunately, not all units have oxygen meters. With these units, the only way to avoid hypoxia is to avoid the circumstances that can lead to it. The most common cause of hypoxia in SCRs is inadequate flow through the mass flow controller. Flow deficiencies can occur in many ways. The first is blockage of the orifice or sonic valve due to corrosion, salt buildup from flooding and inadequate rinsing, or lodged debris. The second is not properly matching the supply gas to the mass flow controller orifice. A third is failure to open the gas supply cylinder. Any flow loss results in less oxygen being added to the breathing loop, so that the diver may not get enough oxygen to remain conscious. This is addressed in greater detail in Chapter 6. Another potential problem is the diver's failure to turn on the supply gas in the SCR or the oxygen in CCRs. This results in the diver breathing increasingly higher concentrations of inert gas as the oxygen in the breathing loop is consumed. In RMV keyed SCRs, a rupture between the discharge and outer counterlungs can cause the breathing gas to become increasingly hypoxic. Failure of the oxygen addition solenoid in mixed-gas CCRs can also lead to hypoxia. With all SCRs and mixed-gas CCRs, hypoxia is more likely to be a problem while diving shallow, as at reduced pressure fewer oxygen molecules are circulating. The most hazardous place to breathe a rebreather is at the surface. This danger can be compounded by a rapid ascent, which causes the PO2 to drop as the ambient pressure is reduced. (Figure 5.4) Finally, heavy exertion using some rebreathers compounds any problem. This is especially true in mass flow SCR units, where the amount of oxygen that is added to the breathing loop is constant. Heavy exertion, such as swimming against a current to get back to the boat, can cause the diver to consume more oxygen than is being

added to the breathing loop. If the mass flow controller for a particular supply gas is replaced with one for a richer gas, this problem is severely compounded. These topics are addressed in Chapter 6. Overall, the key to preventing hypoxia is to either monitor your oxygen meter or avoid actions that can lead to hypoxia. Treatment for hypoxia is simple – get the victim more oxygen. When diving, this can be accomplished in many ways. The three simplest are: 1. Inject fresh breathing gas into the breathing loop, increasing the oxygen content. 2. Go deeper, thereby increasing the PO2 without adding oxygen to the breathing loop. 3. Switch to the bailout bottle (typically an open-circuit scuba system), which should provide adequate oxygen. The worst thing you can do is to ascend without modifying the gas in the breathing loop, as this will further decrease the PO2, increasing the level of hypoxia.

Figure 5.4: Shallow and deep breathing loops. Note: You can see there are relatively few oxygen molecules in shallow water, while many more oxygen molecules are available at greater depths or increased PO2.

CNS Oxygen Toxicity

As stated earlier, hyperoxic breathing gases may result in oxygen poisoning while diving. The more severe form, caused by relatively short exposures to high oxygen partial pressures, is known as central nervous system oxygen toxicity, or CNS toxicity. The probability of having a CNS toxicity problem increases with dosage. Dosage is a function of two factors: the exposure to oxygen as a function of partial pressure, and the time that exposure lasts. [Toxic effects have been observed at PO2 more than 1.2 atm, but for practical purposes in recreational diving are really only a significant hazard at levels above 1.4 atm.] At a lesser PO2, it is unlikely that the dive will approach the duration necessary for problems to develop. Other factors such as individual physiology, hard work, cold or very warm water, CO2, and some medications may alter the PO2 at which CNS oxygen toxicity symptoms occur. Symptoms of acute oxygen toxicity include tunnel vision, tinnitus (ringing in the ears), nausea, twitching muscles (primarily facial tics), irritability, dizziness, and convulsions. We use an acronym, ConVENTID, to remember these symptoms, as shown in Figure 5.5, page 100. The symptoms may appear in any order. However, in about 50% of the cases in one study, nausea was the first symptom reported by divers experiencing this malady. Another study indicated that twitching or tics were the first symptom in just more than 50% of the cases. The time for symptoms to progress varies from seconds to minutes or longer, and not all symptoms may occur. The most severe symptom, convulsions, occurs in a two-phase sequence. The diver is unconscious during both phases. Initially, the affected diver's muscles contract and he becomes completely rigid. This has resulted in victims being described as "stiff as a board." The

airway is closed, and the diver does not breathe. This stage lasts one-half to one minute, and is known as the contraction or tonic phase. As the second phase begins, the victim relaxes or alternates between muscle contructions and relaxation states, and will convulsively inhale (gasp) a breath. Known as the relaxation or clonic phase, this will last for one or two minutes. The affected diver may hyperventilate during this time, and is often semi-conscious. If this occurs above the surface, in a recompression chamber, e.g., it is not life threatening. The seizure will generally last for several minutes, and then leave the person unlikely to remember the event upon regaining consciousness. However, if it occurs under water, the diver will probably lose their mouthpiece and drown. Since it is possible that convulsions may be the initial symptom, this makes the occurrence of CNS oxygen toxicity extremely hazardous. The off-O2 effect is a recurrence of acute oxygen toxicity symptoms. This may occur several minutes after the diver has reduced the oxygen partial pressure (either by switching to another gas mix or by ascending to a shallower depth). The off-O2 effect is manifested by the onset or worsening of CNS oxygen toxicity symptoms.

Figure 5.5: CNS Oxygen Toxicity Symptoms.

We avoid oxygen toxicity by tracking oxygen exposure—just as we prevent decompression illness by using the standard air diving tables to track nitrogen uptake. To determine our risk of oxygen toxicity, we use the NOAA Oxygen Partial Pressure & Exposure Time Limits (Table 5.2, page 101). This table tracks exposures for single dives and daily limits, based on the PO2 the diver breathes. The limits in this table have not been rigorously tested by scientific method, but many years of operational use seem to indicate that the limits stated are reasonably effective in preventing CNS toxicity. Single exposure time is used to determine if the time limit for a single dive has been exceeded. For example, if Marvin is breathing EAN32 at a depth of 100 fsw (30 msw), then the PO2 is 1.28 atm. To determine how long he could stay, we would round up to 1.3 atm, finding he may remain at that depth on EAN32 for 180 minutes. His total allowable exposure time for that exposure over a 24-hour period would be 210 minutes. This would allow him, for example, to participate in three dives of 70 minutes each, since none would exceed single dive time limit, and the three dives in combination would reach the daily limit. But what if Marvin's multiple dives involve different exposures to oxygen?

General Rules: 1. If any dive reaches or exceeds the single dive limit, the minimum surface interval is 2 hours. 2. If one or more dives within the 24-hour period have reached the maximum total 24-hour day limit, the minimum surface interval is 12 hours.

Table 5.2: NOAA Oxygen Partial Pressure & Exposure Time Limits for Oxygen Toxicity1 PO2 Single Exp 24-hour Max %CNS/MIN OTU/MIN 1.60 45 150 2.22 1.92 1.55 83 165 1.20 1.85 1.50 120 180 0.83 1.78 1.45 135 180 0.74 1.70 1.40 150 180 0.67 1.63 1.35 165 195 0.61 1.55 1.30 180 210 0.56 1.48 1.25 195 225 0.51 1.40 1.20 210 240 0.48 1.32 1.10 240 270 0.42 1.16 1.00 300 300 0.33 1.00 0.90 360 360 0.28 0.83 0.80 450 450 0.22 0.65 0.70 570 570 0.18 0.47 0.60 720 720 0.14 0.27 In this case, we determine the cumulative exposure using Equation 5.1. This is often called the Percent-CNS method, or "tracking the oxygen clock." Using this method, the dive time actually spent at a particular PO2 is converted to a percentage of the total allowable time. The percentages from multiple dives are then totaled to yield a cumulative exposure, which should not exceed 100%. The more conservative NOAA single dive time limit is used to incorporate a safety margin, as this procedure has also never been rigorously scientifically verified.

% CNS TD TT

Percent Central Nervous System (oxygen toxicity) Actual time spent at depth (min) Allowable time at depth using NOAA single dive time limit (min)

Example 5.1 Determine Marvin's percent CNS if he does one dive on EAN32 to 107 fsw (32.5 msw) for 50 minutes. D = 107 fsw T = 50 min.

D = 32.5 msw

NOAA limit at PO2

Example 5.2

= 1.36 atm

FO2 = 0.32

= 150 min

Determine Marvin's percent CNS if he does a second dive to 84 fsw (25.6 msw) using EAN36 for 45 minutes. D = 84 fsw T = 45 min.

D = 25.6 msw

FO2 = 0.36

NOAA limit at PO2 = 1.28 atm = 180 min

Total percent CNS = 33.3% + 25% = 58.3% Depending on the dive profile or series of profiles, there are minimum surface intervals. These provide an air break sufficient to minimize the chance of a CNS oxygen toxicity "hit." The recommendations for minimum surface intervals should be followed if any of the threshold limits in Table 5.3 are reached. Table 5.3: Minimum Surface Interval Table for Oxygen Exposure

THRESHOLD EVENT

Single dive reaches at least 80% of NOAA PO2 time limits

MINIMUM SURFACE INTERVAL

45 minutes

Single dive reaches 100% or more of NOAA PO2 120 minutes (2 hours) time limits One or more dives result in reaching or exceeding the 24-hour NOAA PO2 time limits

12 hours

Many factors may predispose a diver to CNS oxygen toxicity. Stress creates a higher risk. Studies indicate that probability increases at water temperatures of 49°F (9C) and 87.5°F (31C) compared to water at 65°F (18C). Exertion also reduced the time of onset of CNS oxygen toxicity symptoms. Interestingly, divers who are immersed in water have far more episodes of CNS hits than divers pressurized in dry hyperbaric chambers. Diver age may also be a factor, with younger divers being more resistant to oxygen toxicity. Most of these factors may be related to carbon dioxide exposure, which is probably the largest variable contributing to CNS oxygen toxicity, after PO2 and time. All people retain some CO2 in their bodies at a cellular level. Exertion, cold, skip breathing, and other factors can cause this amount to increase. Any increase in retained CO2 adds to the risk of a CNS hit. In at least one case history, a diver who was apparently well and problem-free kicked only two or three times at depth, and immediately began convulsing. It is speculated that the effort required to halt his sinking was sufficient to cause the onset of an oxygen toxicity reaction. Rebreather scrubbers may contribute to CO2 issues leading to CNS toxicity. One study examined CNS symptoms in rebreather uses. Issues identified as lending to increased CO2 levels included poor canister packing, strenous activity, and water leaking into the canister. Both under and over–packing the canister with absorbent was shown to reduce scrubber effectiveness. Coupled with this is variation among individuals, or in a single individual from day to day. One diver may tolerate an oxygen exposure far greater than one that induces a strong oxygen toxicity response in another. Tolerance appears to be unrelated to diving

history, experience, or other definable variables. Even in the same diver, an exposure tolerated one day may not be on another. Some medications have possibly contributed to CNS oxygen toxicity events. These include cialis (tadalifil), epinepherine, darvocet, ventolin inhaler, lomotil, marax, transdermscop, decongestants, mylanta, tylanol, sudafed (pseudoephedrine), phenylpropanolamine, antihistamine, and birth control medication. If you are using these or other drugs, you should modify your oxygen exposure limits. Because of the inability to quantify the factors, and the variability in response to oxygen exposure, most agencies recommend that divers follow a maximum PO2 limit of 1.4 atm exposure for their diving. Tables and tools for higher exposures are provided in their materials only to facilitate contingency planning, or to cope with unexpected events. If any of the aforementioned contributing factors apply to a dive you are making, you may wish to decrease your exposure limit or time, or both, to reduce your risk.

Pulmonary Oxygen Toxicity

Pulmonary oxygen toxicity, also known as whole body oxygen toxicity, is also caused by hyperoxic exposures, but over a much longer time at lower PO2 levels than with CNS oxygen toxicity. As mentioned earlier, the PO2 must be at 0.5 atm or above before this becomes a concern. In the commercial diving industry, the practice is to maintain 0.35 atm PO2 in deep saturation diving chambers. Typically, pulmonary oxygen toxicity only becomes problematic during saturation dives, extremely extended scuba dives, or during treatments in recompression chambers. Whole body oxygen toxicity affects the lungs before other parts of the body. Physiological reactions include alteration of lung surfactant, inflammation in the lungs, congestion, edema, bronchitis, swelling of the alveolar walls, and thickening of the walls of the pulmonary arteries. Prolonged exposure has produced hemorrhage in laboratory animals. The most easily measured change is in vital capacity. Vital capacity is the maximum amount of gas a person can

exhale after a full inhalation. Pulmonary oxygen toxicity reduces vital capacity, which decreases as the condition progresses. The lung damage is reversible to a point, but can be permanent if too severe and prolonged. Symptoms that may be noted early in the cycle all involve breathing. Initially, they may manifest as a dry cough, discomfort at the end of an inspiration, increased breathing resistance, shortness of breath, or any combination of these. The diver may feel discomfort in the chest upon inhalation. If the oxygen exposure is not reduced, these symptoms will progress to severe pain in the chest on inspiration, and then constant substernal pain or burning. Again, treatment is simple—reduce the exposure to oxygen. The best way to do this is to ascend to the surface and terminate diving. If that is not possible, switching to a gas resulting in a lowered oxygen partial pressure may alleviate symptoms. We can prevent whole body oxygen toxicity by tracking exposure. This is done differently than for CNS toxicity. To track whole body exposure, we use the REPetitive EXcursion, or REPEX Method, developed by Dr. Bill Hamilton, a diving physiologist. This method involves calculating the number of oxygen tolerance units (OTUs) accumulated by the diver by using Equation 5.2. In practice, however, tables are used in lieu of the equation.

Where: OTU = Oxygen Tolerance Units T = Time (min) PO2 = Oxygen partial pressure (atm) Table 5.4 shows the number of OTUs accumulated per minute based on the PO2. A dive is calculated at the maximum PO2 for the

dive. The shaded area of the table indicates that these exposures are beyond recommended exposure limits, but are included for contingency purposes. Calculating OTU accumulation is of no use unless we have a yardstick against which to compare it. Maximum daily OTU limits are contained in Table 5.5. As can be seen, the daily limit varies based on the number of consecutive days of diving. The fewer the number of consecutive days of diving, the greater OTU exposure permitted. The average diver should not experience problems with whole body oxygen toxicity if the cumulative OTU exposure is less than the limit for the number of days diving conducted. Table 5.4: (after Bill Hamilton2,3) Tracking OTUs with REPEX PO2 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25

OTU per min 0.00 0.15 0.27 0.37 0.47 0.56 0.65 0.74 0.83 0.92 1.00 1.08 1.16 1.24 1.32 1.40

1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00

1.48 1.55 1.63 1.70 1.78 1.85 1.92 2.00 2.07 2.14 2.21 2.28 2.35 2.42 2.49

Table 5.5: (after Bill Hamilton2,3) Multiple Day Allotment of OTUs Exposure Days Average Dose Total Dose 1 850 850 2 700 1400 3 620 1860 4 525 2100 5 460 2300 6 420 2520 7 380 2660 8 350 2800 9 330 2970 10 310 3100 11 300 3300

12 13 14 15-30

300 300 300 300

3600 3900 4200 As Req'd

Example 5.3 If Mary makes two dives using EAN38, the first to a depth of 88 fsw (26.8 msw) for 60 minutes, and the second to a depth of 79 fsw (24.1 msw) for 75 minutes, what is her accumulated OTU value for the day? Dive #1:

D = 88 fsw FO2 = 0.38

OTU = (1.63/min)(60 min) = 98 OTU Dive #2 D = 79 fsw D = 24.1 msw FO2 = 0.38 T = 75 min

D = 26.8 msw T = 60 min

OTU = (1.48/min) (75 min) = 111 OTU Total OTU = 98 OTU + 111 OTU = 209 OTU Example 5.4 How many consecutive days may she dive this profile? Since her exposure of 209 OTUs is less than the most conservative allowable average daily limit of 300 OTUs per day, she can dive this profile for an unlimited number of days, based on whole body oxygen toxicity concerns. Example 5.5 How many consecutive days may Marvin dive if his daily accumulation is 438 OTUs? Marvin's exposure falls between an average OTU dose of 420 and 460 OTUs. If he dives for five consecutive days his total exposure would be 2190 OTUs, which is within the allowable limit of 2300 OTUs. If he dives a sixth day, his cumulative exposure is 2628 OTUs (six days x 438 OTU/day). This exceeds the allowable limit for

six days of 2520 OTUs, so he should not dive for a sixth day at that exposure level.

Hyperoxic Myopia

Hyperoxic myopia (also known as lenticular oxygen toxicity and ocular toxicity) is oxygen toxicity that affects the lens of the eye. It has been suggested that extended exposures to PO2 greater than 1.2 atm may cause myopia (near sightedness). The degree of involvement or severity of the problem may vary based on a variety of factors, including the number and duration of the dives, the PO2 during the dives, the number of consecutive diving days, duration and frequency of surface intervals on normoxic breathing gas, age of the diver, and prior history of eye or vision problems. Hyperoxic myopia is usually seen in patients undergoing repeated hyperbaric oxygen treatments in hyperbaric chambers, but it has been anecdotally reported by several CCR users. The CCR divers suffered from hyperoxic myopia after a series of dives lasting 12 or more days with greater than 45 hours exposure time at a PO2 of 1.2 atm or greater. Onset was reported to occur towards the end of the diving period and persisted for several weeks after the diving was concluded. The primary symptom is a loss of distance vision with good preservation of near vision (assuming that the diver was not farsighted beforehand, in which case distance vision might actually improve). Vision typically returns to normal several weeks after cessation of hyperoxic exposures, but severe cases may result in cataract formation and require cataract extraction to restore vision. Hyperoxic myopia is a very poorly studied phenomenon, and the information in this area is tentative. Other anecdotal reports by CCR divers who have similar dive histories report no problems with hyperoxic myopia. If you are planning to conduct a long series of dives, you may wish to consider maintaining a maximal PO2 of 0.7 1.0 atm, or plan your dive sequences with surface intervals longer than those required for adequate inert gas discharge. It is impossible

to predict at this time if these measures will be completely effective for all CCR users involved in this type of extended diving.

Hypercapnia

For every liter of oxygen we consume, we produce about 0.8 liters of CO2, a waste gas. Normally, on the surface, the CO2 is expelled to the atmosphere, to later be recycled into oxygen by plants, or captured by rainwater and incorporated into rocks, soil, or even the ocean. With open circuit, the CO2 is exhausted with our exhalation and the gas ends up in the same places as if we were breathing on the surface. But on rebreathers, the CO2 is exhaled into the breathing loop. The gas is absorbed by the scrubbing compound and immediately turned into calcium carbonate or some other inert mineral material. This assumes everything is working properly. What if it is not? Another insidious hazard faced by rebreather divers is the possibility of hypercapnia, or carbon dioxide poisoning. Elevated levels of CO2 may occur if the scrubbing compound is close to being completely used up, or if the gas passing through the system is not uniformly passed through the CO2 absorbent. It may also occur if the diver forgets to fill or refill the scrubber canister with absorbent. Use of full-face masks with rebreathers may also increase the potential for hypercapnia. More information on absorbents and emergency procedures for hypercapnia is provided in Chapters 6 and 12. As with oxygen toxicity, the human body is affected by the partial pressure of carbon dioxide (PCO2), not the percentage or fraction of CO2 in the breathing gas. This means that a gas that contains a small percentage of CO2 might be tolerated at shallow depths, but could become highly toxic at increased depths. For this reason, breathing gas in the rebreather that contains as little as 0.5% CO2 (PCO2 of 0.005 at 1 ata) is unacceptable under most diving agency standards.

Figure 5.6: Effects of elevated PCO2

Hypercapnia symptoms include shortness of breath, increased respiration rate, strong anxiety bordering on panic, inability to concentrate, general malaise or discomfort, headache, nausea, narcosis, and dizziness. Extreme cases may lead to unconsciousness and death. The effect depends on the PCO2 and the duration of exposure. Figure 5.6 shows the effects of carbon dioxide exposure over time. As may be seen, the longer the exposure (i.e. the duration of a dive), the lower the PCO2 required for the onset of symptoms. Hypercapnia is similar to hyperoxia in that there is significant individual variation to susceptibility, further indicating the need for caution in this aspect of rebreather use. Most rebreathers have no capability to measure PCO2 level. Several manufacturers are working on the development of PCO2 sensors, but at the time of this writing none of them are in general use. The only method we have of preventing hypercapnia is to ensure that no errors are made during the pre-dive setup of the rebreather, by packing the scrubber properly, logging the duration, and using the scrubber in accordance with manufacturer's specifications.

This is one area where adequate testing of rebreathers is vital. Testing can determine the time it takes for the absorbent to become chemically reduced, thereby allowing CO2 to pass the through the scrubber canister to be breathed again by the diver. This testing must be done at different temperatures and work rates to be effective in predicting absorbent use under different conditions. And, again, manufacturer testing should be confirmed by an independent third party.

Figure 5.7: Recompression chamber.

Should you recognize a problem or suspect hypercapnia, immediately reduce the PCO2 in the inspired gas. Flushing the breathing loop or switching to another gas supply will do this. Breathing pure oxygen on the surface will help alleviate residual symptoms.

Decompression Sickness

The majority of the gas we usually breathe is nitrogen. As you remember from entry-level scuba training, uptake of nitrogen while under pressure can lead to decompression sickness (DCS). The same is true with rebreathers. Even though the oxygen-rich mixtures used in rebreathers will reduce the risk, it is still possible to get DCS (except with oxygen CCR, in which there is no inert gas if used properly). Symptoms and treatment of DCS are just as you learned in earlier diving courses. Symptoms may include itching skin, numbness, paralysis, joint pain, or in serious cases, unconsciousness and respiratory distress. Symptoms generally progress if not treated. First aid includes immediate administration of oxygen on the surface, placing the victim in a prone or supine position, treating for shock, and transport to a recompression chamber. Prevention of DCS is accomplished using air or EANx dive tables or computers for open-circuit scuba. Rebreathers use these tools, but in a different manner. Detailed procedures for use of tables for rebreathers are covered in Chapter 6.

Inert Gas Narcosis

Nitrogen can affect our bodies while diving. If the PN2 reaches a critical level, you may experience nitrogen narcosis, or "rapture of the deep." This feeling of drunkenness results from the anesthetic effects of breathing nitrogen at depth. Symptoms may include loss of motor control, inability to reason, inability to cope with stress or task loading, perceptual narrowing or fixation, and light-headedness. You might experience feelings of euphoria, invincibility, dread, fatigue, or anxiety. The effects of narcosis are extremely variable, among individuals and in the same individual from day to day. Marvin, while on an open-circuit air dive to 110 fsw (33 msw), may be severely affected by narcosis, while Mary feels completely normal. On another day, while at the same depth and PN2 exposure, Mary may be totally

incapacitated. Factors such as CO2 loading, exertion, cold, and personal comfort levels may also impact the onset and severity. Nitrogen narcosis can also affect rebreather divers. Any nitrogen exposure comparable to diving air at a depth of 100 fsw (30 msw) or greater may result in narcosis in rebreathers. With mass flow SCRs, the possibility of nitrogen narcosis is lessened, as the user is generally limited to depths shallower than those at which narcosis is likely to occur.

Figure 5.8: The Evolution CCR may be used with a trimix dilvent to reduce nitrogen narcosis. Courtesy of National Park Service, Brett Seymour

Other "inert" gases may also cause narcosis. Some will have narcotic effects that affect a diver sooner than nitrogen, others at depths or partial pressures significantly greater than nitrogen. This is the primary reason helium is often used as a replacement for nitrogen in diving. Helium is not narcotic, and thus allows divers to

dive deeper without the risk of narcosis. While some divers advocate using helium in rebreathers to allow deeper diving, only rebreathers that have been specifically approved by the manufacturer for use with gases containing helium should be used in this manner. The use of helium based breathing gases in rebreathers is addressed in Mastering Rebreathers, Volume 2. Treatment of inert gas narcosis is simple—reduce the partial pressure of the inert gas. Ascend to a shallower depth until symptoms are alleviated.

Arterial Gas Embolism

Arterial gas embolism (AGE) is probably the most serious malady an OC diver can experience. In AGE, pressure imbalances cause ruptures in the lungs, allowing air bubbles to enter the arteries. In the worst cases these bubbles circulate to the brain, blocking blood flow and causing symptoms similar to those of stroke victims. Symptoms may include chest pain, difficulty breathing, bloody sputum or froth from the mouth, paralysis, unconsciousness, and cardiac arrest. These symptoms usually occur within ten minutes of surfacing from the dive, and often within a minute or two. If not rapidly treated, AGE can lead to death. In open-circuit scuba, embolisms may be avoided by maintaining an open airway during ascents. However, in rebreathers, exhaled gas is not completely expelled, but is retained in the counterlung. If you ascend rapidly without venting gas, the gas expanding in your lungs and the counterlung can over-pressurize the system, causing a rupture in the weakest part of the breathing loop. Unfortunately, this is usually your lung. Most counterlungs contain a pressure relief valve, similar to the one commonly seen on a buoyancy compensator, to allow expanding gas to escape. During rapid ascents the overpressure relief valve may not vent sufficient gas to prevent an AGE, so ascent rate is important. Other counterlungs are made of elastic material, allowing the entire counterlung to expand as pressure within the system increases. This provides a warning to the user, indicating that gas needs to be expelled from the loop.

The most hazardous rebreathers are those which have no means of automatically relieving excess gas from the breathing loop. Many military units are designed NOT to automatically vent gas (bubbles hitting the surface may reveal the diver to enemy soldiers). Because of this, military divers using rebreathers have embolized. The most hazardous conditions are in heavy swells or in surf zones, where water pressure varies rapidly as waves pass overhead. In unprotected units, even if the diver does everything correctly, it can be difficult to prevent embolisms during wide and rapid pressure fluctuations. First aid includes placing the victim in a prone position, administering oxygen, treating for shock, and transporting to a recompression chamber. If the victim is not breathing or is in cardiac arrest, then artificial respiration or cardiopulmonary resuscitation should be administered. These treatment skills are very important for all divers to have, and may be learned in classes from the American Red Cross, American Heart Association, or in a American Safety & Health Institute (ASHI) Dive First Aid/CPR class.

Figure 5.9: Pressure imbalance between counterlung and lungs in different body positions.

Squeezes

Another problem potentially created by pressure imbalances during diving is squeezes. Usually, squeezes occur during descent when the diver has difficulty equalizing. Increasing depth and water pressure compress air spaces in or around the body. When those spaces reach their limits of elasticity, pain results. The problem is remedied by introducing additional gas into the affected air spaces during descent to equalize the pressure. Another type of squeeze called a reverse block may occur if the diver cannot equalize during ascent. In this case, gas introduced into air spaces during descent cannot escape, causing overpressurization within the body. Both of these types of squeezes may occur to rebreather divers. Also, because of rebreather design, another type of pressure imbalance can occur. This imbalance is caused by the difference in depth of the counterlung and the diver's lungs (Figure 5.9). If the counterlung is deeper than the lungs, inhaling will be easy and exhaling difficult, since the pressure on the counterlung is greater than that on the lungs. Conversely, if the counterlung is shallower than the lungs, inhalation will be difficult and exhalation easy. As body position in the water column changes (swimming on one's stomach versus swimming on one's back, for example) the breathing characteristics of the rebreather will change. Breathing resistance caused by pressure differentials may contribute to both squeezes and air embolisms. Optimally, the counterlung and lungs should be at the same depth.

Figure 5.10: Counterlung induced pressure imbalance in ears.

The same problem occurs with the ears. In most normal diving positions on all rebreather designs the counterlung is deeper than the ears and a pressure differential exists (Figure 5.10) with the interior pressure pushing outward on the eardrum throughout the dive. This may lead to sore ears, especially after long dives. It also can force bacteria through the Eustachian tube into the middle ear, causing ear infections. The best way to avoid these problems is to minimize the pressure differential by swimming in a position that minimizes the depth difference between the ears, lungs, and rebreather counterlung. You may need a rebreather of different design. Crotch straps or other harness enhancements may help maintain the rebreather in a stable position on the body.

Middle Ear Oxygen Absorption Syndrome

We are all familiar with the need to equalize our ears during descent to avoid barotrauma due to squeezes. Occasionally during ascent a diver may experience a reverse block and sustain a pressure injury. But we are all used to thinking that when we hit the surface after a dive, the risk of ear injury due to pressure imbalances is past. This is not true with some rebreathers. Many rebreathers can provide breathing gas with a very high FO2 to the diver. For example, mass flow controlled SCRs may use supply gas containing as much as 80% oxygen, constant PO2 CCRs can provide as much as 95% oxygen, and, of course, oxygen CCRs provide 100% oxygen. As you descend, you equalize your ears with the breathing gas available to you, in these cases gas with high oxygen content. This is not a problem during the dive, or immediately after surfacing from the dive. Your ears will feel normal, and you will have no indication that anything is other than normal. However, when you

surface from the dive, you will have high FO2 gas in your Eustachian tube and behind your eardrum. As you remain out of the water, the oxygen in these gas filled spaces will be absorbed by the surrounding tissues. Unfortunately, that absorbed gas is not being replaced as quickly with nitrogen. The result is a net loss of gas molecules from the middle ear, with consequent volume loss. The pressure outside the tympanic membrane becomes comparatively greater, and exerts a pressure on the membrane. This can result in a painful squeeze known as middle ear oxygen absorption syndrome (sometimes called O2 ear), with a barotrauma injury that is identical to that which might be experienced on descent if you forget to clear your ears. Because it is such a slow process, it is not uncommon for the diver to be completely unaware of it until noticeable injury has been sustained. This is especially true if you go to sleep soon after diving. One symptom you may notice is hearing impairment due to stretching of the tympanic membrane. To avoid this injury, it is necessary to periodically equalize after the dive while on the surface. Any standard manual technique for equalization, such as a Valsalva or swallowing, will accomplish this. Some divers find themselves having to equalize multiple times up to several hours after surfacing to prevent damage to their ears. Divers who have difficulty equalizing during dives experience this type of surface barotrauma more commonly than others.

Figure 5.11a: Middle Ear Oxygen Absorption Syndrome. Before the dive, the middle ear is filled with oxygen depleted air. Courtesy of Debra Moysychyn, CMI

Figure 5.11b, 5.11c, 5.11d: Middle Ear Oxygen Absorption Syndrome. Courtesy of Debra Moysychyn, CMI

Figure 5.12: Happy germs living in a corrugated breathing hose.

Disease Transmission

Using a rebreather is essentially like having "open-lung surgery." Your "lungs" now consist not only of those with which you were born, but also the counterlung, hoses, scrubber canister, water traps, and other parts of the apparatus through which your breath is circulated. Unlike your lungs, these mechanical "add-ons" have no natural defenses against bacteria or fungi. Quite the opposite is true, as many of the components of a rebreather provide marvelous breeding sites for microorganisms. Since you are only as strong as your weakest link, consider your rebreather as a limiting factor when contemplating your resistance to disease. Every time you open your rebreather to work on it, add absorbent, connect fittings or hoses, etc., consider that you are, in effect, opening your body and exposing your lungs. So, avoid placing rebreather parts on dirty surfaces, handling them with dirty hands, or having others breathe onto open parts while you perform routine maintenance. Like a surgeon, you want to maintain a clean environment. Otherwise, you stand a possibility of contracting any airborne disease that may be present.

Figure 5.13: Cleaning your rebreather between users and at day's end will help prevent disease transmission.

The risk of disease transmission increases if divers share a rebreather. Any disease that one diver has spreads to and is concentrated within the rebreather. If another person uses the system, the risk of contracting any disease that is normally airborne or transmitted by contact, especially by contact with saliva, is very real. So, all parts of the breathing loop should be thoroughly cleaned and disinfected before being used by another diver. The only exception is the scrubber. Since the environment within the scrubber is extremely alkaline, it is unlikely that microorganisms will be able to live in this sub-unit. While greater contamination may be expected in the exhalation side of the breathing loop (hoses and counterlung) before the gas has passed through the absorbent, all parts of the breathing loop should be disinfected. Proper cleaning procedures are covered in Chapter 10.

Allergic Reactions

A wide variety of substances may cause allergic reactions in people. In some respects, people allergic to common substances

deal with their allergies better, as they have learned to identify and avoid contact on a daily basis. People with allergies to uncommon items may not be as practiced. Exposure may be airborne or contact. Most airborne material is not a concern for divers, as the breathing gas is cleaned and filtered. Allergic reactions vary with the extent of contact, and with the individual. Symptoms may include itching, rashes, swelling, or tenderness. If the agent is inhaled swallowed, or otherwise internalized, then the symptoms may lead to bronchial spasms, swelling of the throat occluding the airway, or anaphylactic shock. These more serious symptoms, if untreated, can rapidly cause death by asphyxiation. Two substances in rebreathers have been reported as often causing allergic reactions in divers. One is rubber. Many people have an allergy to one or more forms of rubber or latex. Since much of the breathing loop in some rebreathers is made of rubber, divers who tolerate contact with rubber poorly probably should not use these rebreathers. The second problem involves the disinfectants used to clean rebreathers. For example, some people are intolerant to iodine. Two common disinfectants used for cleaning breathing loops are Wescodyne® and Betadine®. The active ingredient in each respectively is poloxamer-iodine and povidone-iodine (10%), which may have undesirable effects on these individuals. Other disinfectants have reportedly caused allergic reactions ranging from persistent nasal discharge to difficult breathing. If you experience adverse reactions to one cleaner, consider shifting to another with a different active ingredient. For example, you might switch from an iodine to an alcohol based agent. Cleaning procedures are covered in Chapter 10. If a diver experiences allergic reactions using a rebreather, the dive should be immediately terminated. Exposure to the offending agent should be ended, and affected areas of the body flushed with fresh water. If breathing difficulties are present, competent medical personnel should treat the affected diver as soon as possible. If severe problems persist and medical attention is not readily

available, then epinephrine injections ("Epi Pen") or Benadryl® may be used by qualified personnel to alleviate symptoms while the victim is transported to a medical facility.

Carbon Monoxide

Carbon monoxide levels as high as 810 parts per million (ppm) have been measured in some closed-circuit breathing systems after use.4 Levels above 10 ppm are considered hazardous. In the referenced study, it was theorized that the carbon monoxide was released from hemoglobin molecules in which it was being carried after smoking. The molecules remained in the closed system wherein they were released. For this reason we suggest smoking is to be avoided even more with rebreathers than with open-circuit scuba.

Figure 5.14: Rebreather skills must be overlearned to develp the muscle memory needed in emergencies. Courtesy of Ramon Llaneza

Kinesthetic Considerations

Scuba diving requires motor skills not used above water. This is why many skills are practiced over and over again in entry level diving, to build up a degree of muscle memory allowing safe diving. The more times a skill is repeated in practice, the more it becomes second nature. This becomes especially important when unplanned

events occur. In extreme emergencies, it is often only muscle memory that prevents a situation from becoming a tragedy. The same is true in rebreather diving. Some of the skills you learned in your open-circuit diving are the same with rebreathers. Some skills are new, and will have to be repeated many times before becoming automatic or even comfortable. All new skills should be over-learned, so that no thought is required to perform them. It has been demonstrated that even for a simple skill, as many as 17 repetitions must be performed before it is adequately learned. More repetitions are better. It is not economically feasible to have this level of repetition of all skills in a training course, so you must practice these skills after your training program is completed to build your muscle memory.

Figure 5.15: Rebreather divers may have many additional gauges to monitor while diving, adding to task loading.

Some of the skills you have learned in your open-circuit training are improper and even unsafe for rebreather diving. These will be the most difficult to internalize, because you must unlearn habits that are now "bad." These skills, such as over-clearing a mask, must be practiced many times before they are internalized. There is the additional concern that in the event of an emergency, it will often be the earlier learned skill that will be unconsciously performed.

Finally, all skills will require regular practice to maintain. This will be most critical immediately after you complete training, but is a concern any time you have a break from rebreather diving. If you experience a hiatus from rebreather diving, plan your first dives in extremely benign environments, and practice all of your skills before progressing to more challenging dives.

Psychological Considerations

As you begin rebreather diving, you will find that there are many new chores that must be accomplished. This can lead to task loading, where the sheer number of items you must deal with becomes an impediment. This leads to mental stress, which may impact dive safety. A typical response to task loading is fixation on one task, such as monitoring the oxygen sensors, to the exclusion of other necessary jobs, like watching your depth, time, or buoyancy. If you see this happening to yourself or your buddy, stop, calm down, and continue or terminate the dive as necessary. Future dives should be less ambitious, until you gain or renew skills and muscle memory sufficient to cope with the activity level. Before continuing with your training, ask yourself, "Why you are diving rebreathers?" Is it to reduce the weight of open-circuit gear? To be able to dive longer? To get involved in technical diving? Regardless, your motivations and personality will become an part of your dive planning process. As you have no doubt surmised, rebreather diving is in many respects more complicated and potentially more hazardous than open-circuit air diving. It also entails additional pre-dive planning and post-dive maintenance. Self-discipline is the key to success in rebreather diving. Nobody is watching over your shoulder to see if you complete crucial planning tasks. If you are not willing to make the additional effort, rebreathers may not be the best gear for you. In this type of diving, the "Seven-Ps" are more critical than ever before. "Proper Prior Planning Prevents Piss Poor Performance" is an old military expression that sums up the importance of planning in a nutshell— especially when we remember that "poor performance" in this case potentially means "death." At least part of a diver's

attention to pre-dive planning stems from the driving motivation and attentiveness to detail inherent in his or her basic character. The self-discipline needed in the pre-dive planning process spills over into the dive as well. With rebreathers it is extremely critical that you stick to the parameters established during planning. It is often very tempting to go "just a little deeper" or "stay just a little longer." In OC scuba you can sometimes succumb to these whims with few to no consequences. Surrendering to these same impulses can have "grave" repercussions with a rebreather. The key is to develop a safe attitude for rebreather diving in yourself and in your buddy. You have learned to seek out a dive partner who has the same dive objectives as you. This leads to harmony during the dive. Rebreather diving is the same. Instead of looking merely at the dive objective, however, look at the basic motivations for desiring to utilize rebreathers in any potential buddy. If those motivations reflect your own, that is one indication that you will work well together. Continue working together to gain the patterning to dive rebreathers in a responsible and prudent manner.

Figure 5.16: Diving with a buddy who has similar interests to yours will lead to more harmonious and safer dives. Courtesy of Elaine Jobin

1. The following variables affect the amount of oxygen metabolized by a diver: a. Exertion level, thermal stress, depth, and FO2. b. Thermal stress, basal metabolism, PO2, and exertion level. c. Basal metabolism, thermal stress, exertion level, physical condition, and depth. d. Physical condition, basal metabolism, thermal stress, exertion level, and body size.

2. Draw an oxygen partial pressure scale, labeling the following: a. Level at which a diver becomes hypoxic. b. Point of normoxic gas blend. c. Onset of pulmonary oxygen toxicity. d. Maximum recommended PO2 exposure. e. High risk of CNS oxygen toxicity.

3. Rebreather divers may experience hypoxia when: a. Using a mass flow controlled SCR with a partially blocked orifice.

b. Ascending to the surface rapidly while swimming hard against a current. c. Forgetting to monitor oxygen meters in mixed-gas CCRs. d. Improperly matching the gas supply and mass flow controller orifice. e. All of the above.

4. CNS oxygen toxicity is dangerous because: a. Oxygen seizures can kill you. b. The lungs may not recover full functionality after the event. c. Drowning may result secondarily from CNS oxygen toxicity symptoms. d. CNS oxygen toxicity is not a major potential hazard to rebreather divers.

5. Pulmonary oxygen toxicity is best avoided by using: a. The NOAA oxygen partial pressure & exposure time limits table. b. The Repex method of tracking oxygen exposure. c. The IANTD Calculator.

Pulmonary

Oxygen

d. All of the above may be used.

Toxicity

Exposure

6. The chances of disease transmission in rebreathers may be minimized by: a. Disinfecting the breathing hoses, counterlung after the day's diving.

mouthpiece

and

b. Disinfecting the breathing counterlung between users.

mouthpiece

and

hoses,

c. Avoiding contact of breathing loop components with dirty surfaces or materials. d. All of the above.

7. The following may cause an allergic reaction in a diver using a rebreather: a. Iodine. b. Rubber or latex. c. Oxygen. d. A and B

8. What is the maximum partial pressure of oxygen exposure that most agencies recommend not be exceeded? Why has this limit been selected? _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________

_____________________________________ _____________________________________ _____________________________________ _____________________________________

9. Why is self-discipline important while diving rebreathers? _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________

10. Middle ear oxygen absorption syndrome may be prevented by: a. Equalizing regularly after the dive for several hours. b. Breathing 100% oxygen after the dive. c. Breathing 100% oxygen during the dive. d. None of the above.

Theory Overview

This chapter is divided into five sections. Four cover rebreather operation, each dedicated to a different type of rebreather. This will make it easy for you to turn to the section that covers the type of rebreather you will use in training. Everyone should read Section Five, which discusses absorbents. The sections are:

Objectives After reading this chapter, you will be able to: 1. Explain the importance of purging the breathing loop of an oxygen CCR prior to diving. 2. Explain the principle behind the operation of a mass flow controller. 3. Calculate the theoretical PO2 for various work levels in mass flow controlled SCRs. 4. Illustrate the hazards of diving a mass flow controlled SCR with mismatched gas supply and sonic valve orifices. 5. Calculate the maximum operating depth of a mass flow controlled SCR based on supply gas and interstage pressure. 6. Explain how a RMV-keyed SCR operates. 7. Determine the minimum set point for a mixed gas CCR based on dive depth.

8. Determine inert gas status using dive tables for different types of rebreathers. 9. Illustrate the differences between constant PO2 closed-circuit and constant FO2 systems with regard to dive tables. 10. List different factors affecting absorbent efficiency and their ramifications.

Section One Section Two Section Three Section Four Section Five

Oxygen CCRs Mass Flow Controlled SCRs RMV-Keyed SCRs Mixed Gas CCRs Absorbents

While other rebreather designs have been available, they are not covered in this chapter, as none are currently available in the recreational diving industry. Rebreathers can fail in many ways, some relatively minor, others quite significant. One of your tasks as a rebreather diver will be to determine what is happening with your unit as you are diving. The way you set up your rebreather is also important, and subject to several options. Understanding the basic operating theory of your rebreather will aid you in these tasks. Your rebreather's operating theory also determines proper and improper uses of your unit. It is possible to use rebreathers improperly, far beyond the recommended design specifications. Much of the information provided in this chapter will enable you to evaluate the safety and soundness of suggestions made by other rebreather divers.

Section One OXYGEN CLOSED-CIRCUIT REBREATHERS

Mechanics and Oxygen Addition

Oxygen closed-circuit breathing apparatus is the simplest form of CCR, consisting of a counterlung, scrubber canister, supply cylinder, and breathing hoses. There is little to fail mechanically. These CCRs generally use one of two basic designs. In the first, the gas is added via a valve that is either manually operated or triggered by a loss of gas volume in the breathing loop, caused by descent or metabolic consumption. Automatic addition occurs when the counterlung bottoms out prior to the diver drawing a full breath. This causes a negative pressure that activates automatic oxygen addition. A Schraeder valve or second stage type regulator provides the oxygen. The second design continuously injects oxygen into the breathing loop, typically at a flow rate of 0.7 lpm. This is sufficient to replace the metabolic consumption of a diver at low work rates. A manually activated bypass valve allows the diver to add additional oxygen as needed.

Figure 6.1: Breathe shallowly when using an oxygen CCR in the surf line or heavy swells to avoid air embolism.

Most oxygen CCRs are manufactured without an overpressure relief valve in the breathing loop. This is beneficial if you want to ascend without releasing any bubbles, but presents a potential problem for the complacent user. If you ascend slowly, you can surface without over-expansion of the system, since you can metabolize the oxygen to keep the volume constant during ascent. However, if you ascend rapidly, then you must consciously vent gas from the system, either by exhaling from your nose or allowing gas to escape around the mouthpiece. It is possible to "embolize" the CCR itself, even if you are not using it. If you change from the CCR to the bailout cylinder at the bottom, proper procedure is to first close the mouthpiece of the CCR. If you leave the counterlung full, then gas expansion during ascent may cause the counterlung to rupture. Proper procedure for this switch using the oxygen CCR is to inhale all of the gas from the loop prior to ascent. The most hazardous places to use oxygen CCRs are in the surf line, or close to the surface in high seas. While the diver may not change depth with respect to the bottom, the depth from the surface may vary rapidly as waves pass overhead. Since it takes as little as 4 fsw (1.3 msw) pressure change to cause an air embolism, relatively small waves can be very hazardous. Rapidly moving waves are more dangerous than long, slow swells, as the variations in depth occur more quickly. It is also possible to "squeeze" the CCR, and damage it. If you descend without breathing from the CCR, volume in the counterlung and breathing loop will decrease. If the counterlung is empty prior to descent, then it is possible to fracture rigid components of the unit due to the increase in water pressure and consequent decrease in gas volume.

The second major hazard encountered by oxygen CCR users is hypoxia. Even though 100% oxygen is being injected from the supply cylinder, if the unit is set up improperly, a user may become hypoxic. When the unit is assembled, it contains air: 21% oxygen and 79% nitrogen. This gas must be flushed from the system, or else the diver will not be breathing 100% oxygen. If the gas in the system's breathing loop is not purged and replaced, then the oxygen content may easily drop below 10%. This occurs because the user depletes the oxygen via metabolic use, but the small volume loss is insufficient to trigger gas addition. The original 79% nitrogen is now a greater proportion of the remaining gas, leading to a hypoxic mixture. The hazard is greatest near the surface. In shallower water, improperly purged CCRs have a larger volume of inert gas. The oxygen in the unit may be consumed before the volume drops to a point sufficient to trigger automatic addition of fresh gas from the oxygen supply cylinder. As you descend, two factors work to prevent this. Firstly, as the depth increases, even low FO2 mixes become capable of sustaining consciousness as increasing water pressure increases the PO2 of the low oxygen content gas. Also, during descent oxygen is added to the loop to maintain a minimum breathing volume, further improving the breathing gas. To prevent the problem at the surface, before entry or descent, purge the system by repetitively inhaling all of the gas in the loop through the mouthpiece and exhaling through the nose. Once the loop is as empty as possible, refill the loop with oxygen from the supply cylinder. Do not inhale from the ambient atmosphere (the air around you) during this procedure. Repeat this three times without removing the mouthpiece. Once this is done, the unit is ready to dive.

Figure 6.2: Normally before a dive, nitrogen enters and leaves the body at equal rates (upper). On an oxygen rebreather, more nitrogen leaves the body, resulting in less nitrogen than normal in the body.

If you take a breath of air, for example to speak to someone, you will have filled your lungs with a quantity of inert gas. Before resuming use on the CCR, you must purge your lungs of the inert gas by repetitively inhaling from the rebreather and exhaling through your nose. If you exhale into the rebreather without doing this, you will add inert gas to the system, creating the potential for hypoxia. At the surface, our bodies are saturated with inert gas. We carry a significant quantity of nitrogen dissolved in our tissues. While diving an oxygen CCR, the equilibrium of the inert gas is altered. There will be no nitrogen in the breathing gas. This results in a flow of nitrogen from our tissues into the breathing loop (Figure 6.2). If this is not a

repetitive dive, the amount of nitrogen leaving your body and being trapped in the breathing loop will not be sufficient to cause hypoxia. However, if you have conducted a dive within the previous 12 hours on OC (open-circuit) scuba using air or another gas with a large fraction of inert gas, loss of the inert gas into the breathing loop occurs much faster. More inert gas will be dissolved in your body after a repetitive dive if the fraction of inert gas in the previous dive was high, the dive was deep or long, or the surface interval short. In this case, you must purge the loop under water 15 minutes into the dive, and again at 30 minutes into the dive.

Depth Limits and Dive Tables

The nature of oxygen CCRs constrains their use to very shallow depths. Breathing close to 100% oxygen, our maximum operating depth is determined (using Equation 4.6, page 75) to be 15 fsw (4.5 msw). This is based on a maximum PO2 of 1.4 atm and a FO2 of 0.95. It is very difficult to purge all of the inert gas from the breathing loop even if proper procedures are followed, so 0.05 inert gas is a reasonable estimate of FO2. Other agencies and older texts state depth limits ranging from 20 fsw (6 msw) to 50 fsw (15 msw). These limits were determined based on the mission of the agencies involved, the levels of CNS toxicity risk they were willing to accept, and less historical data on CNS toxicity incidents. Military diving in particular uses procedures very different from those considered acceptable in recreational diving. Inert gas loading is not a problem with oxygen CCRs. In fact, a diver will have less nitrogen in his system after a dive than if he had remained on the surface without diving. Thus, inert gas loading tables (dive tables) are not needed for oxygen CCR use. Flying after diving an oxygen CCR is also permitted if no dives using other equipment have been made during the previous 24 hours. Again, this is because there has been no additional nitrogen loading of the tissues during the dive.

Dive time is constrained by oxygen exposure limits. Limits for both CNS toxicity and pulmonary oxygen toxicity must be determined and respected. As with nitrox diving, the NOAA and REPEX oxygen exposure limits presented in Chapter 5 should be followed. Example 6.1 Mary conducts a dive using an oxygen CCR to a depth of 10 fsw (3 msw) for 40 minutes. What is her %CNS and OTU loading? FO2 = 1.0

D = 10 fsw

D = 3 msw

From Table 5.2 (page 101) and Equation 5.1 (page 102):

From Table 5.4 (page 106): OTU = (1.48/min) (40 min) = 59.2 OTU

Section Two MASS FLOW CONTROLLED SEMI-CLOSED REBREATHERS

Mechanics

Currently, mass flow controlled rebreathers are the most prevalent type of SCR on the market. They are relatively simple to set up, operate, and maintain. However, several issues are critical to their safe use. In a mass flow SCR, nitrox from the supply cylinder is continously injected into the breathing loop in a controlled amount, or dosage. A mass flow controller accomplishes this. The heart of the controller is a sonic valve, or acoustic jet (Figure 6.3). This valve is a precisely machined orifice, or hole, which passes a stable, exact amount of gas. It is called a sonic valve because gas flows through it at the speed of sound. Dirt, debris, or corrosion can compromise proper functioning of the sonic valve by reducing the orifice size and gas flow. This would reduce the amount of oxygen available to the diver, and could cause hypoxia. As part of the pre-dive checks (Chapter 8), measure the amount of flow through the orifice to ensure it is within manufacturer's specifications. If it is not, you must have the unit serviced before you use it. Some mass flow controlled SCR's have a single orifice meant to be used with a single nitrox blend, like the DrägerRay®, which uses EAN50. However, most mass flow controllers have several different sized orifices that can be used. As an example, the Dräger Dolphin® has four orifices available in the United States, and a fifth that is only available in Europe. These orifices must be matched to the proper nitrox gas. If the wrong orifice is used, the amount of oxygen injected into the loop may be inadequate for the diver.

Figure 6.3: Sonic valve and orifice insert from a mass flow controller. This passes a constant mass of gas into the breathing loop at a set rate, regardless of depth or diver workload. Remember that a blockage can be hazardous to your health, so keep it clean and check it regularly!

Other mass flow controlled SCRs have a variable flow rate valve, like the Azimuth®. The flow rate on this unit must be adjusted to match the nitrox supply gas in use. The mass flow controller injects gas continuously into the breathing loop. The diver consumes some or all of the oxygen, but none of the inert gas, despite its continuous inflow. The counterlung has a maximum volume that cannot be exceeded without rupture, and the accumulating inert gas would cause such an incident. To prevent this, the counterlung is fitted with an overpressure relief valve. Thus, gas flow in the overall system is balanced between the gas injected through the sonic valve and gas lost by diver consumption and through the overpressure relief valve.

Oxygen Addition

We can look at this balance mathematically. If we consider gas volumes in a steady state or equilibrium condition, we can say that the quantity of gas being vented from the system is equal to the amount added, less the volume of oxygen metabolized by the diver (Equation 6.1):

Where: Qv

Qs VO2

Vent rate of gas (lpm) Flow rate of the supply gas (lpm) Metabolic consumption of oxygen (lpm)

We could also examine gas flow with respect to oxygen only. In this case, the total amount of oxygen entering the system would be the injection rate times the fraction of oxygen in the supply cylinder (FsO2). (Equation 6.2 below) The amount of oxygen leaving the system is that being consumed by the diver (VO2), plus the amount being exhausted from the overpressure relief valve. That amount is the vent rate times the FO2 of the exhausted gas. A close approximation of the FO2 is that inspired from the counterlung (FiO2). Thus we arrive at Equation 6.3, showing oxygen lost from the system:

Where: Os OL Qs Qv FsO2 FiO2 VO2

Amount of oxygen added to the system (lpm) Amount of oxygen lost from the system (lpm) Flow rate of the supply gas (lpm) Vent rate of gas (lpm) Fraction of oxygen in the supply gas Fraction of oxygen in the inspired gas Metabolic consumption of oxygen (lpm)

Note: • is a symbol calling for multiplication; i.e., 2 • 3 = 6 is identical to 2 x 3 = 6 Since the amounts of oxygen added to the system and lost from the system are equal, we can show:

This equation has two unknown quantities, the vent rate of the gas (Qv), and the fraction of oxygen in the inspired gas (FiO2). We can eliminate one of those by replacing Qv with the value in Equation 6.1 (page 133), arriving at Equation 6.5 below. By rearranging this, we have a formula that yields the FiO2, a value that will aid us immensely in our dive planning (Equation 6.6), called the mass flow steady state equation:

The principle underlying the operation of mass flow controlled rebreathers is that just enough gas is added to the breathing loop to replace the oxygen consumed by the diver. But the amount of oxygen consumed is proportional to diver workload. Because the mass flow controller injects a fixed amount of gas into the breathing loop, we need to plan for the worst case scenario – hard work under water. Typically, we consider this to be an oxygen consumption rate of three liters per minute (lpm). We can now determine the FO2 in the counterlung (FiO2). As we see, FiO2 varies with three factors – the fraction of oxygen in the supply gas (FsO2), the gas injection rate (Qs), and the oxygen metabolism of the diver (VO2). We can measure the FsO2 with an oxygen analyzer. We can measure Qs with a flow meter. We assume a hard work level for VO2. Let's look at an example: Example 6.2 Mary is using a Dräger Dolphin I® SCR. Her supply gas is EAN54, and the measured flow is 7.3 lpm. What should the gas mix be under hard work conditions?

FsO2 = 0.54

Qs = 7.3 lpm

VO2 = 3 lpm

In this example, Mary's lowest FiO2 would be 0.22. To prevent hypoxia, a FiO2 of 0.21 or greater is acceptable. If Mary works less on the dive, her FiO2 will be higher. Thus, this is an acceptable combination of supply gas and orifice. One error often made by divers using mass flow controlled SCRs is selecting their gas supply and orifice based on their anticipated work level. They select a smaller orifice to maximize their supply gas duration. While this may provide a reasonable oxygen content if the dive progresses as planned, it may not be adequate if the diver encounters a situation requiring more effort than anticipated, such as having to fight an unexpected current to return to the boat. Let's look at another example: Example 6.3 Marvin is using a Dräger Atlantis I® SCR on a dive to 33 fsw (10 msw). He selects an orifice with a measured flow rate of 7 lpm, using EAN50 as the supply gas. He will be taking close-up pictures, so figures his oxygen consumption will not exceed 1.5 lpm. During the dive, his BC fails, and he finds he has to kick hard to reach the surface, unexpectedly increasing his oxygen consumption to 3 lpm. What were his planned and his actual FiO2 during ascent? PLANNED ACTUAL FsO2 = 0.5 FsO2 = 0.5 Qs = 7 1pm VO2 = 1.5 1pm

Qs = 7 lpm VO2 = 3 lpm

Marvin will experience no problems with his setup while taking photographs at depth, as his SCR will deliver gas equivalent to EAN36 on OC scuba. Even if he increases his work rate at the bottom, he will be all right from a minimum oxygen standpoint, because his inhaled PO2 will be 0.24 atm—the FiO2 is doubled at his depth of 33 fsw (10 msw, or 2 ata). However, as he ascends the ambient pressure and the PO2 will drop. As he reaches the surface, the PO2 will approach the FiO2. The end result is that Marvin may lapse into hypoxia-induced unconsciousness. The key to preventing hypoxia in a mass flow controlled SCR is to calculate the minimum FiO2 before the dive. Always measure the actual flow rate, using either a calibration bag or a flow meter. Always analyze your own supply gas mix. And always plan on a metabolic oxygen consumption of three lpm for the worst case contingency.

Depth Limits

In mass flow controlled SCRs, two factors limit maximum operating depth. The first is an engineering issue, based on the regulator supplying gas to the mass flow controller. The second is a physiological parameter, based on the FO2 of the supply gas and potential problems with CNS oxygen toxicity. In order for the sonic valve to operate properly, the pressure on the input side (interstage pressure) must be at least 2.2 times the pressure of the output side of the valve (ambient pressure) (Equation 6.7). If the input pressure drops below this threshold, then gas flow through the valve is no longer at sonic velocity and insufficient fresh gas will be added to the breathing loop. This may result in hypoxia and death.

Where: Pi Po

Input pressure (interstage pressure, psi or bar) Output pressure (psi or bar)

Since the output pressure is essentially the same as the ambient pressure, this relationship controls the depth at which a unit of this type may be used. For this reason it is extremely important to measure the interstage pressure before using a mass flow controlled SCR. By combining Equations 4.1 (page 69) and 6.7 we arrive at Equation 6.8, which provides MODIP, maximum operating depth based on the input pressure:

Where: MODIP Pi

Maximum operating depth based on input pressure of SCR (fsw or msw) Input pressure (interstage pressure, psi or bar)

(Note: This simplified equation may provide results which vary by a small amount from exact calculations, due to rounding error.) As an example, the Dräger Dolphin® has a nominal input pressure of 240 psi (16.5 bar). Using Equation 6.7 above, we find that would support a maximum output pressure of 109 psi (7 Bar). With Equation 6.8, we find this would correlate to a maximum depth of 208 fsw (64 msw). This is an ample margin of safety for a unit that has a manufacturer specified maximum operating depth of 130 fsw (40 msw). What occurs if the regulator supplying gas to the mass flow controller is adjusted improperly, or has corrosion that reduces the interstage pressure? Let's suppose the input pressure is only 130 psi (9 Bar). In this case, we would find the maximum depth at which the

unit could function properly would be 98 fsw (30 msw). This malfunction would not be detected by verifying the flow rate at the surface, because the sonic valve would be operating correctly. Without checking the input pressure, we would not know the unit was malfunctioning until the symptoms of hypoxia set in. Some users of the Dräger Dolphin/Atlantis® have suggested using tri-mix in place of the nitrox in the gas supply cylinder. This would prevent the problems associated with high PO2 at depth as well as increased inert gas narcosis. However, the mass flow controller is designed for use with gas of a specific density. The densities of EAN32 to EAN80 are part of the design considerations when engineering the size of the sonic valve orifice. Because the density of trimix is significantly less than nitrox, the actual orifice flow rates would differ from design, and could lead to hyperoxia, or CNS oxygen toxicity. Supply gas provides the second depth limit. As we have seen, the FiO2 with a mass flow controlled unit can fluctuate widely, based on diver work level. At the high end, the FiO2 approaches the same level as the FO2 of the supply gas. This occurs when a quantity of fresh gas is added to the breathing loop–during an intentional loop flush, automatic addition of supply gas during a rapid descent, or fresh gas addition following a mask clearing. If you use the supply gas as one of your bailout options, then your FiO2 would be exactly the same as the FO2 of the supply gas. Because the FiO2 is nearly identical to the FO2 of the supply gas under some conditions, we must follow the conventions for maximum operating depth for the use of any nitrox mixture. Essentially, we use our analysis of the nitrox in the supply gas as the upper limit of what we may breathe at depth, and calculate our maximum operating depth (MOD) using Equation 4.6 (page 69). This is the same as in your nitrox certification course, except that you are using much richer blends. Thus, your MOD will likely be significantly more restrictive than that to which you may be accustomed. When discussing SCR limits, this is often referred to as MODO, or maximum operating depth based on oxygen limits.

Example 6.4 Marvin is using a mass flow controlled SCR using EAN32 as the supply gas. He measures his interstage pressure, as 150 psi (10 bar). What is his MOD, and what is the controlling factor? Maximum allowable PO2 = 1.4 atm FO2 = 0.32

Therefore, in this case MOD is determined by the FO2 of the gas to be 111 fsw (33.7 msw) This is one trade-off you make with the use of mass flow controlled SCRs. The benefit of the SCR is gas economy, but the limitation is depth. As an example, typical duration with the Dräger Dolphin® using EAN32 as the supply gas is 45 minutes, but you can dive to 111 fsw (34 msw), using a maximum PO2 of 1.4 atm. If you use the same unit with EAN60, typical duration is nearly two hours, but you are limited to a depth of 44 fsw (13 msw). Of course, you will also have less venting and therefore fewer bubbles with the EAN60, so this benefit is tied to depth as well. Some users will be able to dive to greater depth if they use an oxygen display system with oxygen sensors. Uwatec's inhalation hose mounted display and the NiTekX® computer with optional oxygen sensor are two examples. By using discipline in gauge

reading, they can avoid exceeding PO2 limits during most dive activities. However, some practices must be rigorously avoided, such as rapid descents, mask clearing at depth, and loop flushes at depth. Otherwise PO2 limits can be exceeded despite the use of oxygen sensors. This practice entails considerably more risk than the procedure discussed above.

Dive Tables

The use of dive tables to calculate inert gas loading is based on the FO2 of the gas in the breathing loop. Since FiO2 may vary widely in mass flow controlled SCRs, we use the most conservative figure. This is calculated at an oxygen metabolic rate of three lpm, the same as the minimum FiO2 was determined above. This figure may then be used as the basis to calculate an equivalent air depth (EAD), enabling the use of standard air tables. If you are not exercising at the maximum rate, as is usually the case, then this technique will add an additional safety margin to your exposure. Since there will be less nitrogen in the counterlung than you have calculated, your nitrogen uptake will also be less. Thus you will have a lower risk dive profile than actually computed. If you do not wish to use the EAD approach, another option is to follow standard air tables or an air dive computer. Each of these approaches requires that you first determine that your minimum FiO2 will equal or exceed 0.21 at a maximum work load of 3 1pm VO2. Oxygen exposures and maximum time limits are calculated using conservative values. We use the FO2 of the supply gas (FsO2) for these calculations. Then the NOAA and REPEX oxygen exposure limits presented in Chapter 5 can be utilized to arrive at acceptable exposures. Example 6.5 Using the information from Example 6.2 (page 134) and the EAD concept, what is Mary's repetitive letter group after a 70

minute dive to 51 fsw (15.3 msw)? What is her %CNS and OTU exposure? FsO2 = 0.54 D = 51 fsw (15.3 msw) Time = 70 min Minimum FiO2 = 0.22 Maximum FiO2 = 0.54

Thus, Mary would read the 50 fsw (15 msw) line from a set of air tables for this dive, finding her repetitive group upon surfacing to be "J" on the U.S. Navy tables.

From Table 5.2 (page 101) and Equation 5.1 (page 102):

From Table 5.4 (page 96): OTU = (1.63 OTU/min) x (70 min) = 114.1 OTU

Section Three RMV-KEYED SEMI-CLOSED REBREATHERS

Mechanics

The biggest drawback to mass flow controlled SCRs is the wide fluctuation in FiO2. This is primarily because oxygen addition to the breathing loop is the same regardless of diver work. RMV-keyed systems mitigate a large part of this variation by linking the addition of supply gas to work. This is possible because breathing rate is generally proportional to work effort. By using nested counterlungs or a proportional valve, every time a diver inhales, some of the gas from the loop is expelled. This results in a more constant FiO2. With this type of system, maximum variations of two to eight percent are normal, compared to 50% variation possible in mass flow controlled SCRs. In most RMV-keyed systems, the counterlungs govern the amount of gas replaced. The larger the size of the discharge counterlung compared to the total bellows counterlung, the more gas exhausted. This ratio is known as the volume ratio (R1). It is fixed by the design of the SCR, since once constructed the size of the two counterlungs does not change. Because the ratio is fixed, these systems are also known as constant volume ratio SCRs. If the volume ratio is large, then the SCR will deliver a more stable FiO2. However, as more gas is exhausted by every breath, gas economy will drop and rebreather efficiency is reduced. Variations in FiO2 are greater at shallow depths, due to fewer oxygen molecules in the breathing loop. Another way of stating this is that less gas needs to be exchanged at deeper depths to maintain equivalent FiO2 stability. Constant volume ratio SCRs become less efficient at depth because they exchange more gas than needed. Some RMV-keyed SCRs circumvent this via a depth compensation mechanism. In this design, the size of the discharge counterlung is reduced as depth increases, resulting in increased gas economy. The Halcyon II, for example, accomplishes this via two

mechanical depth sensors. These sensors contract proportionately with increasing depth due to the increase in ambient pressure. This contraction pulls a wire that reduces the internal volume of the discharge counterlung. This type of RMV-keyed system is known as a variable volume ratio SCR. Both of these types of RMV-keyed SCRs are passive systems. This means that no gas addition occurs unless powered by the breathing of the diver. This is different from mass flow controlled SCRs, in which gas is always being added. Mass flow controlled SCRs are active systems, since the rebreather adds gas automatically. A hybrid system utilizes both a mass flow controller, and the RMV-keyed concept. This results in better gas economy in what is called a constant mass ratio SCR. Gas injection in the constant mass ratio SCR takes place via a dosage chamber. In turn, this chamber is controlled by movement of the counterlung as the diver breathes. Since counterlung movement controls both inlet and outlet valves of the dosage chamber, and gas is provided at a fixed pressure above ambient, a constant mass of gas is added at intervals in amounts keyed to breathing rate proportional to the work effort. While this SCR has attributes of both active and passive designs, it is considered an active system. Generally, active systems are more prone to problems not immediately apparent to the diver. A reduction in gas flow cannot be detected unless measured with a calibration bag or flow meter. This must be done prior to the dive, and cannot predict problems occurring during the dive. Thus, the diver may become unconscious with no warning! Passive systems, on the other hand, usually provide a clearer indication of trouble during the dive before the onset of unconsciousness. Like OC systems, when passive SCRs cease working, they usually stop delivering breathing gas. This is a sure sign of trouble! But at least it happens before the breathing gas in the loop becomes sufficiently hypoxic to cause unconsciousness.

Figur 6.4: RMV-keyed SCRs improve gas stability by tying gas injection to work effort. Coutesy of David Rhea

Two notable exceptions to this generalization involve the discharge counterlung. If the gas line leading to the discharge counterlung becomes blocked, or if the bag or membrane separating the outer and discharge counterlungs develops a hole, then the discharge counterlung ceases proper operation. No gas is removed from the system. As a consequence, after a few breaths no gas will be added to the system either, since gas injection depends on the loss of volume from the breathing loop via the discharge counterlung. In essence, the passive SCR has become a nonfunctioning closed-circuit system. The end result of either failure, if not noticed quickly, is hypoxiainduced unconsciousness. If this type of failure occurs, the only ways to discern it are to listen for gas addition into the SCR, listen for exhaust bubbles, or to recognize a change in breathing effort. Gas injection and exhaust bubbles should come at regular intervals, with the frequency varying with depth and work effort. Breathing effort actually becomes easier if the discharge counterlung is

compromised. A diver using a passive SCR must develop an awareness of gas injection, as well as a feel for "normal" operation. These are very subtle signs. A lapse in awareness of only a few minutes can result in unconsciousness. Task loading, concentration on an objective, or other forms of stress contribute to the problem. As with mass flow controlled SCR's, the shallower you are, the less time you have to identify and deal with the problem. A significant portion of your training dives on this type of SCR should deal with building a subconscious monitoring of gas injection, exhaust bubbles, and breathing effort under varying environmental and work conditions. Fortunately, with the RMV-keyed SCR models currently available in the recreational market, the likelihood of these problems is low. Constant volume ratio SCRs are mechanically more complex than mass flow controlled systems. Mechanical complexity further increases in variable volume ratio SCRs, and even more in constant mass ratio SCRs. This is the trade-off necessary to achieve a relatively constant FiO2 in SCRs. As a result, pre-dive and post-dive activities will take longer and servicing requirements increase.

Oxygen Addition

Just as with the mass flow controlled SCR, oxygen addition in the different RMV-keyed systems can be expressed mathematically. Each different type of RMV-keyed SCR has its own steady state equation. Derivation of each of these is outside the scope of this text, but the interested reader may find this information in the hyperbaric physiology literature.8,9 Let's look at the constant volume ratio steady state equation (Equation 6.9) as an example:

Where: K R1

Ventilatory equivalent for oxygen, = VE / VO2 Volume ratio, = VD / VE

Data FsO2

Depth (ata) Fraction of oxygen in the supply gas

VO2 Note: •

Metabolic consumption of oxygen (lpm) is a symbol calling for multiplication; i.e., 2 • 3 = 6 is identical to 2 x 3 = 6

PiO2 VD VE

Partial pressure of oxygen in the inspired gas (atm) Gas vented from the discharge counterlung (lpm) Gas exhaled, or respiratory minute volume (lpm)

The fraction of oxygen in the supply gas (FsO2) is self-evident, and can be measured with an oxygen analyzer before the dive. The ventilatory equivalent for oxygen (K) is a constant that represents the fact that the harder one works (increasing metabolic consumption of oxygen, VO2), the faster one breathes (respiratory minute volume, VE). This "constant" varies among divers. It is usually a measure of personal fitness, since fit people have a lower respiratory rate for equivalent amounts of work than unfit people. Typical values for this constant are 14 to 28, with the lower figure representing a physically fit individual. The volume ratio (R1) was described above, and is constant in this system. The depth in atmospheres (Data) can be determined using Equation 4.1 (page 69). Example 6.6 Marvin is using a constant volume ratio RMV-keyed SCR on a dive to 33 fsw (10 msw). The volume ratio is 0.7, and he is using EAN50 as the supply gas. He is not in very good shape (ventilatory equivalent for oxygen of 25), but will only be taking close-up pictures, so figures his oxygen consumption will not exceed 1.5 lpm. During the dive, his BC fails, and he finds he has to kick hard to reach the surface, unexpectedly increasing his oxygen consumption to 3 lpm. What were his planned and actual FiO2 during ascent? PLANNED ACTUAL D = 33 fsw (10 D = 0 fsw (0 msw)

msw) FsO2 = 0.5 R1 = 0.7 lpm

K = 25 VO2 = 1.5 lpm

FsO2 = 0.5 R1 = 0.7 lpm

K = 25 VO2 = 3 lpm

To get depth in atmospheres (Data), use Equation 4.1 or 4.1a (page 69).

To get FiO2, use Equation 4.5 (page 72), slightly rearranged:

Figure 6.5: Constant volume ratio RMV-keyed FiO2 for different diving conditions.

We can see that under these circumstances, Marvin's FiO2 dropped by only 0.015, less than two percent. You may also have noticed that this difference was not due to Marvin's working harder, but rather was due to the decrease in depth. Even though Marvin was metabolizing twice the amount of oxygen than he had intended, he was still in the same physical condition at the end of the dive as he was beginning it. His ventilatory equivalent for oxygen (K) did not change. The only result of his increased exertion was that he consumed more gas at the end of the dive. This example may look familiar. It is the same as Example 6.3 (page 135), except that in that instance Marvin was using a constant mass flow SCR. If we look back at those results, Marvin's FiO2 dropped from 0.36 to 0.12. Switching rebreather models reduced the

variation in oxygen fraction from 24% to less than two percent. It also prevented him from breathing a hypoxic mix, which in real life might have resulted in his drowning. Examining Equation 6.9 (page 143), we find that four variables affect the inhaled oxygen fraction: supply gas (FsO2), the diver's physical condition (K), the volume ratio (R1), and depth (Data). How much will each impact the final FiO2? Figure 6.5 above depicts the variation in oxygen levels by varying each of these factors. This graph was developed using Equation 6.9 with a series of different constraints. FiO2 can be read on the Y-axis (vertical axis). The values for each of the variables are reasonable extremes for recreational diving on this type of rebreather: (1) Supply gas (FsO2) of EAN50 (rich) and EAN32 (lean) (2) Diver's physical condition (K) of 14 (fit) and 28 (unfit) (3) Volume ratio (R1) of 0.7 (large) and 0.2 (small) (4) Depth of 0 to 130 fsw (0 to 40 msw or 1 to 5 ata) The first result is immediately apparent. Oxygen variation is greatest at shallower depths. The effect of any of the variables becomes less as depth increases. All of the curves demonstrate this fact. As depth increases, FiO2 becomes nearly the same as FsO2. The difference between inspired and supply fractions at depth in most conditions is only one to five percent. This type of rebreather, like the others we have examined, is more hazardous to use in shallow water than in deep. To see how any other single variable makes a difference, select two curves where the other factors are identical. For example, if we wish to look at volume ratio impact, we could look at the top curve (unfit diver, large ratio, rich mix) and the third curve from the top (unfit diver, small ratio, rich mix). We see that at all depths, the small ratio has a greater variation from the supply gas, delivering a lower FiO2. The variation range across depths is also greater for the smaller volume ratio (i.e. maximum range of 2% (47-49%) for the large volume ratio vs. 7% (41-48%) for the small volume ratio).

Rich supply gases (the four upper curves) demonstrate less variability overall than lean supply gases (the lower two curves). Using lean supply gases, especially with a unit having a small volume ratio, risks providing a hypoxic mix, as may be seen in the bottom curve (oxygen content of eight percent near the surface). It is imperative to use a supply gas appropriate for the volume ratio of your SCR. The most counter-intuitive results involve diver fitness. If we look at the second and fourth curves from the top (unfit/fit divers using EAN50 and a large volume ratio), we see that the fit diver shows much more variability and a greater overall potential drop in FiO2 (14% vs. 7% maximum). This is because a fit person metabolizes more of the oxygen in every breath they inhale, especially when they are exercising hard. The ventilatory equivalent for oxygen constant (K) compensates for this. So is it better to be in poor physical condition if using this type of unit?

Figure 6.6: Halcyon RB80® RMV-keyed SCR.

No! Remember that because more oxygen from every inhalation is metabolized, the fit diver will not have to breathe as often an unfit diver. The fit diver's gas consumption rate will be much lower, and therefore their supply cylinders will last them much longer.

Depth Oxygen Variation (fsw) (msw) (%) 0 0 7 10 3 6 20 6 5 30 9 4 40 12 4 50 15 3 60 18 3 70 21 3 ≥80 ≥ 24 2 Go to exact or shallower depth, read oxygen variation to right, and subtract that figure from the FO2 of your supply gas to get FiO2 Table 6.1: Fraction of Inspired Oxygen using a Halcyon RB80®17

The time any given volume of supply gas will last depends on three factors. As just discussed, the diver's physical fitness is one. The other two are depth and work effort. As with OC equipment, the deeper you dive and the harder you work, the faster your gas is used. This is different from a constant mass flow SCR, that will operate for the same duration regardless of these variables. We know how to determine gas supply duration for these two types of equipment, but how do we do so with a RMV-keyed SCR? Common RMV-keyed rebreathers currently available in the recreational market are the RB80® and Halcyon II®, manufactured by Halcyon. Halcyon provides a chart for determining gas usage, and one to determine the FiO2 of the breathing mix at different depths. Because the variable volume ratio design of this unit

compensates for some of the factors discussed above, their tables are simpler than those for a constant volume ratio would be. Table 6.1 provides FiO2 information. To use it, look in the left column for the depth to which you are diving, then to the right to find the variation from the supply gas. Subtract that percentage from the percent oxygen in your supply cylinder blend to get the FiO2. Be aware that this information is provided by the manufacturer. Recent tests by the Swedish Navy18 indicate that these variations may be much greater. As an example, tests using a Halcyon RB80® at a depth of 100 fsw/30 msw have indicated a possible drop in FiO2 of nearly 10%, compared to the 2% stated by the manufacturer. To determine how long your gas supply will last use Figure 6.7. The curves represent different SAC rates (RMVs) in cubic feet or liters per minute. Find the curve that is closest to your personal consumption and follow it down to the horizontal line indicating the depth of your dive. From where the two intersect, go straight down to read the duration in minutes of an 80 cf (11 L @ 207 Bar) supply cylinder.

Figure 6.7: Supply Gas Duration Chart for use with a Halcyon II SCR10.

Example 6.7 Mary is diving a Halcyon II® rebreather to 51 fsw (15.3 msw), using EAN54 as her supply gas. Her SAC rate is 0.8 cfm (22.5 lpm). Calculate her inspired fraction of oxygen and the expected duration of her 80 cf (11 L @ 207 Bar) cylinder. D = 51 fsw (15.3 FsO2 = 0.54 msw) SAC = 0.8 cfm (22.5 lpm)

Depth Limits

Depth limits with RMV-keyed systems are based solely on the supply gas. On all of these systems it is possible to breathe 100 percent supply gas, for example after a loop flush or rapid descent. In fact, some systems are designed to inject the supply gas into the breathing loop just as the gas flows into the inhalation hose, to help prevent hypoxia. This keeps the FiO2 even closer to the FsO2. A maximum operating depth determination is necessary prior to diving to avoid problems with acute oxygen toxicity. To find it, we use the same MOD equation we learned in the nitrox class (Equation 4.6, page 75). Base the MOD on the fraction of oxygen in the supply gas, not the FiO2 calculated from Table 6.1, page 147. Follow, of course, all manufacturer's recommendations on the maximum depth for their units. Example 6.8 Mary is diving a Halcyon II® rebreather to 60 fsw (18 msw), using EAN40 as her supply gas. What is her maximum operating depth?

FsO2 = 0.40

Maximum allowable PO2 = 1.4 atm

Dive Tables

RMV-Keyed SCRs are capable of maintaining greater FiO2 stability than mass flow controlled SCRs, so the calculated FiO2 may be used when determining nitrogen loading in the body. Use this FiO2 and the equivalent air depth equation (Equation 4.7, page 77) with a set of air diving tables to determine inert gas status. If you use a nitrox computer in your diving, program the calculated FiO2 prior to the dive. The resulting nitrogen loading information will be correct, although the oxygen clock information may allow you slightly more time than based on FsO2. As discussed earlier, the FiO2 in the breathing loop may be as high as the FsO2. Therefore, to be conservative, we use the FsO2 to determine oxygen exposure status. The tools presented in Chapter 4 may be used here as well. Example 6.9 Using the information from Example 6.7 (page 148) and the EAD concept, what is Mary's repetitive letter group after a 70 minute dive? What is her %CNS and OTU exposure? FsO2 = 0.54 D = 51 fsw (15.3 msw) FiO2 = 0.51 Time = 70 min

Thus, using U.S. Navy air tables, Mary would read the 20 fsw (6 msw) line (or the shallowest line on the table if shallow depth data is not included) from a set of air tables for this dive, finding her repetitive group upon surfacing to be "D." To determine %CNS and OTUs, we first use Equation 4.5 (page 74) to find the maximum inspired PO2 based on the FsO2:

From Table 5.2 (page 101) and Equation 5.1 (page 102):

From Table 5.4 (page 96): OTU = (1.63 OTU/min) x (70 min) = 114.1 OTU

Figure 6.8: Constant volume ratio RMV-keyed FiO2 for different diving conditions.

Figure 6.9: Mixed-gas closed-circuit rebreathers offer long duration, opimal gas mixes, and no bubbles. Courtesey of Aleph Alighieri

Looking at the results in Example 6.9, you will note that using the U.S. Navy air diving tables results in a group letter of "D." Some agencies use air tables that are more conservative than the U.S. Navy version. This was done specifically to provide a safety margin for recreational divers. However, there may be times when use of the U.S. Navy air diving tables may be preferable. Bühlmann tables provide another option. These tables are provided in an easy to use format in Appendix B. Your instructor will discuss which tables you will use in your training. Again, this example parallels that of Example 6.5 (page 139), using the mass flow controlled SCR. As can be seen, the RMVkeyed SCR diver gets a significant benefit in nitrogen loading (post dive "D" group versus "J"). However, oxygen clock information for the two units is identical, as in each case we must utilize the FsO2 to determine it. This is one of the benefits of using mixed gas CCRs.

Section Four - Mix-Gas Closed-Circuit Rebreathers

Mechanics

Mixed-gas closed-circuit rebreathers are in many ways the most versatile available. They offer the user wide depth range, long duration, silent operation, optimal inert gas loading, and high gas efficiency. However, having the most complex design, they have the greatest risk of failure due to user error and mechanical or electrical problems. There are three main subsystems in mixed gas CCRs: (1) breathing loop, consisting of mouthpiece, hoses, scrubber, counterlung(s), and water traps; (2) compressed gases, regulators, gas addition valves, gas accumulator, and associated pipe work, and (3) electronics system, comprised of oxygen sensors, sensor displays, electronics package, batteries, cabling, and computers. Not every mixed gas CCR will have all these subsystem components. The breathing loop is generally identical to other rebreathers, and is discussed elsewhere in the text. The other subsystems will be discussed further here.

Mixed gas CCRs have two gas cylinders, one of pure oxygen and the other of a diluent, usually air. As the diver descends, volume in the counterlung decreases due to increasing ambient pressure. At a minimum volume, the collapsed counterlung triggers a Schraeder or similar demand valve that injects diluent into the counterlung to replace lost volume. Because pure oxygen is used, parts of the rebreather must be maintained in an oxygen clean condition. In some systems, both the diluent and the oxygen share the pipe work carrying gas from the regulators to the counterlungs. These designs should use hydrocarbon clean air as the diluent to avoid contaminating the piping.

Figure 6.10: Oxygen sensor electronics and displays are a critical part of CCRs. Courtesey of Shearwater Research

Mixed gas CCRs are very efficient, conserving gas well. As the volumes of gas injected into the breathing loop are generally quite small, compared to OC breathing, some manufacturers use unbalanced, low performance regulators. While these work perfectly well in the rebreather, they are unsuitable for OC use. Thus for bailout purposes, it is a poor idea to attach an OC second-stage to a

CCR regulator that was not designed to provide gas in large volumes. Mixed gas CCRs are designed to maintain a stable minimum PO2 level. This is called the set point. In most mixed gas CCRs, the set point is user selectable, before, or in some units, during the dive. If the PO2 in the breathing loop falls below that set point, then oxygen is automatically added. However, in most mixed gas CCRs if the PO2 in the breathing loop is above the set point, then the unit does nothing to alter the gas content. This is highly significant! Even if the PO2 becomes extremely high, as might occur if the oxygen injection system were to stick open or the diver descend very rapidly, the unit will do nothing to bring the PO2 to a safer level! This can lead to CNS oxygen toxicity if the user does not properly monitor the PO2 level. Most mixed gas CCRs have multiple oxygen sensors that check against one another to help ensure that PO2 in the loop is as reported. Usually, three or more sensors are incorporated. These sensors must compare their values in some manner. The two most common modes of comparison are voting protocol and averaging protocol. Another logic system used with multiple oxygen sensors is the elimination method. These operational modes are discussed in the next section on oxygen addition. To work, the sensors must be in the breathing loop. This is a harsh environment for electrical components. Condensation and saliva are normally there, and fresh or seawater may enter while diving. Any of these liquids may contact the absorbent and become extremely caustic. All can corrode or short out sensor contacts or other wiring, impairing function. Thus safe mixed gas CCR operation depends on the oxygen sensors, and any problem that impacts the sensor readings may lead to hypoxia or oxygen toxicity. Because of this, many systems have redundant mechanisms to read PO2 and control oxygen levels. In the automatic mode, the electronics control gas addition. In manual mode, the user reads the oxygen sensors and controls gas addition. In all systems having redundancy, the design goal is to

have no single component that could adversely impact operation simultaneously in both manual and automatic modes in the event of failure. Another concern in these units is reliance on batteries. All mixed gas CCRs use batteries to control oxygen addition in automatic mode. Some also use batteries to display the oxygen sensor readings in manual mode. If these batteries fail, then some units simply will not operate. Not surprisingly, therefore, battery placement is an important aspect of unit design. Batteries that are independent of the breathing loop and sealed off from other compartments that may flood have greater reliability. Because of their complexity, multiple potential sources of failure and widely differing designs, specific training in the use of mixed gas CCRs by a qualified instructor is imperative. Instruction should include design parameters, failure modes, remedies, emergency protocols and procedures, as well as diving the unit under normal conditions.

Oxygen Addition

Diluent addition into the breathing loop will alter the PO2 of the circulating gas. Oxygen sensors continually measure the PO2. When it drops below the set point, oxygen is added either via an electric valve in eCCRs or manually in mCCRs. Several types of valve mechanisms are used in eCCRs.

Figure 6.10: Solenoid cross-section: The solenoid is an electromagnetic valve that opens when a current is applied. In this mode, it is used to add oxygen to the breathing loop.

One type uses a solenoid. This electromagnetic device opens the valve against spring pressure when an electric current is applied. (Figure 6.10) The unit's electronics control the application of current. The valve opens for a fixed period of time, usually about one second, and then closes. It remains closed for a longer period, typically 4-5 seconds, and then reopens if the PO2 is still below set point. The cycle continues until sufficient oxygen is added to bring the breathing loop gas composition above the set point. The U.S. Navy Mark 15 series rebreathers use this type of mechanism. A design refinement in other rebreathers keeps the valve open for a longer period of time, allowing more oxygen to be added with less power drain.

Figure 6.11: The accumulator in the SM1600® adds 50cc of oxygen to the breathing loop when the solenoid opens, making oxygen addition more efficient.

Generally, if a solenoid fails, it normally leaves its valve in the closed position. However some failures will result in the solenoid valve sticking open–for example, if it has been flooded and has significant internal corrosion, or a broken spring. This is a very serious potential problem, as excess oxygen injection may lead to CNS toxicity. To mitigate this potentiality, the piping providing oxygen to the breathing loop is often of small diameter, restricting gas flow from the regulator. This keeps the oxygen addition rate low even if the solenoid valve fails in the open position. Since such restriction effectively limits gas flow when it is desired as well, an accumulator is built into the system to increase oxygen injection when the solenoid operates. (Figure 6.11) The accumulator provides a fixed volume of oxygen, usually about 50cc, during the injection cycle. It takes 4-5 seconds to completely load, so it fills between solenoid operation cycles to increase oxygen addition efficiency. The second type of control device is the piezo-electric valve. This also remains open as long as current is applied. The duration is again governed by the main electronics. Once the oxygen sensors

indicate that the PO2 is at or above set point, the valve is turned off, ending oxygen addition. The U.S. Navy Mark 16 uses this type of mechanism. A third design uses a step controller valve, also called a sequential controller. This is a stepper motor that controls a needle valve. It works on the premise that the diver will maintain a relatively stable metabolic oxygen consumption (VO2) rate for several minutes at a time. The electronics indirectly measure the VO2, and open the needle valve via the stepper motor just enough to add sufficient oxygen to maintain the PO2 at a stable level. The step motor then turns off, conserving power. When the VO2 changes, the motor is turned back on, moving the needle valve to a new position before being turned off again.

Figure 6.12: Gas control tests measure how well oxygen partial pressures are maintained. Note that CCR1 maintains a more stable PO2 than does CCR2 at equivalent workload and depth.

If a constant depth is maintained, then the counterlung volume will not be affected by depth changes–volume will vary only as the diver's oxygen metabolization varies. Generally, such volume loss is insufficient to trigger the diluent addition valve. Before that occurs,

the PO2 level falls below the set point, triggering an oxygen injection. Optimally, the variation from set point before the injection mechanism operates is small. However, the type of sensors used, electronics design, depth changes, work levels, moisture, sensor location, breathing loop volume, gas injection location, oxygen injection mechanism, and temperature will all affect oxygen injection. This is why gas control testing is important with these CCRs. (Figure 6.12, page 156.) Some variation from set point is normal in all mixed gas CCRs. The less variation, the better the unit design. In steady state conditions, PO2 will slowly drop until the electronics signal an oxygen addition, causing the PO2 to elevate. Then the cycle repeats, with PO2 dropping until the next oxygen addition is triggered. The greater the VO2, the more frequent the oxygen addition cycle (Figure 6.13, page 155). In most units, the electric valve that controls oxygen addition is typically the single largest power draw. If the electronics are used to control oxygen addition in automatic mode, the battery powering the electric valve will be depleted relatively rapidly. The heavier the workload, the sooner the battery will be exhausted. If the user controls oxygen addition manually, the battery will last much longer, sometimes by as much as four to five times. For this reason, some instructors will have students "fly" their CCRs manually slightly above the set point, using the electronics as a "safety net" should the diver forget to watch the oxygen level. Besides extending battery life, this also builds the muscle memory necessary for the user to run the unit in manual mode should the electronics fail.

Figure 6.13: The diver's work level will dictate the frequency of oxygen addition.

The way in which the electronics will control PO2 in the event of one or more sensor failures depends on the electronics control protocol used. With electronics using an averaging protocol, the values from the sensors are totaled and then divided by the number of sensors to arrive at a final reading. If all sensors are functioning properly, then slight variations are incorporated in the total. This yields a figure that is representative of all the sensors, and the electronics add oxygen to the breathing loop based on this composite figure. If a voting protocol is used, then the readings from each of the sensors are taken and examined individually. If any single reading is significantly different from the others, then that value is ignored, and oxygen addition is controlled based on the values from the remaining sensors. Again, if all three sensors provide essentially equivalent readings, the unit is governed by all of the sensors. Table 6.2: Actual breathing loop PO2 with oxygen sensor failures, set point = 1.2 atm.

Sensors Failed Averaging Protocol Voting Elimination 0 1 2

1.2 1.8 3.6

1.2 1.2 Data

1.2 1.2 Data

The elimination protocol takes the values from all sensors, and then ignores the readings from the highest and lowest sensors. It bases oxygen addition on the remaining intermediate sensor data. Differences arise when more sensors fail. Table 6.2 shows what would occur in a CCR using a typical array of three sensors. In any example, a set point of 1.2 atm is used. The breathing loop PO2 is provided in the table. In any system, if all sensors are functioning properly, the PO2 in the breathing loop is identical to the set point, subject to normal variation. Let's postulate problems up to a worst case scenario of complete sensor failure in order to examine what will occur. If one sensor fails in an averaging system, the breathing loop PO2 will become greater than set point. The electronics will increase the response of the two operational sensors to replace the lost input from the third sensor, yielding a gain equal to that which would have been provided had all three sensors been working normally. Basically, both operational sensors will cause extra oxygen to be injected. This could lead to oxygen toxicity problems, depending on the initial set point. The CCR using a voting protocol will maintain proper PO2, as the reading from the two operational sensors will override the malfunctioning one. An eCCR using an elimination protocol model will also maintain proper PO2. If two sensors fail, an averaging system will provide excessive amounts of oxygen. The electronics will attempt to boost the PO2 in the breathing loop so that the figure from the remaining sensor compensates for the lack of input from the two failed sensors. This will, if not observed and compensated for by the diver, rapidly lead to oxygen toxicity problems. A CCR using voting electronics will discard the anomalous reading. Since in this case two sensors are reading 0

atm, and one reading 1.2 atm, the latter figure is ignored. Similarly, the elimination protocol relying on three sensors will also dismiss the valid reading of 1.2 atm. These systems will add oxygen continuously. Theoretically, the system PO2 would equal the depth in atmospheres absolute. Obviously, at most dive depths this will rapidly lead to oxygen toxicity if not noticed and corrected. If all three sensors should fail, the end result is variable. With some electronics packages, no oxygen will be added. In others, oxygen will be added as rapidly as the system can. Obviously, either situation will lead to problems if not corrected. Some CCRs will turn themselves off and do nothing. Since the diver has no means of determining PO2 in the breathing loop, the only viable courses of action are to either use the diluent in the system in SCR mode, or to switch to the bailout cylinder and ascend to the surface. These procedures are described in Chapter 12. Not all eCCRs rely on multiple sensor inputs. One control design uses a single primary sensor, with a sensor validation protocol. To validate proper sensor function, the electronics regularly add a small amount of diluent, filling the sensor cavity. The output reading is then checked to see that it matches the expected value (based on the known composition of the diluent and the depth of the diver). If a significant discrepancy is found, an alarm alerts the user to change to OC bailout as an alternate mode of operation. Another potential hazard with oxygen addition arises when a gas other than oxygen is used in the oxygen cylinder. This stems from two factors. Firstly, most mixed gas CCR pre-dive procedures call for calibration of the oxygen sensors. In some cases the user does this manually; in others the electronics do it automatically. In either case, the gas used for calibration is typically that in the oxygen cylinder. If that gas is not 100% oxygen, then the calibration will be incorrect. The oxygen values provided by the sensors will be interpreted as being higher than the actual oxygen content of the gas in the loop. This could lead to hypoxia, or problems with decompression sickness. The second factor to consider is that when "oxygen" addition does occur, not all of the gas will be oxygen. Additional nitrogen (or

whatever the inert gas component in the oxygen cylinder) will be added to the breathing loop with every gas addition from the cylinder. That volume of gas must escape somewhere, as the maximum breathing loop volume is fixed. Typically, excess volume is vented through the counterlung overpressure relief valve. In effect, the unit will be operating as an electronically controlled semi-closed rebreather. The above discussion applies to eCCRs. mCCRs control oxygen addition differently. In the basic mCCR design, the user must add all oxygen manually based on observed PO2 levels in the breathing loop. An improved design uses a mass flow controller or an adjustable needle valve to inject a constant amount of oxygen into the loop automatically, usually amounting to 0.5 to 0.8 lpm. This continuous injection reduces the possibility of hypoxia. If exertion level is increased, then the user must manually add additional oxygen to maintain set point. In any mCCR, frequent sensor display monitoring is critical to avoid hypoxia, as the rebreather will not add oxygen sufficient to maintain consciousness in all operating conditions.

Depth Limits

Depth in mixed gas CCRs is limited by the narcotic effect of the inert gas component. This is called MODN, or the Maximum Operating Depth based on narcosis. Since air is most commonly used as the diluent in recreational mixed gas CCR use, we become concerned when the partial pressure of nitrogen (PN2) exceeds that which is considered reasonable for recreational diving. This limit is 4 atm PN2 (132 fsw/40 msw equivalent air depth). The maximum operating depth based on this limit using air as the diluent may be found by using Equation 6.10:

Equation 6.10 can be further simplified to:

Where: MODN SP

Maximum operating depth based on narcosis (fsw or msw) Set point or partial pressure of oxygen (atm)

Example 6.10 What is Marvin's maximum operating depth based on narcosis if he is using a Titan® mixed gas eCCR with an air diluent and a set point of 1.3 atm? SP = 1.3 atm

Remember that one major advantage of some mixed gas CCRs is that gases other than air may be used as diluent. If a helium/oxygen mixture is used as the inert gas, then the narcosis depth limit is essentially unlimited, because helium has negligible narcotic effect. However, factors such as the work of breathing on the unit at depth, high pressure nervous syndrome (HPNS), bulk loading effects, and absorbent effectiveness become limiting factors. These considerations are beyond the scope of entry-level training. If you are interested in learning more about these limits, then you should consider taking an advanced CCR class. A final consideration regarding maximum depth involves descent rate. With constant PO2 rebreathers, oxygen toxicity should not normally be a problem. This assumes a proper rate of descent during which the electronics and gas addition valves in your rebreather can compensate for depth changes, keeping the PO2 within safe bounds. If you descend substantially faster, it is possible for your PO2 to climb above reasonable limits, due to the rapid increase in ambient pressure. As this may increase the potential for oxygen toxicity, you are cautioned to watch your descent rates, and

monitor your oxygen sensor displays especially closely whenever you change depths. Depth limits with mixed gas CCRs may also be constrained by oxygen limits. This should not be surprising, as we saw in Chapter 4 that all OC nitrox mixes constrain depth to prevent CNS oxygen toxicity. Unlike OC nitrox use, when properly operating a mixed gas CCR you should never exceed a safe PO2, as your set point should be established below that level. With mixed gas CCRs, we are concerned with minimum oxygen levels. One option we have for dealing with emergencies under water on OC scuba is an emergency swimming ascent (ESA). Using CCRs, we maintain that option only if we are diving at 130 fsw/40 msw or shallower following no decompression stage profiles. These constraints limit the extent of your training and competence after initial open water CCR training. The difference between OC and CCRs is that with OC scuba, FO2 is fixed with a minimum level of 0.21. This may not be true with CCRs. In CCRs, FO2 varies with depth. At low set points at depth, you may be breathing a hypoxic mix. While the gas mix may be perfectly capable of sustaining consciousness at depth, an ascent on that gas may result in you becoming unconscious: We use Equation. 6.11 to determine MODO, the maximum operating depth based on oxygen level:

Where: MODO SP

Maximum operating depth based on oxygen level (fsw/msw) Set point, the partial pressure of oxygen being maintained in the CCR (atm)

Example 6.11 If Marvin is diving an Evolution® eCCR and forgets to change set point from his low set point of 0.7 atm to his high

set point of 1.3 atm, what is his maximum operating depth based on oxygen levels? SP = 0.7 atm

Table 6.3: Maximum operating depths based on maximum nitrogen partial pressures and minimum oxygen fraction. Setpoint MODN (Narcosis) MODO (Oxygen) PN2 - 4 atm FO2 - 0.21 atm (atm) 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

(fsw) 122 125 129 132 135 139 142 145 149 152

(msw) 37 38 38 40 41 42 43 44 45 46

(fsw) 77 93 93 124 140 156 171 187 203 218

(msw) 23 28 28 38 43 47 52 57 62 67

Figure 6.14: Graphical representation of maximum operating depth based on nitrogen narcosis limit (PN2 = 4 atm) and minimum FO2 (0.21) for CCRs at various set points.

As we can see, when using mixed gas CCRs, we have two MODs to determine. Both may be calculated by using the set point in use. MODs based on a range of set points are provided in Table 6.3 and displayed graphically in Figure 6.14: Generally, divers plan their dives to a maximum depth, not a predetermined set point. For that reason, it is better for dive planning purposes to select a set point based on our intended maximum depth. If we rearrange Equation. 6.11, and combine it with Equation. 4.1 by substituting maximum depth for MODO, we derive Equation. 6.12. This allows us to determine a minimum set point:

Where: minSP D

Example 6.12

minimum set point (atm) maximum dive depth (fsw or msw)

Mary is using a KISS® mCCR to visit a wreck at 92 fsw (28 msw). What is the minimum set point she should use for this dive?

Dive Tables

When air is used as the diluent, a diver using a mixed gas CCR is essentially breathing some form of nitrox. As with OC enriched-air nitrox (EANx) use, normal air dive tables may be used to provide a physiological advantage to the diver. The difference between OC and CCR nitrox use is that the mix is continuously changing in the latter. As the diver is not breathing a fixed FO2 mixture, at some depth the use of air diving tables becomes unsafe. The maximum depth for air table use is based on the PN2 (partial pressure of nitrogen) that would be found with a diver breathing OC air at the same depth. The PN2 in a CCR breathing loop may be found using Equation 6.13:

Where: PN2 Data SP

Partial pressure of nitrogen (atm) Depth in atmospheres, or pressure at depth (ata) Set point, or partial pressure of oxygen (atm)

If we substitute Equation 4.1 (page 69) for depth in atmospheres (Data) and rearrange, we arrive at Equation 6.14. This may be used to determine the maximum depth for using air diving tables at any set point. As we can see, this is identical to Equation 6.11, page 160. MODO defines both our ability to conduct an ESA, and to use air diving tables without modifications. Likewise, Equation. 6.12 (page

161) can be used to determine a minimum set point that allows a CCR diver to use air dive tables.

Where: MODO SP

Maximum depth at which air tables may be followed (fsw or msw) Set point, or partial pressure of oxygen (atm)

Example 6.13 What is the deepest depth at which Mary may use air diving tables without an EAD conversion if she is using a Megalodon® eCCR with a set point of 1.3 atm? SP = 1.3 atm

Figure 6.15: Duplicate dive profiles comparing nitrogen and oxygen partial pressures for OC air and CCR scuba (set point =

1.4 atm) at the same depths. Note that inert gas loading does not begin for the CCR diver until a depth of 39 fsw/12 msw is reached.

One advantage of using OC nitrox is that bottom times may be extended with no increase in DCS risk. This is done using the equivalent air depth (EAD) concept, or tables generated based on EAD conversions. The same advantage is true with constant PO2 CCRs, only to a greater degree. When a diver is breathing a constant FO2 mix, some nitrogen loading always occurs (except at very shallow depths). This is not true with a constant PO2 breathing gas. At shallower depths, the diver may not be breathing any nitrogen at all. Then, at moderate depths, the PN2 may be much less than if air or fixed FO2 EANx were used. Figure 6.15 depicts dives to 66 fsw (20 msw), using both OC air and a constant PO2 CCR with a set point of 1.4 atm and air diluent. The figures show depth (fsw/msw), and the partial pressures of nitrogen and oxygen during the dive. As is expected, when OC air is used, the PN2 and PO2 increase at a constant and linear rate during the descent, remain constant while at the bottom, and decrease at the same rate as the diver surfaces. At all times while under water, the OC diver is breathing an increased PN2 and is ongassing nitrogen. Air diving tables are used to track this gas loading. The maximum depth in this example is 66 fsw (20 msw), so the 70 fsw (21 msw) schedule would be followed. In contrast, if using a CCR with basic electronics, the diver breathes no nitrogen at the surface. The CCR will attempt to maintain the 1.4 atm set point as closely as possible. Since the ambient pressure at the surface is only one ata, the unit will run 100% oxygen in the loop for a total PO2 of 1 atm. As the diver descends, the PO2 increases, but still no nitrogen is added to the loop. It is not until the diver gets to 13 fsw (4 msw) that the PO2

reaches 1.4 atm, and diluent (air containing nitrogen) is added. As the diver continues to descend, the PN2 increases, but the PO2 remains constant. When the diver ascends the same changes occur in reverse. Note that for the ongassing of nitrogen, the dive for the CCR user does not even begin until a depth of 39 fsw (12 msw) is reached. At this depth the PN2 is 0.79 atm, the same as if the diver were breathing air at the surface at sea level. The dive "ends" when the diver reaches the same depth during ascent. The true maximum depth is still 66 fsw (20 msw), but the EAD based on the PN2 is only 34 fsw (l0.3 msw), so the 35 fsw (l0.7 msw) schedule would be used. An OC diver would have to use EAN53 to achieve the same table benefits, and could not achieve the same true physiological benefits at all. EAD can be calculated while using CCRs using Equation 4.6 (page 75). However, you must first determine the FO2 of the mix. This is not as straightforward as with OC scuba, because with the CCR you breathe a constant PO2. The FO2 at any given depth may be calculated using the following equation:

Where: FO2 SP Data

Fraction of oxygen (atm) Set point, or partial pressure of oxygen (atm) Depth in atmospheres, or pressure at depth (ata)

If you use the deepest depth of the dive as your depth, then the value you calculate for FO2 can be used to calculate your EAD for use with air diving tables. This two-step process can be shortened to a single step if Equations 4.6 (page 75) and 6.13 (page 162) are combined. If you do so, then you get Equation 6.16, which allows you to determine EAD with CCRs with a single computation:

Where: EAD SP Data

Equivalent air depth (fsw or msw) Set point, or partial pressure of oxygen (atm) Depth (ata)

Example 6.14 What is Marvin's equivalent air depth if he is using an Inspiration® eCCR at a set point of 1.3 atm at a depth of 115 fsw (35 msw)? SP = 1.3 atm D = 115 fsw D = 35 msw

While examining Figure 6.15 (page 163), you may have noted a curious fact. Even though our CCR diver was under water, for part of

the dive he was not ongassing any nitrogen. It was not until he reached a depth of 39 fsw (12 msw) that the PN2 reached levels equal to breathing air on the surface. Physiologically, with respect to nitrogen ongassing and the dive tables, this is where the dive "began." This observation leads to operational possibilities difficult to achieve with OC scuba. In this example, while the CCR diver is at a depth of 20 fsw (6 msw), his PN2 is only 0.2 atm. This is less than the PN2 of 0.79 while breathing air on the surface. He is actually offgassing nitrogen at this depth, even if it is his initial dive. Because the PN2 is less than 0.79, the time spent here may be considered "surface interval," despite the fact that the diver is under water. Likewise, if he was anticipating flying after diving, this dive would not physiologically be considered a "dive" at all. During the dive he is off-gassing nitrogen. Were he to board a plane, even immediately after diving, he would be less likely to experience decompression sickness symptoms than if he had not been diving. Using a CCR may allow you to dive and fly on the same day, which is not recommended when using OC air scuba. We can use Equation 6.17 to determine the depth at which a dive starts physiologically with respect to nitrogen loading. Another indication that a dive is within these limits is an EAD less than zero.

Where: D0

SP

Depth considered as surface (fsw or msw) Set point, or partial pressure of oxygen (atm)

Example 6.15 Mary is diving a Titan® eCCR at a depth of 13 fsw (4 msw). Her set point is 1.1 atm, what is her EAD? At what depth will she begin ongassing nitrogen if this is not a repetitive dive? SP = 1.1 atm D = 13 fsw D = 4 msw

Mary's dive counts as surface interval because her EAD of -20 fsw (-6 msw) is less than zero. Her PN2 surface depth is 29 fsw (8.9 msw).

Constant PO2 Dive Tables

Constant PO2 dive tables (CPDT) incorporate these changes, and provide the user a greater advantage than do OC EANx tables. A set of constant PO2 tables has been developed for use with mixed gas CCRs using air as diluent. These tables are actually EAD conversions of the U.S. Navy dive tables, from the 2008 revision of the U.S. Navy Diving Manual. A second available set is an EAD conversion of the Bühlmann tables. As such, they do not provide the optimal benefit of true constant PO2 tables, since they do not account for varying mixes during ascent and descent. However, because of that they incorporate a small relative safety margin, and make bailout to OC systems easier.

Figure 6.16: Constant PO2 Dive Tables for PO2 = 1.3 atm, U.S. Navy basis. See page 331 for a full scale view of the above chart.

CPDTs for PO2 ranging from 0.7 to 1.6 atm are provided in this text (Appendix B). CPDTs for PO2 of 1.5 and 1.6 atm are included for emergency contingency planning purposes only, since diving at PO2 > 1.4 atm is not recommended. For each set point, an EAD was calculated for the 10 fsw (3 msw) intervals used in the U.S. Navy (or Buhlmann) tables. This EAD was inserted to replace the air depth indicated. Even though this course is a no stage-decompression diving course, decompression information has been provided for dives requiring stage decompression stops for those taking an advanced CCR course later. The NOAA Oxygen Exposure Limits for

both single dives and 24-hour maximums are provided on the margin of each table. %CNS and OTU per minute values are also provided. Your instructor will tell you which tables you will use during your training. The remainder of this text will refer to the U.S. Navy based tables, although the Bühlmann based tables could be used as well. These tables are used just as air tables. First, find the proper table corresponding to the PO2 set point of the CCR. Next use Table I (see Figure 6.16, Table 1, for an example) to find your actual or next greater dive depth in the left column. Move right until you find your actual or next greater time. Read down to locate your end-ofdive letter group. Table 2 (see Figure 6.16) is the surface interval time table. This is identical to air diving tables, in use and content. Read down until you find the range including your surface interval and read to the left. If you are going to change set point for a repetitive dive, this is the place to do so. Table 3 (see Figure 6.16) is the repetitive dive time table. Use Table 3 from the CPDT for the set point you will be using on the repetitive dive. Again, it is identical in use to air tables. Enter from the right, and read to the left until you reach the column for the depth of your next dive. Two figures are provided. The upper figure is your repetitive nitrogen time, the lower your adjusted no-decompression time. (When the adjusted no-decompression time was greater than the NOAA oxygen time limits for a single dive, then the repetitive dive time was terminated at the oxygen time limit.) You are still responsible for tracking your oxygen clock separately, although that becomes easier. Since the entire dive is conducted at a constant PO2, the NOAA limits can be followed for the dive without the need to calculate for multi-level depths. The percent CNS method may be used for repetitive dives.

Figure 6.17: Solution for Example 6.16 using Constant PO2 Dive Tables—PO2 = 1.3 atm. See page 331 for a full scale view of the table.

Example 6.16 What is Marvin's EAD, end-of-dive letter group, and percent CNS following a dive to 50 fsw (15 msw) for 70 minutes using a set point of 1.3 atm?

Figure 6.18: Dive computers integrated into some CCRs calculate inert gas loading based on oxygen sensor readings. Courtesey of Elaine Jobin

Solution: use the CPDT for PO2 = 1.3 atm (Figure 6.17). 50 fsw (15msw) falls between the 47 fsw (14.4 msw) and 51 fsw (15.6 msw) lines on Table 6.1. Use the deeper 51 fsw (15.6 msw) line. Read to the right to find the time of 75 minutes, which is the first time greater than the actual bottom time. Read down to find a "D" letter group (Figure 6.17). Some mixed gas CCRs, like the Dive Rite O2ptima®, Titan®, AP Diving Evolution®, and Delta-P Sentinel®, incorporate dive computers within the CCR (Figure 6.18). These calculate inert gas loading based on the readings from the oxygen sensors. Most of these systems use true constant PO2 models. This is appropriate until the diver is forced to do an ascent on the bailout gas supply. Because this is a fixed FO2 mix, the advantages of a constant PO2 are lost as soon as the switch to bailout occurs (Figure 6.19, page 170). If the diver is close to no-decompression time limits, this switch

may create a situation requiring stage decompression. To account for this possibility, I recommend a minimum five-minute safety stop at 15 fsw (4.5 msw) if an ascent is made on the bailout cylinder. Some manufacturers provide separate wrist mounting computers that incorporate a true constant PO2 model. These should be set at the same set point as your CCR. If the wrist computer does not allow for gas switching, for example to a constant FO2 mix, then the same safety stop recommended in the previous paragraph should be followed. If the unit does provide for gas switches, then when you change to your bailout gas supply, remember to change the computer as well.

Figure 6.19: Comparison of CCR versus OC ascent: Note that as the diver switches from the constant PO2 CCR to OC bailout, the PN2 immediately elevates and remains greater at all depths. If this occurs when the diver is close to the no-decompression limits of true constant PO2 tables, the diver may be at imminent risk of decompression sickness due to the increased nitrogen being breathed. To minimize this possibility, a minimum five minute safety stop at 15 fsw (4.5 msw) is recommended whenever an OC bailout ascent is made.

Normal nitrox computers may also be used with constant PO2 CCRs. With a single mix nitrox computer, calculate the FO2 in your breathing loop at the greatest dive depth using Equation 6.15 (page 164). If the value has more than two places after the decimal, truncate the value to two places, but do not round to the nearest hundredth. Input that value into your computer. This will not provide you the extended bottom time benefits of the constant PO2 CCR, but will allow you to use the EAD concept to increase your bottom time compared to air. Multiple mix nitrox computers may be set to a variety of mixes, using air as one endpoint and your FO2 in the breathing loop at greatest depth as the other. As you dive and vary depths, change the gas mix to follow your actual PO2. Computers that display PO2 at depth, such as the Dive Rite Nitek3®, make this simple. Just keep the PO2 on the computer at a figure equal to or slightly less than that showing on your oxygen sensor display—the same as your set point. This technique will closely approximate the benefits you could achieve with a constant PO2 dive computer.

Figure 6.20: Some wrist-mounted computers allow you to switch between constant PO2 and constant FO2 modes, beneficial if you change from using the CCR to OC scuba (during bailout, for example)

Several wrist mounted dive computers will run constant PO2 models, like the Cochran Commander® (Constant PO2 version). Others allow the user to switch between constant PO2 and constant FO2 mixes, allowing the rebreather diver to switch back and forth between their rebreather and OC scuba, for example during emergency bailout situations. The Hydrospace Engineering Explorer®, LiquiVision X1®, and Phoenix Oceaneering VR-3® computers allow this option (Figure 6.20). Of course, an air diving computer may also be used if the maximum depths for any set point as given in Table 6.3 (page 160) are not exceeded. This procedure provides the greatest physiological benefits, but of those discussed, is also the option allowing the shortest bottom times.

Section Five ABSORBENTS The product of oxygen metabolism is carbon dioxide (CO2). Since high CO2 levels in breathing gas are toxic, we must remove the CO2 we produce from the breathing loop. Absorbents do this. Absorbents are mixtures of alkaline compounds. The most common absorbent is soda lime, a mixture of calcium hydroxide [Ca(OH)2] and sodium hydroxide (NaOH). Some manufacturers add potassium hydroxide (KOH) to their formula. When exhaled gas passes through the absorbent or scrubber bed, a series of chemical reactions convert CO2, a gas, to calcium carbonate (CaCO3), a solid. The following section will address the chemical equations that occur. It is not important that you memorize these reactions. You may not even know enough chemistry to understand what the equations mean. That's OK! It is important, however, that you understand why they work, and how you, as a rebreather diver, might inadvertently prevent them from occurring. That information will be included in the text following each equation.

Carbon dioxide combines with water (H2O), forming carbonic acid (H2CO3). Carbonic acid is a very weak acid found naturally in the atmosphere, soils, and ground water. Since carbon dioxide combines with water so readily to form an acid, it is sometimes called an acidic gas. For the first step of the scrubbing reaction to proceed, there must be water available. There is optimal performance in absorbents when there is 15% to 19% moisture present. This water is not visible as a liquid. The manufacturer incorporates the necessary water into the absorbent during its formulation. Absorbents are produced with varying quantities of water, from none to over 20%. The amount of water required is determined by the use to which the absorbent will be put. Most absorbents sold in the civilian market are created for use in surgical anesthesia. During operations, anesthetic gas is recycled in a "rebreather" called an anesthesia machine. For human surgery, these absorbents are used in an environment containing an amount of humidity close to a "normal" amount, compared to the atmosphere. Hence these absorbents typically contain 7% to 15% moisture4. Absorbents used in veterinary applications have less water. Animals such as horses exhale large quantities of moisture, so their formulations have less water in them. Absorbents for this market contain 0% to 10% moisture.4 The gases divers breathe are dehumidified to remove virtually all of the water vapor. This is done to prevent cylinder oxidation and is part of the gas compression or cylinder filling processes, or both. Thus, we need to use absorbent that has a large quantity of incorporated moisture, typically 15% to 18%. Absorbents with this quantity of moisture are often designated high performance or HP grade.

Absorbents that have too little water work poorly, or not at all. Slight decreases in moisture content have severe effects on performance. Absorbents that have too much water also experience performance degradation, but to a lesser degree. It is better to have too much rather than too little moisture. Even if we purchase the proper absorbent, we can inadvertently alter the moisture content. One way of doing this is to allow absorbent to freeze. Freezing makes the moisture in the absorbent unavailable for the chemical reaction to occur. Even if frozen and later thawed, the moisture may no longer be evenly dispersed, and the effectiveness of the absorbent may be variable. Absorbent material that has been frozen should not be used for diving. Allowing the absorbent to bake has equally detrimental effects. Baking drives the moisture off, again making it unavailable. Even if stored in a sealed container, the moisture baked out of the absorbent compound will later condense on the inner walls of the container as a liquid and will not be available for chemical use. Proper storage of absorbent is important! Flushing large quantities of dry gas can also dry out absorbent. As dry gas passes through the scrubber bed, it picks up moisture from the absorbent, drying it out. If you are breathing on the breathing loop, or if the gas is being cycled in the breathing loop, this is not a problem. However, passing a large quantity of gas from a cylinder through the scrubber and releasing it from the breathing loop should be avoided. The rate at which CO2 forms carbonic acid depends on temperature. The reaction proceeds slowly if cold. The next two steps of the scrubbing reaction produce heat, but it is important that this first step begin, or CO2 may build rapidly enough to become toxic before the absorbent starts to operate effectively. This is why it is important to breathe on your rebreather for several minutes before a dive if you will be in cold conditions. This is called "pre-breathing." Even after the reaction has begun, diving conditions play a part in the effectiveness of the absorbent. The differences are notable at temperatures commonly seen in diving conditions. For example, absorbent canisters may be less than 50% as effective in 40°F (4C)

water as in 70°F (18C) water. The warmer the scrubber bed can be kept, the more effective the process. Essentially, a warm scrubber bed will last much longer than a cold one.

Acids and bases (alkalis) react very vigorously to form salts and water. The carbonic acid formed in the previous chemical reaction combines with the sodium hydroxide (and potassium hydroxide, if present) to form sodium carbonate (Na2CO3) and potassium carbonate (K2CO3). This reaction is virtually instantaneous, and produces both water and heat. The heat is beneficial, as it keeps the scrubber bed warm. The water, to some degree, is also beneficial, as it increases the humidity, allowing the first reaction to effectively proceed. Eventually, though, enough water will be produced to begin to condense as liquid. Some of the absorbent will then begin to dissolve in this liquid. The resulting solution is extremely caustic and harmful. Scrubber canister designs that separate the liquid from the scrubber bed are beneficial, and will be addressed in Chapter 7. Sodium and potassium hydroxides are extremely reactive. Because of this, they form a more caustic solution very rapidly. As a result of the reactivity and possible end products, both of these compounds are classified as a hazardous material, and regulations designate how they may be transported, used, and released. While an absorbent could be manufactured using only these compounds, other compounds are less reactive, and thus do not have the same associated problems. Calcium hydroxide is one example.

Figure 6.21: Absorbent Cycle.

The calcium hydroxide combines with both of the carbonates produced in the second scrubbing reaction to form calcium carbonate (CaCO3). It also releases the original sodium and potassium hydroxides. These are again available to drive the second part of the scrubbing process. In a sense, they act as catalysts, allowing CO2 scrubbing to proceed faster. This reaction produces additional heat. The end product of removing CO2 is calcium carbonate. This is a naturally occurring compound. Chemically, it is the same material as birds' eggshells, seashells, coral reefs, limestone, and the chalk used in school classrooms. Calcium hydroxide is manufactured by taking naturally occurring limestone (CaCO3), and baking it to drive off CO2. Essentially, we use absorbent in a closed cycle, using limestone to make absorbent, and absorbent to scrub CO2, producing a limestone-like compound (Figure 6.21, page 175). Theoretically, it would be possible to "regenerate" spent absorbent by baking it in an oven. Practically, the conditions for manufacturing calcium hydroxide must be precisely

controlled, or calcium oxide is formed instead. This is a very reactive and potentially dangerous material. In addition, remember that absorbent moisture content is critical and must be regulated by process controls. Do not attempt to "regenerate" your spent absorbent. Calcium carbonate is completely inert and not harmful to people or environmentally. However, used absorbent will contain some alkaline materials that have not been consumed, and so should be disposed of accordingly. Disposal information for various products is contained in the Material Safety Data Sheet (MSDS) for the material, which the manufacturer is required to make available. MSDSs list other hazards associated with use of the compounds they describe, and first aid and protective measures for the same. Most of this information will be covered in later chapters. The MSDSs for absorbents commonly used in diving are included in Appendix K of this text. Since MSDSs are periodically updated by the manufacturer, you may wish to request a current version for the absorbent you will use.

Figure 6.22: Begin deep dives with fresh absorbent. Courtesey of National Park Service and Brett Seymour

One hazard not discussed elsewhere is that of mixing absorbent with other chemicals. While you might think you would never do that, you could unintentionally. For example, some rebreathers must be cleaned for oxygen service annually. A variety of chemicals could be used to do this. One that was commonly used in the past was freon, or trichloroethylene. Under some circumstances, residual quantities of this cleaning agent could react with absorbents to form phosgene or acetylene. Both are highly toxic. In fact, phosgene is the active ingredient in some gases used in chemical warfare–not a good thing to have in your rebreather! One final problem deals with the use of absorbents at depth. When diving shallow, there are relatively few gas molecules circulating in the breathing loop. CO2 molecules are relatively concentrated. As they pass through the scrubber bed, the chances are high that they will come into contact with a grain of active absorbent and be removed from the system. However, as you descend, the gas molecule density increases due to Boyle's law. But the number of CO2 molecules remains constant, so the concentration of CO2 in the gas is reduced. The chance of any given molecule impacting an active absorbent grain is reduced, as they are "protected" by the other gas molecules circulating in the rebreather. The deeper you go, and the longer the scrubber has been used, the more likely that CO2 will pass through the scrubber bed without being absorbed. This is known as bulk loading. It can be reduced in part by canister design, but in most cases is avoided by limiting the duration of deep dives, and using fresh absorbent for such dives.

OXYGEN CCRs: 1. Hypoxia may be a problem with oxygen CCRs if you: a. Fill the oxygen cylinder with air. b. Do not purge the breathing loop with oxygen prior to diving. c. Use an oxygen CCR soon after conducting a series of repetitive air dives on OC scuba, and do not purge the breathing loop during the dive. d. All of the above.

2. Allowing an oxygen CCR with a deflated counterlung to sink from the surface to the bottom at 100 fsw (30 msw) may result in: a. Oxygen toxicity for the diver. b. The CCR being damaged from "squeeze." c. A high potential for spontaneous combustion, due to the high PO2 in the breathing loop at that depth. d. All of the above.

3. The deepest depth to which an oxygen CCR may be used is: a. 10 fsw/3 msw.

b. 20 fsw/6 msw. c. 30 fsw/9 msw. d. Variable depending on the procedure followed for purging the breathing loop.

MASS FLOW SCRs: 4. A sonic valve may deliver inadequate flow if: a. Dirt or debris blocks the orifice. b. The orifice is corroded. c. The interstage pressure is set too low for the depth. d. All of the above.

5. You are using EAN50 with a measured flow rate of 12 lpm. What is your FiO2 under hard work conditions? a. 0.21. b. 0.33. c. 0.37. d. 0.50.

6. What is the maximum operating depth of a SCR using EAN30 based on an interstage pressure of 110 psi (7.5 bar)?

a. 78 fsw/24 msw. b. 121 fsw/36 msw. c. 132 fsw/40 msw. d. 143 fsw/43 msw.

RMV-keyed SCRs: 7. The amount of gas discharged from the breathing loop increases: a. As the diver works harder. b. With a larger volume ratio. c. As depth increases with variable volume ratio RMV-keyed SCRs. d. A and B above. e. A, B, and C above.

8. All other factors being equal, the FiO2 will be: a. Greater for a fit diver compared to an unfit diver. b. Greater for an unfit diver compared to a fit diver. c. Equal for both divers, as the volume ratio for the SCR is identical, and so is discharge gas volume.

d. None of the above are true, as other variables are more important.

9. The maximum operating depth for a diver who has a ventilatory equivalent for oxygen of 20 using a RMV-keyed SCR having a volume ratio of 0.8 and EAN70 as the supply gas planning a dive to 33 fsw (10 msw) is: a. 1.38 atm (45 fsw/14 msw). b. 1.38 ata (12 fsw/4 msw). c. 2 atm (66 fsw/20 msw). d. 2 ata (33 fsw/10 msw). e. Cannot be determined from the information.

MIXED-GAS CCRs: 10. The electronics in a mixed gas CCR will: a. Add oxygen if the PO2 in the breathing loop drops below set point. b. Add diluent if the PO2 in the breathing loop climbs above set point. c. Warn the diver if the PCO2 in the diluent climbs above set point. d. A and B above. e. A, B, and C above.

11. Assuming that one sensor of three has completely failed: a. Electronics using an averaging protocol will maintain set point better than those using a voting protocol. b. Electronics using a voting protocol will maintain set point better than those using an averaging protocol. c. Electronics using either a voting or averaging protocol will work equally effectively. d. The diver must immediately switch to a manual mode of operation to avoid potential hypoxia. e. None of the above are true.

12. The no-decompression time limit for a diver using a mixed gas CCR with air as diluent with a set point of 1.3 atm diving to a depth of 110 fsw (33 msw) is: a. 20 minutes. b. 25 minutes. c. 30 minutes. d. 40 minutes. e. Variable based on the work rate of the diver.

13. The minimum set point for a dive to 100fsw (30msw) using a mixed gas CCR is:

a. 0.7 atm b. 0.8 atm c. 0.9 atm d. 1.0 atm e. Cannot be determined from the information provided

ABSORBENTS: 14. A diver may use veterinary grade absorbent (0% moisture) content by: a. Adding 15 to 18% water to the absorbent by volume prior to packing the canister. b. Adding 15 to 18% water to the absorbent by weight prior to packing the canister. c. Mixing the absorbent 50:50 with an absorbent containing 30% to 36% water. d. Any of the above will work sufficiently well. e. Veterinary grade absorbent may not be used in rebreathers by divers.

15. Moisture content in diving grade absorbents may be altered to an unsafe degree by: a. Freezing.

b. Setting it in direct sunlight while still in the manufacturer's container for long times. c. Purging large quantities of diluent through the absorbent while it is in the canister. d. All of the above.

16. The Material Safety Data Sheet for an absorbent will provide information on: a. Regulations for disposal. b. Health hazards. c. Handling precautions. d. First aid for exposure to the absorbent. e. All of the above.

Rebreather Design Overview

Even though rebreather designs differ, the various units include many similar components. All have a counterlung, scrubber canister and scrubber, mouthpiece, hoses, and gas supply. Some may include computerized monitoring, oxygen sensors, or other components. Just as major rebreather designs differ, the components may be different as well. Knowledge of component design will help you in several ways. For one thing, some designs are better suited to some types of diving. This greatly facilitates rebreather selection. Understanding design and function will help you with maintenance and troubleshooting. Ultimately, you will be better able to dive within the capability of your system if you know its design limitations.

Objectives After reading this chapter, you will be able to: 1. Draw four scrubber canister designs, and explain how each works. 2. Explain how counterlung placement affects breathing effort. 3. Explain water trap function. 4. List three variations in hose design and explain their significance. 5. Detail five ways in which an oxygen sensor or sensor may be damaged.

6. Select a scrubber based on the type, grade, and other constraints that suit your diving.

Scrubber Canisters

Scrubber canisters are the heart of a rebreather. They control the conditioning of the exhaled gas, removal of the CO2, and aid in mixing the added fresh supply gas. Canister design controls a host of important factors: scrubber efficiency, work of breathing (WOB), channeling, time to breakthrough, scrubber insulation, and, in some cases, protection of the absorbent from water. Scrubber efficiency is a function of temperature, depth, absorbent size, and dwell time (also called residence time) – the amount of time the exhaled gas remains within the scrubber. The longer the dwell time, the more CO2 is removed from the exhaled gas. Dwell time is partially dependent on counterlung design, which will be discussed in the next section. Generally, dwell time is increased with canister length and volume. However, you cannot increase either or both of these without a tradeoff. WOB (work of breathing) is a measure of the effort required to inhale and exhale. The longer the path through the scrubber, the higher the WOB. If the WOB is too high, merely breathing will fatigue you. The rebreather manufacturer should make WOB testing results at various depths available to you, so you can make an informed purchasing decision. You can reduce the length of the canister, reducing the WOB, by increasing the cross-sectional area through which the gas travels. This involves a trade-off. Normally, gas flow through the scrubber bed is relatively uniform, with the gas diffusing equally through all parts of the scrubber canister. Channeling occurs when the exhaled gas passes through the scrubber bed in a non-uniform manner. Typically, this occurs because there is a "path of least resistance" in the absorbent packing. For example, an area where the absorbent is less compacted, or even absent. (Figure 7.3) The greater the crosssectional area, the more chance that the absorbent may not be

uniformly packed. The shape of the canister, how it is designed to be filled with absorbent, and good gas flow design may reduce this risk.

Figures 7.1 and 7.2: Typical WOB curves – good (left) and bad (right).

Figure 7.3: Settling can allow gas to bypass the absorbent. This is called "channeling." Channeling can lead to early canister breakthrough, which occurs when the gas that has passed through the canister still contains a significant amount of CO2. Most breakthrough definitions are based on a 0.5% CO2 (or higher) at sea level equivalent. If the absorbent is uniformly packed, then the time until breakthrough occurs is based on the quality and quantity of absorbent, canister flow design, work level of the diver, depth and temperature of the scrubber. Most canisters are designed to last from one to 10 hours, with two to three hours being the norm. While breakthrough time

varies based on these factors, a general rule of thumb is that a pound (0.45 Kg) of high-grade scrubber will allow 30 minutes to one hour of diving in warm water doing moderate work.

Figure 7.4: The Dräger Dolphin canister uses springs to compress the absorbent, reducing the possibility of channeling.

Figure 7.5: Scrubber canister design affects the work of breathing, efficiency, and other important rebreather characteristics. Absorbent works more efficiently and will last longer at warm temperatures. Some canisters are designed to insulate the absorbent by maintaining an air space around it, or wrapping it in neoprene rubber. Others use materials that conduct heat poorly, such as plastic, to minimize heat loss. Canisters made of metal that have direct contact with the absorbent on the inside and the water on the outside generally provide the poorest thermal protection. Another test result set of great interest is the time until breakthrough for a unit's canister measured at a variety of temperatures. Finally, some canisters are designed to minimize the possibility of water coming into contact with the absorbent. Some do this with passive design measures, like placing the absorbent within the canister where water will not pool if partial flooding occurs. Others use active measures, such as using special materials (similar to Goretex®) that allow gas to flow, but not liquids. Sponges are commonly used to trap absorbent dust and small quantities of water. All approaches work in conjunction with the counterlung design, and some add water traps to minimize the possibility of water coming into

contact with the absorbent. Some approaches can lead to an increase in WOB. Here are several scrubber canisters designs: Axial flow is the simplest in concept and construction (Figure 7.5). In axial flow, the gas passes in a straight line from one end of the scrubber to the other. Axial flow canisters are usually easy to fill properly, but because of the straight path flow from one end to the other, may use absorbent inefficiently. In testing, some axial canisters have been found to be less than 10% efficient. Short duration axial flow canisters have acceptable WOB characteristics, but as the canister length increases, so does the WOB. Thus, this design is generally not optimal for rebreathers requiring long dive duration. Axial flow canisters ordinarily offer poor to moderate insulating capability compared to other designs. Pendulum axial flow canisters pass the gas back and forth through the canister (Figure 7.5, page 185), allowing the absorbent at both ends to effectively scrub the gas. However, this design has a large dead space in the breathing hose, and its use has been discontinued in modern rebreather designs. Annular axial flow canisters provide an improvement over axial flow design by dramatically decreasing WOB. (Figure 7.5, page 185) Because the cross-sectional area is large, WOB is low and the dwell time is improved. However, the increased surface area decreases the insulating capability of the canister. Depending on the design, properly filling these canisters with absorbent can be relatively straightforward to very difficult. Co-axial flow canisters incorporate an air jacket around the canister, providing good insulation (Figure 7.5, page 185). The gas flows in through the cylindrical scrubber bed, around the end, and then through the hollow center. The long travel path of the gas improves dwell time, but at the cost of high WOB. This is one of the better canister designs for cold water diving. Radial flow canisters offer perhaps the broadest range of good operational attributes. Canister design is similar to the co-axial flow canister, being a hollow cylinder. However, the gas flow path enters the hollow inner cylinder and flows radially outward through the

scrubber. (Figure 7.5, page 185). This offers the lowest WOB, because of the short pass-through distance, and high efficiency of absorbent use before breakthrough. Because it is also incorporates an air jacket, insulating properties are good. Proper design is extremely important with this type of canister, with length to diameter ratios being so important the data is deemed proprietary by manufacturers. As you can see, there are many factors to consider in canister design. This is probably the single most important component in the rebreather, and probably the most difficult to design well. There is no single "best" design. The canister design that best suits your needs will depend on your diving requirements.

Counterlungs

Counterlungs, or breathing bags, are categorized by their location on the diver. Placement affects the work of breathing, and the overall packaging of the rebreather. Counterlungs are designed with different capacities, which may affect utility for some divers. If the counterlung volume is smaller than the actual lung volume, then gas will be unnecessarily vented from the system on every breath. Every inhalation will require that additional gas be injected to make up for that lost volume. A counterlung that is too small for its user will dramatically decrease efficiency and increase WOB.

Figure 7.6: Body, mouthpiece, and counterlung positioning relative to depth. Small changes in relative depth differences can have a large impact on WOB. Back-mounted counterlungs are situated on the back of the user, close to normal scuba cylinder placement. Thus the average recreational diver readily embraces this configuration. Back-mounted counterlungs create a hydrostatic head, or negative pressure, at the mouthpiece, resulting from the difference in depth between the diver's lungs and the counterlung. The slight "suction" makes it easy to exhale, but difficult to inhale. The counterlung's position on the back will affect the amount of the hydrostatic head, with higher positioning increasing the pressure differential. Most back-mounted units are designed for the diver who is swimming at a 45° angle in the water. This minimizes the hydrostatic head (Figure 7.6). When the diver swims on his/her back with a back-mounted unit, it is easy to inhale but difficult to exhale. This is more fatiguing for most divers. The hydrostatic head experienced by the ears and Eustachian tubes in this position is such that gas is forced into the

ears. Even in a normal swimming position, the ears and Eustachian tubes experience constant positive pressures. This may lead to problems with equalization, squeezes, and ear infections. Front-mounted counterlungs are carried on the chest and stomach region of the diver. This has the advantage of allowing the diver to easily observe and manipulate the apparatus. The breathing characteristics are identical to the back-mounted counterlung when the diver is face up. It is easy to inhale, difficult to exhale, and generally fatiguing. Ear problems are common with this type of rebreather.

Figure 7.7: Depth differences between the counterlung, mouthpiece, and ears can lead to squeezes, infections, and other ear problems. Over the shoulders counterlung designs attempt to minimize hydrostatic heads by allowing the gas to travel to the point of minimal pressure difference. This is similar to current BC design. Regardless of the diver's position in the water, the buoyant gas in the counterlung can rise to the point, most likely for a minimum pressure differential between it and the lungs. Such units provide more flexible trim control as well, by allowing the gas to move to either side of the body. Some counterlungs incorporate a split counterlung design comprised of an inhalation and exhalation bag. This is different from most designs, which have only a single bag. The split design allows

increased dwell time of the respired gas in the canister, because the driving pressures are reduced. This increases the scrubber's ability to remove CO2. The paired bags also feature water trapping benefits not available in single bag designs, as water pooled in the exhalation bag cannot easily travel to the inhalation bag to be breathed or ingested by the diver.

Figure 7.8: Front mounted counterlung. Courtesy of Scott Cassell

Because gas in counterlungs is subject to expansion during ascent, they should have some means of venting the excess. This overpressure valve is similar to those in BCs, and helps prevent air embolism while diving rebreathers. In addition, some counterlungs are made of an elastic material that will place some back-pressure on the system when slight overexpansion occurs. This signals the diver that gas must be vented from the system.

Figure 7.9: Scrubber canister designed to trap water, preventing contact with absorbent.

Water Traps

Water traps are necessary in rebreathers to minimize problems caused by small quantities of water in the breathing loop. The diver may inadvertently admit water to the breathing loop, by allowing some leakage around the lips and mouthpiece for example. Saliva generated during the dive may also pool as free liquid in the loop. Since any water, regardless of source, may form a caustic solution if it comes into contact with the absorbent, rebreather designers strive to prevent such contact. One technique is to design the scrubber canister to prevent contact. This can be done by having the absorbent held in an inner container within the canister, providing an area for moisture to pool before it can contact the absorbent (Figure 7.9). This approach can handle only very small volumes of water.

Figure 7.10: Counterlungs designed to trap water (left) can be defeated by poor body positions.

Figure 7.11: Maintaining a moderate head up, feet down position will allow you to utilize the counterlungs of many rebreathers as a sizable water trap.

A more common technique capable of handling more liquid uses the counterlung as the water trap. Since most counterlungs are vertical on the diver, water entering the counterlung will pool at the bottom. Such counterlungs have the inhalation hose at the top of the counterlung, so that the water collected at the bottom cannot easily travel into the hose and reach the mouthpiece. Some counterlungs have a purge device through which the diver can expel any accumulated liquid. When counterlungs are positioned between the exhalation hose and the scrubber canister, water can be trapped before it can contact the absorbent. Of course, body positioning can defeat these designs. (Figure 7.10) If the diver turns head-down, the water will stream into the hose and possibly the scrubber canister or diver's lungs.

Figure 7.12: The Poseidon Discovery Mk 6 uses two sponges as water traps. Finally, some rebreathers utilize an auxiliary water trap. This is a smaller bag positioned in the breathing loop for the sole purpose of trapping water. Again, when using a system incorporating this design feature, you must remember that some body positions during the dive can defeat its design objectives, and minimize activity in those positions.

Mouthpieces

A rebreather mouthpiece is not as simple as it looks. It performs several functions an open-circuit mouthpiece does not. Like the OC (open-circuit) mouthpiece, it provides gas to and collects gas from the diver. Instead of exhausting the gas to the environment, though, it contains it and controls the direction of its flow within the rebreather. It also provides the isolation mechanism that prevents water from entering the breathing loop when the rebreather is not in use by the diver. Because it has two different operating positions, one for when you are diving and another when it is not in use at the surface, it is also called a Dive/Surface Valve (DSV). Knowing mouthpiece design will enable you to recognize and solve potential problems before entering the water. Mushroom valves (also called check valves) located at either end of the mouthpiece chamber provide directional control of the gas. Some rebreathers position the valves in the breathing hoses next to the mouthpiece, rather than in the mouthpiece itself. These mushroom valves are similar to the exhaust valves in OC regulators' second stages, and are arranged sequentially. When you inhale, the valve on the inhalation hose opens, allowing gas to enter your mouth, and the exhalation valve closes, preventing you from inhaling gas exhaled from your last breath. As you exhale, the inhalation valve closes, and the exhalation valve opens. These valves control the direction of gas flow in the breathing loop. Unlike OC apparatus, the entire breathing loop is at ambient pressure. There are no interstage pressures or valves directing gas flow. Most rebreathers are designed to inhale from the left and exhale to the right. If you install the mouthpiece backwards, then you will direct the flow in the opposite direction called for by the design. This error can lead to a marked increase in the WOB and loss of rebreather effectiveness.

Figure 7.13: A rebreather mouthpiece incorporates mushroom valves to control gas flow direction.

Figure 7.14: Schematic cross section of mouthpiece showing inhalation and exhalation phases. Not all rebreathers are designed to flow in this direction. The manufacturer's specifications should be checked prior to installing or using any rebreather mouthpiece. Dirt or debris can hold a valve open, and may compromise o-ring seals in the mouthpiece. Older mushroom valves and those that have been improperly cleaned and dried may retain a permanently curled edge that can also prevent closing. If this occurs, gas may flow in the wrong direction. Test both mushroom valves prior to every dive. If operating improperly, they should be cleaned or replaced before the rebreather is used. Older, stiffened valves may work, but will increase WOB. These should also be replaced.

All mouthpieces have some way of preventing water ingress when not in use. The mechanism may vary by manufacturer. One design locates a rotating chamber inside the mouthpiece housing. It is opened and closed by grasping the mouthpiece at both ends and rotating one hose clockwise, the other counter-clockwise, relative to each other. This design creates a very streamlined mouthpiece, with no protruding levers, but has a few disadvantages: it takes both hands to open or close the mouthpiece, you cannot see if it is open or shut, and the diver must remember to manually close it when necessary (Figure 7.15).

Figure 7.15: Some rebreather mouthpieces are very streamlined, but require two hands to operate them. One common design uses a lever to open and shut the mouthpiece. The lever protrudes from the front of the mouthpiece, and rotates up (usually the open position) and down (usually the closed position). This mouthpiece's status is visually apparent to you or your buddy, but it still has the disadvantage of relying upon the diver to close it (Figure 7.16). A third design links an open/close mechanism to the mouthpiece bite tabs. Biting the mouthpiece opens it, and it automatically closes when the diver releases bite pressure. This prevents diver error leaving the mouthpiece open. However, malfunction may make it impossible to close, and some divers may experience jaw fatigue during the dive.

Figure 7.16: Mouthpieces that can be operated with a single hand offer convenience in many circumstances. Courtesy of Darren Fox, Ocean Legends

A few mouthpiece features are optional. A purge device will make it easier to clear the mouthpiece of water. Such a device usually consists of a small hole or passage that opens the chamber of the mouthpiece to the ambient environment while it is in the closed position. By blowing into the chamber, you can easily displace water from it before opening the mouthpiece.

Figure 7.17: Steam Machines uses weighted rings to attach hoses to the mouthpiece, to counter bouyancy. Note the purge holes on the upper mouthpiece. Another feature incorporates weights into the mouthpiece. Because the hoses and mouthpiece are filled with breathing gas, they are buoyant. They pull at the diver's mouth, causing jaw fatigue. Weights reduce or eliminate the buoyancy, greatly increasing diver comfort. If your unit does not have such weights, you may wish to add them. If you do, make sure you do not compromise operational features with your modifications, such as the ability to open or close the mouthpiece. Some mouthpieces incorporate neck straps. When tightly drawn, a neck strap may prevent the mouthpiece from being dislodged from the mouth in the event of unconsciousness.

Bailout Valves

Some rebreather mouthpieces have functionality beyond the standard mouthpiece. The most common of these is the bailout valve (BOV). The BOV has an OC second stage regulator or valve incorporated into the mouthpiece. When the mouthpiece is in rebreather mode, it works as a standard mouthpiece. However, when it is changed to the surface (closed) position, you can use it as an OC second stage. Because of this, some manufacturers call their valve an Open Circuit Bailout Mouthpiece (OCB). With a BOV you no longer have to remove the mouthpiece from your mouth in an emergency to change to an OC gas supply. This reduces task loading, and in most cases adds a safety factor. Gas is supplied via a standard intermediate pressure hose. The source of the gas may be the diluent cylinder in a mixed gas CCR, the supply cylinder of a SCR, or a separate bailout cylinder used with any type of rebreather. If a separate bailout cylinder is used, some people prefer to use a high flow quick-disconnect fitting to attach the intermediate pressure hose to the BOV.

Figure 7.18: Bailout valves incorporate an OC second stage regulator, allowing you to breathe OC gas when the mouthpiece is in the closed position. Coutesy of Elaine Jobin

Automatic Diluent Valve

In manually operated rebreathers, the user must add gas to maintain sufficient volume to breathe. If you fail to do this, especially during descent, it is possible to suffocate. An automatic diluent valve (ADV) adds gas to the breathing loop when the volume is depleted. When you are no longer able to take a full breath, gas is supplied automatically. In a mixed gas CCR, the gas supplied is diluent. In a SCR a similar valve uses the nitrox supply gas. ADVs reduce task loading by ensuring you have adequate breathing volume without having to manually attend to the task.

Figure 7.19: Automatic diluent valves add gas to the breathing loop without user intervention when loop volume falls beow a

critical threshold. Courtesy of Elaine Jobin

Automatic Diluent Bailout Valve

A further evolution of rebreather mouthpieces integrates OC bailout capability and the automatic diluent valve into an automatic diluent bailout valve (ADBOV) (Figure 7.20, page 198). Other designs further combine components like the manual diluent add, overpressure, and fluid drain valves. Besides the obvious benefits of such combined units, they also offer the advantage of placing several components that must be manually operated in one easy-to-reach location.

Nor Any a Drop to Breathe

A very experienced OC technical diver began using rebreathers to extend his deep diving activities. A short time after completing his initial training class, he planned a dive on a wreck he had dived numerous times before. The wreck was in 280 fsw (85 msw). Because of strong currents, divers enter the water with no air in their BCs, and sink to the bottom as quickly as possible so they are not swept past the wreck. Our diver equipped himself as he normally did—with a crowbar, hammer, and hacksaw used to recover brass artifacts clipped to his harness, and carrying an expensive camera to document the wreck. He used the excess weight to assist him in a rapid descent. Fully equipped, he jumped off the dive boat and started down. When last seen by his dive buddies, he was descending very quickly, holding his camera with one hand and fumbling with the other. He was later recovered from the bottom, mouthpiece out of his mouth. His camera was a short distance away. It is theorized that during his descent, he failed to add diluent to his breathing loop. During his normal OC descents, this was never an issue. His regulator supplied whatever gas volume he needed

regardless of descent rate. The rebreather did not do that. Lacking an automatic diluent valve, he had to manually add gas. We think that he was concerned with not losing his ancillary equipment, particularly his camera. It was not until he could not inhale that he realized his difficulty and tried to find the manual diluent add control... unfortunately too late. This accident could have been avoided: (1) Had the rebreather been equipped with an automatic diluent valve, it would have added the gas volume he needed. (2) If he had not task-loaded himself with the extra gear, he would have been more focused on his life support equipment. (3) Had he gained more experience on rebreathers before attempting an advanced dive (even though this one was well within his capabilities using OC equipment), he would have been able to manipulate his equipment without fumbling, or even thinking about what he was doing.

Figure 7.20: ADBOVs combine the mouthpiece, OC bailout capability, and automatic diluent valve in one unit.

Hoses

The hoses channel the gas from the scrubber/counterlung to the mouthpiece and back again. They are corrugated to be flexible, and

so minimize jaw strain. The most important operating variable is hose diameter. Small diameter restricts gas flow, increasing WOB. The type of corrugations will impact several factors in rebreather use. Helical corrugations facilitate water drainage after washing, as there is a continuous channel down which the water can travel. The downside is that once manufactured, hose length is fixed, and cannot be modified by the diver. Annular corrugations permit the diver to cut the hose to a better length, or to cut away holes located near a hose end. Hoses of this design trap and hold water, however. The longer the hose, the greater the WOB. You want hoses long enough to allow you to move your head and look side to side without restriction. Any beyond that increases drag, buoyancy, and WOB.

Figure 7.21: Helical and annular corrugation hose designs. The manner in which the hose is attached to the rebreather varies. Some hoses use connectors that clamp to the hose. These allow hose length to be modified. Others use a cuff-type fitting, which is part of the injection molding design. This type minimizes chances of leaks at the connector, but cannot be modified. O-rings are used in some connectors. They are prone to the same problems as other orings, such as loss, or foreign matter compromising integrity. Some designs utilize a double o-ring, providing a safety margin.

Figure 7.22: Weights on the AP Diving Evolution and Inspiration CCRs counteract breathing hose buoyancy. Courtesy of Elaine Jobin

Some breathing hose materials are prone to punctures or crushing. Very soft hose material, easy to manipulate and move, is easily damaged. Other hoses made of crush-proof material are very resistant to external damage, but are stiffer and less comfortable to use. Like mouthpieces, hoses are buoyant. Ring weights may be added to the hoses to keep buoyancy within comfortable levels (Figure 7.17, page 194). Weight location is a matter of personal preference. Experiment with weight placement between or even during dives to maximize your comfort.

Gas Supply Apparatus

Rebreathers use a variety of approaches to provide an acceptable breathing mix, each with design considerations. Because of the complexity of this aspect of rebreather design, it is a separate chapter in this text. Some theory and design considerations have already been discussed. If you have not yet done so, read Chapter 6 for details on this aspect of rebreather design. One feature that many designs may provide is a manual override. This provides the diver a means of manually flushing the breathing loop with fresh gas, or adding gas from the gas supply cylinder(s). Since the ability to flush the breathing loop is necessary

in many diving situations, this is a very desirable feature for some types of diving. However, this type of control also increases system complexity, and may not be appropriate for all users.

Gas Supply Cylinders

As with OC scuba, many cylinder types are available for rebreathers. Some manufacturers make proprietary cylinders, but most use cylinders that are otherwise available. Cylinders are usually made of either steel or aluminum. The choice of material is based on physical size, buoyancy characteristics, and volume. Aluminum cylinders may be damaged if they come into contact with absorbent. The chemistry is such that the aluminum will pit very rapidly, potentially affecting structural integrity. If you use aluminum cylinders, store and transport them away from any absorbent material. Some rebreathers now use composite materials for cylinders, fiberglass or carbon-fiber wrapped. These cylinders are very light. Consequently, their use may require additional lead while diving to compensate for buoyancy. They also have the advantage of having working pressures to 4500 psi (300 bar). This increases available gas volume for a given size cylinder. Composite cylinders are not as robust as steel or aluminum, so you should exercise due care with their use. They have a finite lifespan, with cylinder retirement recommended 10 to 15 years after manufacture date. The most important factor to consider with rebreather cylinder selection is that the cylinder matches the scrubber canister. Either of these may provide the limiting factor for dive duration, but if the scrubber canister will only provide for a dive of an hour, it would be pointless to have a cylinder with a 10-hour gas supply. The best approach is to match scrubber capacity with gas supply, and then add sufficient reserve to the gas supply to allow for bailout capability. Cylinders may be worn in many positions, with placement generally dictated by rebreather design. Cylinders may be carried at the bottom, sides, or atop the rebreather based on buoyancy and trim consideration, to provide as close to neutral buoyancy and trim as possible.

Cylinder valves may be either yoke-type or DIN configuration. DIN valves capture o-rings, which minimize chances of o-ring extrusion, especially at higher pressures. These are usually preferred. However, because yoke-type valves are more far more available in some geographic regions for OC scuba systems, some rebreather designs use these valves to facilitate interchange and availability of parts. Some rebreathers are now using regulators and valves keyed to the gases contained in the cylinders, minimizing the possibility of attaching the cylinders improperly.

Absorbent

As we saw in Chapter 6, the absorbent chemically combines with and removes CO2 from the breathing gas the diver has exhaled. Several chemicals are used in absorbents, all of which will adequately remove CO2. The selection of the chemical is a balance between the volatility of the chemical, efficiency in removing CO2, cost, temperature constraints, and potential hazard to the diver should the absorbent become wet during the dive. Chemicals that scrub CO2 include barium hydroxide (Ba(OH)2), lithium hydroxide (LiOH), sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2), and potassium hydroxide (KOH). These may be used in their pure form, but usually they are blended or mixed to gain the benefits of several chemicals. The most commonly used absorbent material is soda lime, Ca(OH)2 blended with NaOH. This product has the advantages of reasonable price, low hazard, and environmental safety. The end product of soda lime used as an absorbent is common chalk. Soda lime is manufactured and sold under a variety of trade names (Table 7.1). Only manufacturer-recommended or diving grade absorbents should be used in rebreathers. While many hospital or medical grade absorbents are available, they may not have been tested for diving applications, and their use may be contraindicated for diving. In general, medical grade absorbents have different manufacturing specifications for moisture content, hardness, grain size uniformity,

dust content, and other characteristics. Because of these different characteristics, medical grade absorbents usually do not provide the qualities necessary for rebreather use. Table 7.1: Rebreather Absorbents Trade Name Manufacturer Composition Mesh Density Hardness Size (g/cm3) (%) Sofnolime® Molecular Soda lime 4-8, 0.90 90±3 Products, 8-12 Ltd. SodaSorb® W.R. Grace Soda lime, 4-8, 1.18 92-95 Co KOH 6-12 HP W.R. Grace Soda lime, 4-8 1.18 92-95 SodaSorb® Co KOH DiveSorb® Dräger Soda lime 5-9 0.83 ≥75 DrägerSorb® Dräger Not approved for use in diving systems Intersorb® Intersurgical Not approved for use in diving systems Spherasorb® Intersurgical Not approved for use in diving systems Amsorb® Armstrong Not approved for use in diving Medical systems Lithium Molecular LiOH 6-14 0.38 ≥95 hydroxide Products, Ltd. Lithium hydroxide (LiOH) is used when weight, bulk, and temperature are prime considerations. Unlike other absorbents, LiOH will chemically scrub even in very cold environments. LiOH is used as the absorbent in spacecraft because of the large quantities of CO2 it will remove per unit of weight, and the favorable temperature considerations. It is a powerful alkali, and is much more efficient than

soda lime at scrubbing CO2. However, it is also much more expensive, and more hazardous. LiOH dust is exceptionally corrosive, and if breathed may cause serious injury. This is also the case if the rebreather floods, as a LiOH caustic cocktail is particularly damaging. For this reason, LiOH is rarely used in diving applications. Dust content primarily determines how corrosive an absorbent will be, either in dry form or solution. Dust, because of its small size, will migrate easily around a system and readily dissolve in water. If a manufacturer's absorbent has a high dust content and floods, a very corrosive liquid will result. However, if the same absorbent has the dust removed first, and then is mixed with water, the result may well be a solution that is only mildly caustic initially. The solution will become increasingly caustic as more absorbent dissolves. So, manufacturers producing absorbents for diving applications have stringent control measures to remove dust from the absorbent prior to packaging and shipping.

Figure 7.23: Color indicators allow one to observe absorbent status (unused on left, used on right) Courtesy of Elaine Jobin

However, even products that have no dust initially may contain some by the time of your purchase. During packaging, transport, and canister filling, mechanical agitation occurs. The greater the agitation, the more powdering will occur as the grains of absorbent grind together. If the absorbent is produced in a softer formulation, then more dust will be formed than if the end product is harder. Hardness is measured as a percentage of material that remains after a proscribed mechanical agitation test. The closer to 100%, the harder and better for diving applications the absorbent. Absorbent grain shape will also factor into dust production, as grains with sharp edges will have a higher propensity to fracture. To combat post-manufacture production of dust, some rebreather manufacturers incorporate dust traps or foam pads in their canisters. As a user, treat unused absorbent with care to prevent crushing or agitation and fill canisters outside in a breezy area, allowing excess dust to blow off during the process. Some absorbents are made with a color indicator that changes color as the absorbent is consumed (Figure 7.23). In some rebreathers this feature adds to the utility of the absorbent. However, if you cannot directly observe the absorbent diving use, color indicators have limited value. Once removed from use, color reversion may occur. Essentially, the pH of the material may change shortly after usage ends, and the color changes back to white. Just because an absorbent containing a color indicator is white after use does not mean it is usable on a future dive. Besides chemical composition, dust content, and hardness, the size of the absorbent grains is important in diving utility. Grain mesh size states the number of absorbent grains that will fit into a given area (Figure 7.25). The higher the number, the smaller the particles. Smaller particles have a greater surface area, increased scrubbing efficiency and greater CO2 absorption capacity. In fact, 8-12 mesh size absorbent can scrub as much as twice as much CO2 as 4-8 mesh size (the two most commonly sold sizes for diving applications), but will cost more.

Figure 7.24: The purple color indicates that this prepacked granular absorbent cartridge is about 1/3 consumed. Courtesy of David Pence

The smaller mesh size has a higher WOB, however, since the space between the grains is smaller. This increase in breathing resistance may be felt at the surface in some canister designs, but in others may not be noticeable until used at depth. You should utilize only grain sizes recommended by the manufacturer of your rebreather. The WOB with all absorbents increases as the absorbent is used. The CO2 scrubbing process causes the grains to increase in volume in such a way as to reduce the interstitial spaces. This occurs at a level too small to see, but impacts WOB. The gas has smaller passages through which to travel, so resistance goes up, as does WOB (Figure 7.26, page 204).

Figure 7.25: Different mesh sized absorbent grains.

Figure 7.26: Absorbent grain spaces before and after use.

Some rebreathers use solid state scrubbers (Figure 7.27). This solid absorbent has none of the problems associated with powdering or dust. It also avoids any potential problems with channeling, as the absorbent material is fixed in a hard matrix. These benefits

substantially improve safety and reduce the potential for user error. Currently, these cartridges are more expensive and less available than traditional granular absorbents, but are much more convenient to use. A hybrid solution for granular absorbents is also being pursued by some manufacturers. Granular absorbent is being pre-packed under tension in plastic sealed units. Even though the absorbent is granular, it is used as a single unit, like the solid scrubbers. This has similar advantages of convenience and reduced user error potential, but the same disadvantages of higher cost and limited availability (Figure 7.24, page 203).

Figure 7.27: Solid state scrubbers, sometimes called Reactive Plastic Cartridges (RPCs), virtually eliminate dust. Courtesy of Micropore

As mentioned, proper absorbent care and handling is vital. Minimize powdering by storing absorbents in hard containers, or if in bags, inside crush-proof boxes. Many manufacturers sell their absorbent only in hard containers for this reason. If you pack for a dive trip improperly, you could find that the $200 you spent on absorbent was wasted, because by the time the airlines get your gear bag to you, the absorbent is literally dust. Stored absorbents should be kept sealed to prevent contamination and to maintain proper moisture content. Most diving grade soda limes contain 17-19% water by volume. If left exposed to

the open air, scrubber compounds can lose or absorb more water (and CO2), reducing their effectiveness. If you leave the absorbent in the sun or another hot place, the required moisture will evaporate, leaving the compound ineffective. As a rule, you should not leave your filled scrubber canister in your rebreather for long periods. This leads to powdering in the canister resulting from transport. Many units inadequately isolate the absorbent. If you must save absorbent in your canister, follow the manufacturer's recommendations for doing so. Most manufacturers either have you seal the canister ends with special caps or plastic, or remove the canister from the rebreather and seal it by redundantly enclosing it in two 4 mil plastic bags.

Lubricants

Some components of rebreathers must operate in a high FO2 environment. A general rule is that any part that may be exposed to high pressures and a FO2 of 0.4 or greater is a concern. These components should be lubricated with oxygen compatible grease, such as Christolube® or Tribolube®. Different lubricants should not be mixed on a single o-ring.

Oxygen Sensors

Some rebreathers have oxygen sensors in the unit. These give a readout of PO2 in the breathing loop so that you can monitor what you are breathing. They allow you to confirm the gas calculations performed prior to and during the dive. (Figure 7.29) Essentially, an oxygen sensor is an oxygen-powered battery. A typical galvanic sensor is comprised of a gold cathode and lead anode submerged in a potassium hydroxide electrolyte. As oxygen passes across the sensor membrane, it combines with the lead to form lead oxide. The higher the concentration of oxygen, the faster the reaction, and the greater the electrical current generated. The current is read on a scale calibrated to show the PO2 at the sensor (fuel cell). The reaction varies greatly with temperature. For that reason, sensors contain a temperature compensation circuit. Typical accuracy of a sensor is ±1% FO2 at 1 ata.

Figure 7.28: Oxygen-compatible grease should be used on all orings used in high pressure, high FO2 environments.

Courtesy of Elaine Jobin

Figure 7.29: Some rebreathers use as many as four redundant oxygen sensors. Courtesy of J. Ian Martin

Sensors are made with different reaction times—the faster the better, generally, but a sensor with fast reaction time usually has shorter life. Reaction times at standard temperature (70°F, 21°C) range from 7 to 30 seconds. If your rebreather has a 30-second

response sensor, then your PO2 can drift substantially before you receive any indication of it. You can drift outside the acceptable PO2 range for 30 seconds or more.

Figure 7.30: Oxygen Sensor Cross-Section.

Many factors can cause sensors to give inaccurate readings. Ambient temperature is one. Although temperature compensation circuits correct for this, the time it takes for the sensor to reach the correct PO2 can vary greatly. A sensor which normally operates with a 6-second response time at standard temperature may take more than 20 seconds at 39°F (4C), or less than 0.5 seconds at 104°F (40C). Thus, placement of the sensor in the rebreather is an important design consideration. Another placement concern deals with gas homogenization. Sensors should be placed so that they are reading after any supply gas(es) have been added and adequately mixed within the breathing loop. Optimally, it would "read" the gas just before it is supplied to the diver to breathe.

In many rebreathers, the sensors are placed to minimize the chance of their getting wet, which could cause false readings. Some manufacturers put a PTFE membrane across the sensor face. This will keep the sensor face dry, but can also slow response time. Many actions damage sensors. Touching the face of the sensor can leave contaminants that impair function. Excessive CO2 will cause permanent damage. Even the amount of CO2 in an exhaled breath is potentially damaging. Prolonged heat exposure (>110°F/43C) may cause electrolyte to vaporize, allowing the moisture to pass through the gas permeable membrane. If electrolyte does evaporate or leak from the sensor, the sensor will not work. Secondary damage from alkaline electrolyte fluid leakage could be extensive. Electrolyte is extremely alkaline and toxic, and should not be ingested or inhaled. Even if there are no leaks, sensors can be permanently damaged by heat. Avoid leaving your rebreather in direct sunlight. Temperature extremes of any kind are harmful. One myth is that sensors should be stored in a refrigerator. This is not true, and will actually necessitate an adjustment period of several hours after removal before the sensor will provide stable, accurate readings. Sensors may be kept sealed in a low PO2 environment when not in use. If stored in an inert environment (as in 100% nitrogen or for a long period in an airtight container originally filled with air), the sensors should be open to atmospheric gas for several hours prior to use. Atmospheric oxygen will contribute to the decay of the sensor, but will nullify the need for a post-storage adjustment period. When shipped from the manufacturer, the sensors may be shorted to prevent decay. Do not remove the shorting pin until you are ready to install the sensor in the rebreather. New sensors will also require a two hour adjustment period prior to use. Depending on their exposure to elevated PO2, sensors must be replaced after 6 to 24 months. Sensors are delicate. The lead anode may be attached with a very fine wire. If you jar the sensor, or throw it around, it is possible to break that wire, ruining the sensor. Even if the wire does not sever, a

small piece of lead may separate from the anode. This will shorten the life of the sensor.

Figure 7.31: Manufacturer sensor test label

Ease of calibration is another feature to consider. Since the sensors should be calibrated as close as practicable to the pressure at which they are planned to operate, determine how easy it is to calibrate them at that pressure. Does calibration require a pressure chamber? Can the FO2 be raised to the appropriate level? How easy is it to access the calibration components? Some sensor manufacturers provide test results with every individual sensor. (Figure 7.31). This shows that they were operational at elevated PO2 (usually 2 atm) at the time of manufacture. Sensors deteriorate over time. The safest way to ensure they are still functioning properly is to test them in a pressure chamber. (Figure 7.32)

Figure 7.32: Pressure chambers may be used to test sensor function to 2 atm PO2.

Courtesy of Robert Landreth

With the number of things that can go wrong with oxygen sensors, some manufacturers believe that the rebreather should include redundant sensors. Some rebreathers have three or more. If more than one is used, the diver needs a means of determining which to believe. Usually, the three readings are averaged, or if not all in close agreement, the reading furthest from the average is eliminated. The accuracy of a single sensor can be determined by comparing a known gas at a known ambient pressure. This will be explained in detail in Chapter 12. A word of caution: Some rebreather divers have used inert gases other than nitrogen in rebreathers, primarily helium. Some sensors are not designed for use in a helium environment. Problems with membranes and other components may occur if helium is used to replace some or all of the inert gas, causing inaccurate readings or ruining the sensor. It is also possible to "bend" sensors during rapid ascents, with nitrogen- or helium-based gas mixtures. This text does

not attempt to address the issues involved in using rebreathers at depths exceeding 130 fsw (40 msw). Before you attempt to use rebreathers at such depths, get training to do so.

Displays

Information from the oxygen sensors is provided to the diver on a display. There are a variety of displays in use, including mechanical, digital, and LED. There are also a variety of forms and placements common on rebreathers, including wrist mounts, HDDs, HUDs, inmask LCDs, and buddy mounts. Mechanical displays work by directly reading the voltage generated by the oxygen sensor. Essentially they are voltmeters, but instead of displaying volts, display atm PO2. They have the advantage of not needing another power source to function properly, but are prone to damage caused by mechanical shock. The readings they provide are commonly affected by orientation, so to read the display it must be in the same position it was in when you calibrated it during pre-dive preparation. Digital displays are much more robust. Because they do not rely on mechanical movements, they are less prone to damage from rough handling. They work essentially like digital voltmeters. Like those instruments, they rely on batteries to power the display. If the battery dies, you are unable to see what the sensors are reading. Some displays use light emitting diodes (LEDs) to indicate PO2. They may provide information on PO2 to 0.1 atm, using different color LEDs, a blinking pattern, or a linear series. Other LEDs may act as "idiot lights," changing from one color to another only when the PO2 in the system becomes unsafe. Generally LEDs are very sturdy, and because they draw low power usually do not die due to power drain. They do, however, still rely on an electric power source to function. Divers are used to carrying instrumentation on their wrists. This is a common place to place depth gauges, dive computers, compasses, or other items. It is also a common place to wear rebreather instrumentation. Primary displays, including mechanical,

digital, or LED-based indicators, are frequently designed to be wristworn. It is easy to refer to a display so placed, which rebreather divers must do every several minutes (or more frequently).

Figure 7.33: Rebreather oxygen displays may be mechanical, digital or use LEDs

OC divers also wear instruments on hoses. Again, hose or pendant-carried instrumentation is common with rebreathers. They have the advantage of being less constrictive than wrist mounted units, but are often more difficult to locate and monitor. If they are not adequately secured, they may drag on the bottom and become entangled, or damaged by striking against objects. To assist divers in frequently monitoring their PO2, indicators are being sited where they are in view of the diver during the entire dive. Head down displays (HDDs) are frequently attached to mouthpieces or breathing hoses where the diver can look down on them from their mask. Head up displays (HUDs) perform the same

function, but are located in the mask itself. They typically display information on the mask faceplate. Usually, both HDDs and HUDs utilize LEDs to indicate the gas content in the breathing loop. Another less common method to display the data uses a small liquid crystal display (LCD) or video screen to show PO2. Some manufacturers actually incorporate these displays into the dive mask. All display options that do not require the diver to manually look at an instrument are potentially much safer than those that must be manipulated to be read.

Figure 7.34: HDDs make it easier to monitor PO2 as the display is within your field of vision at all times. Courtesy of Jill Heinerth

If you forget to regularly monitor PO2 in some rebreathers, it is possible for the gas in the breathing loop to become hypoxic. In this event, you may not have the mental acuity to read your display, or to process the information if you do. Recognizing this, some rebreather manufacturers are installing buddy displays. These displays are

situated where they may be easily observed by your dive partner, who can then assist you should your breathing gas impair your function. Alarms may be incorporated into some rebreathers to alert you of unsafe situations. They may warn of high or low PO2, sensors that do not agree, excessive oxygen or nitrogen, unsafe ascent rates, low batteries, or other hazardous conditions. Rebreathers have been made with audible, visual, and vibrating alarms. Some systems now under development will automatically change from closed to OC when gas in the loop becomes unsafe to breathe.

Carbon Dioxide Monitoring

Excess CO2 (carbon dioxide) has many adverse physiological effects. Besides hypercapnea, CO2 increases susceptibility to CNS oxygen toxicity, deep-water blackout, and other problems. Because channeling, poor absorbent canister packing, temperature considerations, and normal use can cause time to breakthrough to greatly vary, CO2 monitoring has been a priority for many agencies and manufacturers. Two methodologies have been developed to do this, absorbent bed monitoring and direct gas analysis. Absorbent bed monitoring relies on the fact that the chemical reaction which removes CO2 is exothermic. That is, as the absorbent is used, a heat front travels the length of the absorbent bed. A "temperature stick" containing a series of thermistors (digital thermometers) is placed along the length of the bed. By measuring the temperature differentials between thermistors, a rough estimate of used absorbent can be made. There are several problems with this approach. If the entire rebreather is heated prior to use, for example by diving in hot climates, temperature differentials may be insufficient for the system to function. The primary potential problem with this method, however, is that CO2 is not measured directly. In some cases, for example with breakthrough caused by channeling, the heat from the bed may only be 50% complete, but there is already sufficient CO2 bypassing the bed to cause physiological problems. Absorbent bed monitoring will not detect this condition.

Direct monitoring of PCO2 of the gas downstream of the scrubber can detect this situation. Most of the CO2 sensors available use infrared light for detection. Infrared (IR) LEDs generate light at a wavelength that is absorbed by CO2. This IR light is passed through a gas chamber, and is read by a sensor on the other side of the chamber. When no CO2 is present, then all of the IR light is received. The more CO2, the less light is received. This signal is then converted into a corresponding PCO2 display. Traditionally, the challenge to making this system successful has been the fact that the breathing loop is very humid. Moisture commonly condenses on all surfaces. Condensation on the sensor lens leads to inaccurate readings. The surface can be kept dry numerous ways, but most require relatively high power consumption. Advances in sensing technology have recently resulted in an operational CO2 direct gas analysis system for rebreathers. With future refinements, we can expect to see a wider implementation of this technology in the marketplace. An alternate method that has been explored uses a pH indicator. Essentially, a "litmus paper" gel is placed in the breathing loop. When PCO2 begins to climb, the acidity of the gas begins to change. This change alters the gel, indicating the presence of CO2. The indicator must be replaced after a certain level of change has occurred. This method has not yet been successful in rebreathers, but may be in the future.

Full-Face Masks

Two hazards of rebreathers are hypoxia and oxygen toxicity. Either may result in unconsciousness. During the period of unconsciousness a standard mouthpiece will usually fall from the mouth, resulting in drowning. Some rebreather divers are using fullface masks (FFM) to minimize this risk. FFMs have the added benefit of keeping the face warmer, important in exceptionally cold water. However, a FFM must be specifically designed or modified for use with rebreathers. Standard FFMs are designed for use in OC applications, often in a free flow mode. Fresh gas is always renewing

the volume of the FFM. This is not true with rebreathers. As a result, CO2 may be retained, building to unsafe levels within the FFM. The problem is compounded with depth, and is worse for males with beards. At a minimum, a FFM used with a rebreather should have a mouthpiece or oral-mask chamber capable of isolating breath exhalation from the FFM.

Figure 7.35: Full-Face Mask (FFM)

Courtesy of Myfanwy Rowlands, OTS, and AP Diving

Communication Equipment

With practice it is possible to speak into a rebreather mouthpiece and be understood by your dive buddy. However, this only works when the divers are within a few feet of each other. Electronic communications equipment has been developed that eliminates this

limitation. Modern communication equipment will allow divers to speak to each other under water at distances to three miles (4.8 km). This type of gear is commonly used by military, commercial, and search and rescue divers as well as professional underwater photographers. Recent advances in microcircuitry have brought the cost of this equipment into the range of the serious recreational diver.

Figure 7.36: Both divers above are using communication equipment with their rebreathers, the one on the left with a FFM, the other with a half mask (upper). The Buddy Phone earpiece (left) attaches to the mask strap, allowing the diver to hear a

dive partner.

Courtesy of OTS and Marty Snyderman

Electronic communications equipment consists of two primary components, a transmitter and a receiver. These may be combined into a single unit called a transceiver. A microphone must be contained in a volume chamber near the diver's mouth. This may be in a FFM, in a half mask that covers the mouth area, or incorporated into the rebreather mouthpiece. When the diver talks, a signal is generated that is transmitted through the water. The buddy's receiver picks up this signal. A speaker worn near the buddy's ear allows him to hear what was said. Since rebreathers make little bubble noise, if any at all, communications are easily heard (Figure 7.36).

Dive Computers

Calculation of inert gas status is a primary planning consideration. Decompression theory and table use for rebreathers was addressed in Chapter 6. Rebreather dive computer use follows the same basic parameters: Plan for the highest PO2 and PN2 that you might experience. With a mass flow SCR, for example, you would use air to calculate the nitrogen loading, as that is the lowest FO2 likely to be in the breathing bag. Therefore, you could use an air computer for calculating inert gas uptake, even though under resting conditions the actual FO2 in the counterlung would probably exceed that by a comfortable margin. In the same mass flow controlled SCR, you would calculate oxygen exposure on the FO2 of the supply gas. So you would set your FO2 on your nitrox computer to that value to track percent CNS and OTUs. This is obviously much higher than you would generally be breathing, but you must plan as if this is the value. These methods are very conservative, but are required if there is no sensor in the breathing loop to actually measure PO2.

Figure 7.37: Dive Rite's NiTekX® computer has an optional external oxygen sensor and constant PO2 modes for use with CCRs. Courtesy of Dive Rite

Figure 7.38: Liquivision's X1® dive computer allows you to upload different decompression software models. Courtesy of Kevin Lee

Several computers are available that would eliminate this problem. Uwatec's Oxy-2® gauge may contain either one or two oxygen sensors. (Remember that these sensors have all of the same inherent problems as those discussed above.) The sensor(s) are installed on the inhalation hose, the best location for reading the PO2 of the breathing mix supplied to the diver. Using wireless technology, this unit communicates to the Uwatec Aladin Air X-O2® wrist computer, provides several oxygen alarms, and integrates nitrogen uptake. This computer may also be used in OC mode, with the proper transmitter. Cochran's Lifeguard® computer incorporates a single oxygen sensor and a nitrox dive computer. The oxygen sensor must be installed by the user in either the mixing area of the breathing loop or in the counterlung. It communicates to the computer via a pulsed magnetic signal. The sending unit contains a pressure transducer for depth determination. The wrist unit includes the display read by the diver. Optional personal computer interfacing and software is available, allowing the diver to download dive profiles and other data. DiveRite's NiTekX®, VR Technology's VR-3®, and Hydrospace Engineering's Model O Explorer® dive computers have a single oxygen sensor option. The sensor connects to the computer via a wire. All allow you to switch from among as many as 7 to 10 gases, including constant PO2 and constant FO2 modes. One of the most interesting dive computers available is Liquivision's X1® dive computer. It has an open architecture which allows you to program it with software from multiple manufacturers, like Decosoft® or V-Planner Live®. This allows the user the option to select the preferred decompression or modeling algorithm, and change it later if desired. The X1® also has an optional external sensor add-on allowing monitoring of as many as four oxygen sensors, and computes inert gas loading for OC or closed circuit systems. New computers are regularly coming to market, so do some research on your own. Remember that extensive constant PO2 computer databases do not exist, and that other computers may be

developed for rebreathers that rely on little or no actual diving data. Be wary of untested algorithms.

Figure 7.39: A proper fitting harness will keep your equipment secure, even during rough entries.

Case, Harness, & Buoyancy Compensator

The diver must carry all of these components in some way. Usually, the entire system is enclosed in a case or housing. This minimizes damage should you accidentally hit anything. Most manufacturers use a strong, lightweight, plastic shell for this purpose. The harness allows you to carry the case. Regardless of design, it should permit the unit to be carried comfortably, snugly, and securely. A harness that minimizes restriction of movement of the arms is preferable, so that you can easily handle equipment such as lights, reels, lift bags, or bailout bottles. The harness may include attachment points for supplementary equipment. These are often desireable for attaching accessories such as lights, game bags, slates, cameras, etc. The harness may

incorporate quick-release buckles or clips, to facilitate donning and removal. Harnesses often have small bellows pockets high on the shoulders. These facilitate adding small weights (to 5 pound/2.5 Kg per pocket) to the harness. This allows you to counteract some of the additional lift created by the counterlung, and to achieve neutral trim. Finally, all units require the use of a BC. This may be separate from the rebreather, or integrated. Generally, integrated units are preferable, as they are designed to allow you to access all controls without conflicting straps, control placements, etc. The harness may include an integrated weight system. If so, ensure that the weights can be easily dropped in the event of an emergency.

1. Draw an axial flow and a radial flow scrubber canister, showing the gas flow direction.

2. Counterlung placement can affect the diver by: a. Altering breathing resistance. b. Increasing drag while swimming. c. Making it more difficult to equalize. d. All of the above.

3. Work of breathing may be affected by: a. Scrubber canister design. b. Counterlung placement. c. Flooding of the scrubber canister. d. None of the above. e. A, B, and C above.

4. A water trap:

a. Traps water before it enters the counterlung. b. Minimizes the amount of water entering the scrubber canister. c. Prevents the possibility of a caustic solution forming. d. All of the above.

5. Breathing hoses should: a. Have a small internal diameter, to minimize the amount of water if flooded. b. Be flexible to allow for turning and minimize mouth/jaw fatigue. c. Be weighted to improve diver trim. d. Have smooth internal bores to minimize germ and bacteria growth. e. All of the above.

6. Carbon dioxide absorbent: a. Has low reactivity, and may be safely handled with bare hands. b. Is better if it is hard, and thus less prone to mechanical grinding and breakage. c. May lose effectiveness if left in direct sunlight for extended periods.

d. B and C, but not A, above. e. A, B, and C above.

7. Oxygen sensors should: a. Have a slow reaction time, because that extends the usable life. b. Be packed in a manner so as to minimize shocks during transport. c. Be calibrated by blowing on them prior to the dive. d. Be kept in a low oxygen environment when stored for long duration. e. B and D, but not A and C, above.

8. Rebreather mouthpieces: a. Have mushroom valves in them that control the gas flow direction. b. Must have a means to open and close them. c. May not have a purge valve built into them. d. All of the above.

Preparing for the Dive Overview

Pre-dive planning and equipment preparation is, if anything, the most critical part of rebreather diving. Dive planning includes many concerns. With rebreathers, you are responsible for a greater degree of equipment assembly and preparation than with standard scuba gear. Aspects that are identical to those you have learned in previous classes will not be addressed. Different makes and models of rebreathers require different predive assembly and testing. This chapter provides general information on pre-dive equipment checks, but does not substitute for the detailed pre-dive checklists and procedures provided by the rebreather manufacturer.

Objectives After reading this chapter, you will be able to: 1. Explain the importance of utilizing and following a pre-dive checklist. 2. Explain four concerns when filling scrubber canisters with absorbent. 3. List six elements of preparing the rebreather gas delivery system for a dive. 4. Explain the importance of verifying oxygen sensor operation prior to diving.

5. Describe how to perform positive and negative pressure checks. 6. Desribe final buddy checks to perform immediately before entering the water with a rebreather. 7. Relate three briefing items you need to communicate to an OC buddy before diving As this is a generalized overview of pre-dive procedures, some specialized equipment may not be mentioned. Again, it is your responsibility to review the manufacturer documentation that accompanies any specialized rebreather components prior to use. These components may also require additional training from an authorized instructor as well, as such training is beyond the scope of a basic rebreather certification course. Appendices contain pre-dive procedures for many rebreathers. While these are current at the time of this publication, manufacturer changes may alter them. Therefore, utilize these procedures as guidelines only, and always check manufacturers' current instructions.

Pre-Dive Checks

In reviewing accidents that have occurred with rebreathers, one fact consistently recurs: the incident could have been avoided if the diver had properly prepared his unit. In nearly half of all rebreather fatalities, the failure to perform one or more preparation tasks contributed to or directly caused the demise. Using a pre-dive checklist would have prevented these needless deaths. More on accident analysis may be found in Mastering Rebreathers, Volume 2. Conduct your rebreather pre-dive by following a checklist. Do not rely on memory to prepare for the dive. At this level, it is far too easy to inadvertently omit a step. Even as an experienced rebreather user it is not uncommon to forget to conduct some pre-

dive tasks working from memory. Use a checklist and manually mark items off as they are completed! Distractions make it easy to skip part of pre-dive checklists, or forget to complete a given item. Try to control distractions by performing the predive of your rebreather away from other divers who may be unfamiliar with rebreathers. This is especially important when you are diving from open charter boats, where the confusion factor is already high. It is even more important if you are with advanced OC divers, who will most likely be very interested in your rebreather. As an experienced rebreather diver, it is very easy to become complacent about pre-dive checks. After all, you do not use a checklist to prepare OC equipment, so why here? Because rebreathers are much more complex than OC equipment. In fact, rebreathers are closer to aircraft than OC gear in nature. No prudent pilot would step into an aircraft and take off without pre-flight inspections. You should regard rebreather diving the same way. If anything, pre-dive checklists become even more important as you gain experience, as they are an excellent tool to avoid the effects of complacency. Remember— it is your death you are preventing!

Canister Filling

Canister filling is an art. Packing yours properly will allow you to enjoy a long dive. Packing it improperly may lead to early breakthrough, CO2 buildup, or other problems. In fact, the consequences of incorrect filling are so great it led John Brooks, former Diving Safety Officer for the Intermountain Region of the National Park Service, to proclaim, "Packing a scrubber canister is like packing a parachute!" During your course, your instructor will demonstrate the best way to fill your scrubber canister with absorbent. Because of the granular nature of the absorbent, and the tendency for the grains to settle with agitation, you can always expect some variations in precision, but following a consistent protocol will help you fill the canister correctly.

Always begin by placing the canister on a clean surface. As part of the breathing loop, the canister must be considered part of your lungs. If an appropriate surface is unavailable, then a clean towel placed over an available surface will suffice. When possible, canisters should be filled outdoors where there is a mild breeze blowing (Figure 8.2). If no breeze is available, an electric fan may be used to artificially create one. Pour the absorbent from its storage or shipping container into the canister from a height of twelve inches (35 cm). This will allow much of the dust in the absorbent to blow away, like chaff being separated from the grain in wheat processing. The less dust you have in the canister, the more efficiently CO2 will be removed, and the less chance you will have of a problem with caustic cocktail or dust migration to your mouth and lungs.

Figure 8.1: Always use a pre-dive checklist when preparing your rebreather.

Figure 8.2: Allow a breeze to blow away dust while filling your canister with absorbent.

Some individuals are more susceptible to absorbent dust than others. If you find this to be your case, protective clothing like gloves, long pants and sleeves, and dust masks of the type used during painting may insulate you, preventing problems. Everyone should remain upwind of the filling process to minimize exposure. Always wash your hands after handing absorbent, so residual dust from your fingers is not transferred to your eyes or other body parts. Do not allow any moisture to enter the canister at this time. It will increase the possibility of caustic cocktail, and reduce the effectiveness of the absorbent. While filling, take time to lightly pack the grains. At about onethird full, and again at about two-thirds full, gently agitate the canister to allow settling (Figure 8.3). This can be accomplished by tapping the side of the canister with your open palm, or in some canisters by tapping the bottom lightly against a firm but yielding surface (such as a towel on a tabletop).

Figure 8.3: Lightly tap your canister during filling to settle the absorbent.

Some canisters have recesses or holes adjacent to the part of the canister that must be filled that should not have absorbent in them. Since you are filling from a height above the canister, you probably do not have the control to prevent this by your pouring technique alone. You can, however, often prevent absorbent from entering these recesses by blocking them off with a plastic cap or paper towel before starting the filling process (Figure 8.4). Remove all blockages prior to assembly, of course.

Figure 8.4: A paper towel will prevent absorbent from entering the wrong place while filling in some canisters.

Some canisters are designed incorporate a gasket or dust pads to be installed during the absorbent filling process. Gaskets prevent gas channeling along the smooth sides of the canister. Dust pads reduce dust migration through the system. If your rebreather uses such, insert them as recommended by the manufacturer (Figure 8.5). Finish filling the canister, and again pack lightly. If your canister uses a compression pad, spring-loaded lid, or another assembly to hold the absorbent under pressure, install it now. If not, you may need to install the lid, tap lightly against a firm surface, and then reopen the canister to add a small amount of absorbent. Once the canister is filled, shake it gently near your ear and listen (Figure 8.6). Rattling sounds may indicate loose absorbent. Since this could allow channeling, reopen the canister and add more absorbent, or repack it.

Figure 8.5: This rubber spacer gasket must be installed during the filling process to prevent wall channeling.

When you have filled the canister, install it into your rebreather and clean any loose absorbent or dust from o-rings or sealing surfaces. Some axial flow canisters should have the dust removed from the exit side of the unit. To accomplish first wipe the gas ingress side with a clean cloth to remove any surface dust, then blow through that end to blow out any loose dust. Then carefully clean any dust from that end with a cloth. Avoid skin contact with the absorbent dust.

Figure 8.6: Check a filled canister for rattling, a sign of poor packing.

If the ambient temperature outside is less than 40°F (4C), minimize exposure of the absorbent to the cold prior to use. As the temperature drops, absorbent efficiency is reduced. In some cases, if it is too cold, the absorbent will not work at all. This is because the initial temperature is too low to allow the chemical reactions that absorb the CO2 to begin. But if the absorbent is kept warm, the reaction can be started prior to beginning the dive (by pre-breathing from the rebreather for several minutes). The reaction will continue at that point once you enter the water because the heat generated by the scrubbing reactions will keep the absorbent warm.

Gas Supply System

The first step in preparing the gas supply system in your rebreather is to install the breathing hoses and mouthpiece. Remember that the mushroom valves in the mouthpiece control the direction the gas flows. If you install the hoses and mouthpiece backwards, the rebreather will not operate as designed. Some rebreathers have keyed breathing hoses to prevent this problem, but others do not, so be sure to install them correctly! First, open the mouthpiece. Inhale and exhale through it. There should be no odd noises. A "mooing" sound generally indicates a mushroom valve needs replacement. Now cover the open inhalation end of the hose. If you can inhale gas with this end covered (and you are certain you have covered the proper end), it means the exhalation mushroom valve is leaking or there is a tear or puncture in the hose. Cover the other hose end, and exhale. If you can exhale and the hose is fully intact, then the inhalation mushroom valve should be replaced. If both mushroom valves are operating properly, install the hoses onto the rebreather in the proper fittings. Before you install your filled high-pressure gas cylinders, analyze the contents for oxygen content. Using the wrong nitrox or diluent in the system may lead to hypoxia or CNS oxygen toxicity, and subsequent drowning. As with all EANx diving, it is your responsibility to ensure that you have the gas intended for use in

your rebreather (Figure 8.7). Carry a backup analyzer if traveling, in case your primary one fails.

A Tale from the Frozen Hinterlands

The star of this story was diving in Wales, which, at the time of the adventure, was a frozen land, cloaked in snow and ice. Our hero pulled his equipment from the car, piled it on the ground (including his rebreather), and prepared for the dive. Unfortunately, one of the other divers had a problem that took several hours to solve. Our hero's rebreather, in the meantime, grew its own mantle of white, as the snow continued to fall. Finally, the problem solved, our protagonist donned his rebreather and entered the frigid waters of Dorothea Quarry. As he slid beneath the surface, he opened his mouthpiece, and began to breathe. He quickly settled to the bottom in 67 ffw (20 mfw). After a few minutes, he began feeling extremely nauseous and developed a headache. He ascended to the surface and terminated the dive. Upon reaching the surface, his headache had developed into one of raging proportions. Opening his rebreather, he found that the absorbent had frozen, and actually contained crystals of ice. In this case, the absorbent had become too cold for the chemical scrubbing reaction to begin. Two errors were made. (1) The rebreather and its absorbent should have been kept in a warm environment prior to use. (2) The diver should have breathed from the unit on the surface for several minutes prior to beginning the dive, to ensure the reaction had properly begun before going under water. CO2 buildup and a CO2 headache were the result.

If your rebreather uses 100% oxygen in the system, it is especially important to fill properly. A common problem with filling

oxygen cylinders is the use of filling whips that are stored in air. The air inside the whip prior to filling is pushed into the rebreather oxygen cylinder at the beginning of the filling process, introducing nitrogen into the oxygen cylinder. Over time and repeated fillings, the amount of nitrogen can reach significant amounts, especially if the fill lines are long. This problem may be avoided by blowing oxygen through the filling whip just before filling the oxygen cylinder. Confirm that the high-pressure cylinders have sufficient pressure for the dive. As with OC scuba, we strive to end a dive with a set amount of gas remaining. For rebreathers, we surface with at least 500 psi (35 bar) in any gas cylinder. If your rebreather uses more than one gas cylinder, you have multiple submersible pressure gauges to check at this time. Before you completely assemble the unit, check the integrity of all hoses, tubing, piping, and connectors. After pressurizing the system, look and listen for any leaks. If you find evidence of a leak (hissing, bubbling, etc.), purge the unit, and attempt to correct the leak by reconnecting the components in question, or changing userserviceable orings. If this does not correct the problem, terminate diving activity with the unit and have it serviced by a qualified facility before diving it again.

Figure 8.7: Always analyze your gas supply before your dive.

If your SCR uses a mass flow controller, you have additional steps to complete. Most mass flow controllers have several options as to which orifice or sonic valve is used. These correspond to different EANx gas supplies. Ensure that the sonic valve or orifice in use matches the gas in the gas supply cylinder. For example, the Dräger Dolphin® has orifice options for EAN32, EAN40, EAN50, EAN60, and EAN80. If you are using EAN50, use the associated orifice. If you have a nitrox mix between two orifice options, use the orifice for the nitrox with less oxygen (see Example 8.1). Example 8.1 Marvin is using a Dräger Dolphin®, but has EAN 48 in the supply cylinder. Which orifice should he use in the mass flow controller? Solution: The EAN40 orifice. This will allow sufficient oxygen partial pressure for the dive, even though the nitrox he has is closer to the EAN50 option. Because he has EAN48 instead of EAN50, he will have reduced dive time, as the gas will flow at a faster rate.

Figure 8.8: Checking a mass flow controlled SCR with a flow meter.

Once you have installed the proper orifice, check that the flow rate is correct. This may be done in two ways. The most precise is with a calibration bag. This is a specially designed bag of precise volume that is attached to the mass flow controller as a pre-dive check. The mass flow controller fills the bag at a constant rate. Time how long it takes to completely fill the bag. That time must fall within the parameters provided by the manufacturer of the mass flow controller. If it does not, then do not use the SCR until it has been serviced. The second method for confirming gas flow is by using a flow meter, which attaches to the mass flow controller (Figure 8.8). It measures the flow, which must match the manufacturer's specifications. Many divers have problems using the flow meter on SCRs not designed to provide absolutely uniform flow. Small flow variations may cause oscillations in the flow meter. Flow resonance may cause these small oscillations to rapidly become severe,

making it impossible to read the flow rate. This problem may be minimized by opening the gas supply valve on the SCR very slowly, or by inserting a length of rubber tubing between the flow meter and the mass flow controller to dampen flow variation. You can also use your finger to dampen variations by partially blocking the exhaust side of the flow meter for a few moments, until the flow ball remains stable. Then remove your finger to take the reading. As covered in Chapter 6, proper operation of the sonic valve is dependent on the pressure input into the valve. If this pressure is reduced, then the valve will not operate at depth as designed. Check the interstage pressure prior to the dive to verify that the regulator is providing the proper pressure. Or, if your rebreather includes an oxygen meter, this check should be performed monthly. If not, then check the interstage pressure before every dive. You may need to measure interstage pressures for mixed gas CCRs as well. If they are out of range, then the solenoid, ADV, or other devices controlling gas addition to the breathing loop may not function properly. Next, verify operation of all manual gas addition valves. First test to see that the automatic addition valve functions when needed. Then add gas to the breathing loop using the manual override valve. If your system has more than one gas supply, test the automatic and manual injection valves for each gas supply separately. Do not dive with a rebreather that has a malfunctioning valve. If you are using a mass flow controlled SCR and are not immediately entering the water, turn off the gas supply. Otherwise, the supply cylinder will empty, even though you are not diving. Remember, these units flow gas continuously at their set rate regardless of whether or not anyone is breathing on them.

Electronics

If your rebreather has electronics, check them for proper function. Begin by examining all electrical cabling, looking for any damage or wear spots. Then check that all connectors are correctly installed and not loose.

Use a voltmeter to confirm that battery voltages are correct, and install them in the rebreather. It is preferable to test batteries under load. Batteries tested directly (not under load) may appear fine. But as soon as a load is placed on them, the voltage may immediately drop to unacceptable levels. You may be able to test the battery while it is in your rebreather, preferably under highest load conditions (such as with the solenoid firing, if you have one). If you cannot do this, use an appropriate resistor in line to provide a comparable battery load for test purposes. You can also use the voltmeter to confirm output voltage of the oxygen sensors. See that the analog and digital displays are providing reasonable data. In some rebreathers, oxygen sensors and some displays may require calibration prior to each dive. Oxygen sensor calibration should be done at the upper end of the range in which the sensor will be used. First verify that the sensor reads 0.21 in air if you are at sea level (at altitude this figure will differ, as discussed later in this chapter, page 235). Then fill the breathing loop with your highest percentage supply gas available. Inhale all of the gas from the loop, exhaling through your nose. Then completely fill the loop again with supply gas. Repeat this procedure three times. This is called purging the system. Some rebreathers will accomplish a comparable purge for you. It ensures that the breathing loop is filled with the oxygen or nitrox that is in your supply gas, with little or no contamination with the air that was originally in the system. Your oxygen sensor should match the readings you took with your oxygen meters earlier when confirming the gas supply.

Figure 8.9: Oxygen sensor voltage output should be linear over the expected range of use, but may flatten out if the sensor is old.

If you plan to dive using a PO2 greater than 1.0 atm, you may need to calibrate your sensors in a pressurized container. Sensors operate on the principle that the voltage output is linear with increasing PO2. However this linearity flattens out at higher PO2. The point this occurs depends on the design, age, and condition of the sensor. In Figure 8.9, Sensor #2 fails to provide a linear output at a PO2 of greater than 1.2 atm. Any reading of 1.2 atm or above would be inaccurate for that sensor. The others would operate to 1.5 atm PO2. The only way to determine where linearity occurs is to test the sensors in a pressure chamber at elevated PO2 before the dive. Calibrate any displays on your rebreather as necessary. If your system includes a programmable dive computer, check that the gas supply, data logging frequency, tissue model variables, and other parameters that may be set are as you desire.

Buoyancy System

Most rebreathers available for recreational diving have integrated buoyancy compensators. Make sure that yours will indeed hold air prior to diving with it. Confirm that manual inflation and deflation valves operate properly, and that all overpressure valves operate. Connect the low-pressure inflator hose, and try the power inflator to see that it works. If your rebreather also has an integrated weight system, establish that the weights are in place. Check the quick release mechanism to see that it is clear and properly assembled. Many rebreathers have shoulder weight pockets to help provide proper trim while diving. If you use these weights, ensure that they are in place.

Pressure Checks

Two types of pressure check must be performed on rebreathers— the negative pressure check, and the positive pressure check. The objective of these tests is to check the integrity of the breathing loop prior to entering the water. Both are necessary, since some leaks will only react to one condition. Of course, most holes will leak in either condition, but we are trying to maximize safety by catching all leaks. The negative pressure check is done after the rebreather has been fully assembled, and all other tests and checks completed. To perform this test, turn off all supply gas valves, and bleed from the lines any residual gas held under pressure. Then open the mouthpiece, inhale through your mouth, and exhale from your nose. Repeat this until you can no longer inhale, and have drawn a vacuum in the system. At this time, without allowing any gas to enter the breathing loop, shut the mouthpiece valve. This should leave a slight negative pressure in the rebreather. Wait thirty seconds, and then open the mouthpiece valve. You should hear an audible rush of air into the system. If not, repeat the procedure. If you still do not, check all connectors for integrity, confirm that gas supply lines are purged, and look for any leaks in

the breathing loop. Do not dive with a system that will not pass the negative pressure test. The positive pressure test can be done using one of two methods. The easiest involves submersion in water.

Figure 8.10: Bubbles are a sign of trouble during a pre-dive positive pressure check.

Confirm that the supply gases are turned off. Orally inflate the breathing loop until the automatic exhaust or overpressure valve releases gas. Close the mouthpiece valve. Now submerge the rebreather in water (Figure 8.10). If your rebreather has a removable cover or fairing, remove it prior to immersion. Look for any bubbles coming from the breathing loop. Wait a few moments for bubbles caused by dropping the rebreather in water to clear first. If you find bubbling, note the source and correct the problem. If you do not have water readily available, you can perform the positive pressure check another way. Inflate the breathing loop as

before. If you have an adjustable overpressure valve, set it to its maximum setting. Now place a five-pound (2.3-kg) weight on the counterlung for about a minute. The weight should not sink appreciably. If it does, look for leaks (This is less definitive than inwater testing). Note that this alternative positive pressure check cannot be done on rebreathers that have an enclosed counterlung.

General

Some systems include user-serviceable water-absorbent pieces to soak up small quantities of water or condensation. Usually, these are either recyclable chamois pads or disposable paper towels. Water absorbent pieces may be in the scrubber canisters, counterlungs, or battery/electronic compartments. If your rebreather uses them, then dry pieces should be installed as necessary prior to the dive. Some rebreathers contain removable drain plugs or valves on some components. Ensure that these are installed or turned to closed position prior to positive and negative pressure tests (Figure 8.11).

Figure 8.11: Ensure that all drain plugs are in place prior to diving.

O-rings should be greased using appropriate lubricants prior to diving. When assembling o-ring sealed components, be careful to do

so correctly. In some systems it is very easy to pinch an o-ring, allowing water to flood the compartment during the dive. Visually confirm all o-rings are situated properly before continuing in your pre-dive checks.

Altitude Diving

As altitude increases, ambient atmospheric pressure drops. This affects such dive planning factors as inert gas uptake and elimination. As you will learn in the specialty course, altitude diving involves the use of special dive table procedures or tables as a result. We will not discuss special altitude dive tables or related procedures further in this text. However, reduced ambient pressures found at altitude affect rebreather divers in another way as well. Our oxygen sensors measure PO2, not FO2 (i.e., not the percentage of oxygen). Although the FO2 of the air around us remains reasonably constant as we ascend, because the overall pressure is reduced, so is the PO2. Therefore, we must use a conversion table to calibrate our sensors at any altitude greater than 1,000 feet (300 meters). Such a table is provided (Table 8.1). Table 8.1: Oxygen Sensor Conversion Figures for Altitude Diving11

Altitude (Feet) 0-999 1000-1999 2000-2999 3000-3999 4000-4999 5000-5999

(Meters) 0-304 305-609 610-914 915-1219 1220-1524 1525-1828

Ambient Pressure PO2 (ata) 1.00 0.97 0.94 0.91 0.89 0.86

(atm) 0.21 0.20 0.19 0.19 0.18 0.18

6000-6999 7000-7999 8000-8999 9000-9999 10000-10999 11000-11999 12000-12999 13000-13999 14000-14999 15000-15999 16000-16999 17000-17999 18000-18999

1829-2133 2134-2438 2439-2743 2744-3048 3049-3352 3353-3657 3658-3962 3963-4267 4268-4572 4573-4877 4878-5181 5182-5486 5487-5791

0.83 0.80 0.77 0.75 0.72 0.69 0.66 0.63 0.61 0.58 0.55 0.52 0.49

0.17 0.17 0.16 0.15 0.15 0.14 0.14 0.13 0.12 0.12 0.11 0.11 0.10

If you are calibrating your oxygen sensors in air at altitude, then first find your altitude in the left column. Then, read to the right column to obtain the calibration value. If you are calibrating in 100% oxygen, then use the figure in the middle column, as your PO2 will be identical to the ambient pressure for your altitude. If you fail to follow this or a similar conversion chart, it is possible to experience a hypoxic event while diving, and you may also have increased risk of DCS.

Dive Planning

Most of the elements of dive planning for rebreathers and OC scuba are identical. You still need to conduct a site survey, look at conditions, agree on hand signals, establish a dive accident management plan, determine dive objectives and leader, and conduct a buddy check. Some elements differ significantly – specifically, emergency procedures, alternate air source supply, and dive parameter planning. Rebreathers may fail differently than OC scuba systems. Address these potential failures and reactions to them prior to the dive. Emergency procedures are covered in detail in Chapter 12.

If your dive buddy is not using a rebreather, or is not trained in rebreather use, spend additional time briefing him on what to expect, and basic problem solving. In particular, inform him of hyperoxia and hypoxia problems and symptoms. If your rebreather has a display that shows the PO2 in the breathing loop, explain its significance and show how to read it. Tell your buddy that if you experience problems and become unconscious to get you to the surface as quickly as possible, and remove the mouthpiece immediately! Explain how to close the mouthpiece, and why that is important. Otherwise, a successful assist may result in tragedy as you asphyxiate from breathing a hypoxic mix at the surface, or drown due to buoyancy loss. It is nearly impossible to buddy-breathe using a rebreather. Gas supplies in the rebreather may not be adequate for OC bailout. Therefore we carry bailout bottles with an emergency gas supply. Part of dive planning involves determining how large that supply should be (see Chapter 4). Too little gas may result in your being unable to make an ascent to the surface, while too much gas may entail carrying a large, bulky cylinder for the entire dive. The amount of gas you carry will be based on your breathing rate, depth, and any potential overhead obstructions you might encounter (kelp, wrecks, safety stops, etc.). A minimum bailout supply of 20 cf (3 L cylinder at 200 bar) is recommended for recreational, no decompression diving to depths of less than 130 fsw (40 msw) where unobstructed ascents are possible. Dives that do not meet these criteria are outside the scope of this basic text. As soon as you begin diving in environments lacking direct access to the surface, your diving is far more serious. While you may have hours of extra gas based on the anticipated gas consumption in the rebreather, if the rebreather fails you are suddenly very restricted with only your bailout supply. The procedures covered in this text do not cover how much extra bailout gas to carry. (Mastering Rebreathers, Volume 2 addresses these procedures.) Nor do they consider the greater depths involved in decompression diving, the high psychological stress involved, or impacts of increased breathing rates. Even diving at or near the

limits of your dive computer can be considered more hazardous, as momentary inattention or an emergency may cause you to stray into a mandatory decompression profile. For these reasons, selfdiscipline is very important in rebreather diving. Avoid the temptation to exceed your training limitations or your dive plan by "just a little."

Figure 8.12: Select your bailout cylinder based on your SAC rate and the dive depth. Courtesy of the National Park Service and Brett Seymour

As OC divers, we are familiar with the concept of inert gas uptake and use of dive tables. As you learned in Chapter 6, rebreather divers use dive tables differently. In addition, you may have to determine maximum operating depths based on the oxygen content in your gas supply, time limits based on CNS oxygen toxicity, and time limits based on pulmonary oxygen toxicity. If you are diving with a buddy using OC scuba, you may not be able to follow the same dive profiles. Discuss this prior to the dive; compare your two profiles, and follow the one most conservative.

Diving with an OC Buddy

It is common for rebreather divers to partner with OC scuba buddies. They may be unfamiliar with rebreathers, and should be

briefed on a few basics. This will help prevent problems, and in the event of an emergency help them provide move effective assistance. While preparing equipment and during transit to the dive location, you should inform them of any procedures and concerns related to handling the rebreather. In particular, they should be cautioned against setting their cylinders and weight belts on your rebreather, or in a position that may lead to any heavy or sharp objects inpacting your system.

Figure 8.13: You should brief OC scuba partners on basic rebreather emergency procedures before buddying on a dive. Courtesy of Elaine Jobin

You should tell them that during the dive they may see only a few bubbles, or no bubbles at all. You may wish to explain basic rebreather operation to them, so they understand this is normal. The most common diving emergency occurring under water is buddy separation. With OC systems we generally look for bubbles from our partner's exhalations. You must tell your buddy this will not work effectively with a rebreather, and they must surface within a

minute to regain buddy contact. Since the majority of rebreather fatalities occur while the rebreather diver is solo or not in contact with their buddy, the importance of this action cannot be overemphasized. If any other emergency occurs under water, they should be briefed to help you change to OC bailout. You should teach them how to close your mouthpiece, and why it is important to do to maintain bouyancy. If you are found unconscious, you should tell them how to close the mouthpiece, get you to the surface as rapidly as possible, and placed on oxygen on the surface. Finally, they should be briefed to assist you in performing your final immediate pre-dive checklist.

Immediate Pre-Dive Checks

You may have prepared your rebreather for use some time before actually entering the water. As part of that check, you may have left it in a condition inappropriate for diving. This is especially likely with SCRs, where gas supplies are often shut. Thus, you must check your system to see that it is functional immediately prior to diving. This immediate pre-dive check only takes a minute or two, and should be done simultaneously with your buddy after you have donned your rebreathers on every dive. A brief, laminated checklist will aid in this task. Ensure that your gas supply is turned on. If you have more than one gas supply, ensure all of them are on and available. This is especially important for mass flow controlled units, that will have had the gas supply shut off after the pre-dive setup done earlier. If you have manual addition valves, activating them while looking at the respective submersible pressure gauge is the fastest and easiest way to check that the gas supplies are on. Movement of the SPG needle indicates that the gas cylinder may not be turned on fully. Verify that your bailout cylinder is on. Take a breath from the regulator to see that it is functioning. If you look at the submersible pressure gauge while doing so, any needle movement while you are breathing may indicate that the valve is not fully on. Stow the regulator second stage in a location where it is easily accessible and

will not come loose. If you are carrying more than one bailout system, confirm that all are functioning. Locate your buoyancy compensator hose, and place it where it is easily accessible. Confirm that it is functioning by adding a small amount of air. Turn on your electronics, and make a quick check to see that the displays are readable. Look at your oxygen sensor display. Confirm that there is ample partial pressure of oxygen in the breathing loop, generally 0.4 to 0.7 atm. Remember to look at both primary and secondary displays, if present.

Figure 8.14: An open mouthpiece will cause a loss of buoyancy..."glub, glub, blub!"

You should pre-breathe the unit for a minute or so at this time. (Longer if the ambient temperature is low, as long as five minutes.) While doing this verify that the oxygen sensor display is showing appropriate readings, changing as you are breathing, and that the automatic gas or oxygen addition system is functioning. If you are entering the water with your mouthpiece out of your mouth, confirm that it is in the closed position. If you will be using the mouth-piece, insert it in your mouth and open it. Do not remove it

without closing it first! Even if you are not under water, do not leave the mouthpiece open. If you are floating at the surface holding the open mouthpiece above the water, you will have a buoyancy problem. The hydrostatic pressure on the counterlung will expel the gas in the system through the open mouthpiece. This will, in turn, cause a sudden reduction in buoyancy corresponding to the volume of gas lost from the counterlung. This can amount to loss of more than 22 pounds (10 kg) of lift in some rebreathers. Marvin says, "Glub, glub, glub!"

Figure 8.15: Following proper pre-dive procedures will help you enjoy your dive.

Enter the water. Either on the surface, or at a depth of less than 20 fsw (6 msw) perform a bubble check of the rebreather. Carefully look over your partner's rebreather, looking for bubbles. Pay special attention to hose connectors and other connection points of the breathing loop. Then have your buddy do the same for you. Any bubbles are cause to surface and correct the problem.

Finally, it is time to take off, and enjoy your dive.

1. All of the following practices for canister filling should be followed except: a. Keeping the canister clean. b. Capturing all absorbent dust before it blows away. c. Minimize skin contact with absorbent. d. Shaking the grains during filling to settle them. e. All of the above should be followed.

2. Rebreather gas supplies do not need to be analyzed if: a. The dive will be shallower than 20 fsw (6 msw). b. The dive will not require stage decompression. c. You are using only air and 100% oxygen. d. You are using a SCR with EANx procured from a dive store. e. You should always analyze gas supplies.

3. The following rebreather systems should be checked prior to a dive, except: a. Electronics.

b. Gas supply/flow. c. Mouthpiece and mushroom valves. d. Hypoxic flow levels. e. All of the above should be checked.

4. Oxygen sensors should be checked: a. Only if you use mixed gas in your rebreather. b. Before every dive to ensure that they will power the rebreather electronics. c. Before every dive, preferably at a level above the anticipated oxygen setpoint. d. Only if you will be diving deeper than 20 fsw (6 msw). e. All of the above are true.

5. Pre-dive checklists should be utilized to prepare your rebreather because: a. A large percentage of fatal rebreather accidents could have been avoided had a pre-dive checklist been utilized b. To avoid complacency c. To insure that the rebreather is properly prepared to dive d. To minimize the chance that distractions prevent you from completing all pre-dive tasks

e. All of the above are true

6. A negative pressure check should be conducted: a. Before every dive. b. By submerging the unit in water, and looking for bubbles. c. After the unit has been completely assembled. d. A and C. e. All of the above are true.

7. Final immediate pre-dive buddy checks include: a. Confirm bailout supply cylinder is on and operational. b. Confirm rebreather mouthpiece is closed if entering the water with it out of your mouth. c. Confirm rebreather gas supply cylinders are turned on. d. Confirm BC supply hose is connected and operational. e. All of the above are true.

8. If diving with an OC scuba diver, you should first: a. Brief them on what to do if separated under water b. Teach them how to close your mouthpiece

c. Only dive with OC divers who are also certified to use rebreathers. d. A and B above e. Rebreather divers may not safely buddy with OC scuba divers.

Diving Techniques Overview As we have seen, while much diving theory is the same for rebreather and OC scuba systems, there are some significant differences. The same is true with the practical aspects of diving the systems. Much of what you learned in your open water dive training applies to rebreather diving—things like always exhaling on ascent, maintaining neutral buoyancy, and monitoring your gas supply. But some techniques differ, and require new skills to accomplish properly. In fact, some techniques that are perfectly safe with OC scuba can be dangerous if done on rebreathers.

Objectives After reading this chapter, you will be able to: 1. Explain proper mouthpiece protocol, and the problems encountered if improperly followed. 2. Explain the hazards of using rebreathers near the surface. 3. Describe proper descent and ascent procedures diving with rebreathers. 4. List four differences between rebreather and OC diving techniques. 5. List five indications of possible rebreather failures and explain their significance.

This chapter will cover the basic techniques used in diving rebreather systems. However, in this area in particular, the text provides only an introduction. You cannot learn and internalize the skills needed to dive rebreathers by reading about them. The only way to learn this is to dive the systems under the supervision of a qualified rebreather instructor. Of course, since this is the part of training that is the most fun, that should be no problem!

Mouthpiece Protocol

Proper rebreather diving technique begins before you enter the water. One of the most common mistakes that experienced OC scuba divers make is with the mouthpiece. In scuba training you learned to be comfortable removing your regulator second stage mouthpiece from your mouth and replacing it while under water. No special skill was required other than exhaling to clear the small amount of residual water prior to inhaling. In fact, you are probably so comfortable with the skill that you no longer consciously think about dropping your regulator from your mouth at all. This is very different with rebreathers.

Figure 9.1: Proper mouthpiece protocol is essential in rebreather diving.

Courtesy of Elaine Jobin

Rebreather mouthpieces must be manually opened and closed during use. There is a simple but vital rule to follow with mouthpieces: Close the mouthpiece BEFORE you remove it from your mouth!! Even if you are only taking it out on the surface for a second for a quick reply to a question, or taking a single breath of air as a change from your rebreather, you must shut the mouthpiece. This habit needs to be so ingrained that you never fail to observe it. It sounds simple, but because of the habits you have learned scuba diving, you can unthinkingly neglect to do this. If you do forget, you may have to abort the dive and could seriously injure yourself. In order for rebreathers to work, they must have a dry breathing loop. Removing the mouthpiece while under water may allow the loop to flood. This will wet the scrubber, making it ineffective, and can also result in the inhalation of a "caustic cocktail," the corrosive solution formed when scrubbing compound dissolves in water. There are many possible indicators of partial or complete system floods. The first is simply recognizing that you have dropped the mouthpiece while open and allowed it to flood. Failing this selfrecognition, you may hear bubbling or gurgling noises from either the inhalation or exhalation hose after resuming breathing, or a significant increase in breathing resistance. The former comes from water in the hoses, the latter from a flooded scrubber canister. You may find the over pressure valve in the counterlung operating on every breath, indicating inadequate remaining counterlung volume for proper operation. Another sign of counterlung flooding is a marked loss of buoyancy for no other apparent reason (such as descent). A metallic taste or soapy feeling liquid could signify a caustic cocktail caused by flooding. Finally, another indication of system flooding is breathing water. Emergency procedures for these events are covered in Chapter 12.

Surface Use

Another OC habit is breathing from our regulators while at the surface. This is one way we prevent fighting to keep our heads above water, or worrying about water in our snorkels, or kelp entanglement on entries. Yet the surface is possibly the most dangerous place to be using a rebreather. OC gas is supplied at a constant fraction of oxygen; that is, the gas provided will always have at least as much oxygen in it as we normally breathe. With rebreathers, the fraction of the gas varies significantly. With heavy workloads it is possible for some rebreathers to deliver gas with a FO2 of only 0.15-0.17 or less. If we are at depth, this is not a problem. For this example, at 2 ata (33 fsw/10 msw) we would have about 150% of the oxygen molecules available to us that we would at the surface. But at the reduced pressures near the surface we would be breathing a hypoxic mix, and could very easily find ourselves physically exhausted as a result. It is possible for the mixture to become so hypoxic that you lose consciousness. To prevent this, limit the time you spend breathing from the rebreather at or near the surface. When possible, switch to your snorkel while swimming on the surface. If you must use the rebreather on the surface, keep exertion to a minimum. If you have an oxygen sensor display, monitor it very frequently. In the worst cases, you may need to rely on your OC gas supply to swim on the surface during periods of heavy exertion. After a hard surface swim, take a short rest to allow your respiratory rate to return to normal before descending. Mass flow SCRs have another problem associated with surface use. Whenever they are on, they continuously provide gas flow to the system, regardless of whether the diver is using it or not. If you must do a long surface swim while wearing a mass flow SCR, consider turning the supply cylinder off. This is a particularly hazardous procedure, as you will see in the next section. However, in some circumstances it may be preferable to losing a substantial part of the gas supply.

Gas Supply

Another common practice from OC diving is to check diving cylinder pressure prior to leaving the dock. We open the valve, see the cylinder is full, and then close the valve. Occasionally, we jump into the water without remembering to reopen the valve. We find out in a hurry, since we get no gas when we inhale!

(Almost) Sleeping on the Job

Two relatively inexperienced divers using mass flow controlled SCRs were diving from a boat anchored in deep water, with a moderate current running. The two geared up and jumped off the stern of the vessel. In the water, they descended about five feet (two meters), and began swimming hard against the current, to reach the anchor line. Upon reaching the line, the divers started to follow it to the bottom. However, after just a few minutes one diver surfaced and returned to the boat, complaining of dizziness, visual problems and difficulty breathing. The gas mixture and flow setting he had selected prior to the dive could not cope with the heavy exertion in shallow water, even though it was within manufacturer's specifications. The system was injecting insufficient oxygen into the breathing loop, producing a hypoxic mix. The diver's hypoxia could have been avoided if he had used a "granny line" to pull himself against the current to the anchor line, or used his snorkel at the surface instead of his SCR just below it. With rebreathers, checking cylinder pressure is still a good idea. But if you jump in the water with the valve turned off, you may not realize it immediately. The gas you breathe is being recirculated, and the fact that no new oxygen-containing gas is being added may not be immediately apparent. Given this, you could be diving the system unaware that your breathing gas is becoming increasingly hypoxic.

The simple rule here: Confirm the gas supply is on just prior to entering the water!

Descent

During descent, pressures increase rapidly. Some rebreathers work by maintaining a fairly constant PO2. If the set point of such a unit is high, it is easy to exceed a safe PO2 during descent. For example, let's assume Marvin descends rapidly from 10 fsw (3 msw) to 50 fsw (15 msw). The ambient pressure about doubles as he descends. If his initial PO2 was 1.2 atm, it would double to 2.4 atm at depth! To avoid this, descend slowly enough to allow the rebreather to accommodate the pressure changes, keeping the PO2 within safe limits. The rate at which this occurs varies by unit, based on design. Of course it is not a problem with all rebreathers. Rapid descents can also lead to inadequate breathing volume. In this case, you may have to manually inject gas into the breathing loop, using the manual diluent valve or manual gas addition override. Slow descents alleviate this problem if you have an automatic diluent valve or other automatic gas addition device. Mass flow control SCRs have a maximum operating depth based on the FO2 of the supply gas. Since the maximum operating depth is based on the diver breathing the supply gas as it is, it is impossible for this to become a problem. This is another reason that the calculated counterlung PO2 is not used for calculating the maximum operating depth.

Buoyancy and Trim

Experienced OC scuba divers have learned to use their lung volume for finely tuned buoyancy control. Gross buoyancy control is provided with the BC. But when the diver needs to swim over a rock or coral head, or is lifting a small object from the bottom for a closer look, inhaling a little deeper than normal provides a small amount of additional lift. Most divers learn to do this unconsciously after a few

dozen dives. If you try this while using a rebreather, you will run right into that coral head or rock, or plop to the bottom with your artifact!! Lungs function as a BC because they change diver volume. You displace more or less water, causing more or less lift. With a rebreather, the exhaled volume inflates the counterlung. When the diver inhales, the counterlung deflates, negating any buoyancy changes. The overall displacement of the diver remains the same, except for the small volume of CO2 that is scrubbed out of every breath. Thus, the only buoyancy change experienced with rebreathers is a slow, gradual loss of buoyancy, until breathing volume is automatically made up by the rebreather. With mass flow SCRs, this occurs continuously; with other designs, breathing gas replenishment is episodic.

Figure 9.2: Avoid using additional bouyancy devices like drysuits in your early rebreather dives. Courtesy of National Park Service and Brett Seymour

Thus you will need to spend more time manually adjusting your buoyancy with rebreathers than with OC scuba. You can do this in either of two ways. One method is to add a little additional gas (nitrox or oxygen) to your breathing loop. This has the same effect as breathing off the top of your lungs. It has the disadvantage of being closer to maximum volume of the counterlung, resulting in automatic venting of the system to prevent rupture. While this works for very small, temporary buoyancy changes, it generally is not the preferred way to achieve proper buoyancy. The best way to control buoyancy is with the BC. With rebreathers it is better to use the low pressure or power inflator to fill the BC, rather than orally inflating it. This minimizes the chance of flooding from improper mouthpiece protocol, and also maintains the gas in the counterlung. The BC feed hose can be run from the SCR gas supply cylinder, CCR diluent cylinder, or the bailout bottle. If you run it from the SCR gas supply cylinder, remember that with mass flow SCRs the cylinder continues to empty even if not in use. So, if you are doing a surface swim and turn your gas supply off to conserve air, you will not have power BC inflation.

Figure 9.3: Avoid vertical trim postion (left). Most rebreathers are designed for use in a nearly horizontal trim position.

In OC diving, most divers have only one buoyancy system to worry about. Rebreather divers have a minimum of two (the BC and the counterlung). This increases task loading, and makes it more difficult to control the dive. It takes most divers six to eight dives to learn to control a second buoyancy system. If you wear a drysuit, you have a third buoyancy system to manage. Given the option, you should minimize the number of buoyancy systems you wear, especially during training dives. If you have limited experience using a drysuit, you should not use one while learning to dive rebreathers. Mass flow controlled SCRs are designed to be used at or near maximum loop volume, i.e., the breathing loop is always completely or nearly completely full. All other rebreathers should be used with the least possible gas volume in the breathing loop, called minimum loop volume. When used properly, you should find that your counterlung is completely deflated at the end of a normal inspiration. This will improve your trim and bouyancy, and will minimize drag while swimming. The ability to establish trim is as important as buoyancy control. Trim refers to the angle at which you swim through the water. You can be neutrally buoyant, and yet find yourself floating completely upright when relaxed. Or, you could find your natural resting body position is horizontal in the water. The latter is far preferable. Horizontal trim reduces swimming drag, makes observation easier, and with most rebreather designs minimizes WOB (Figure 9.3). Placement of the counterlung and weights are the two most critical factors in achieving proper trim. The counterlung, BC, and weights can be considered as forces acting on a seesaw, with gas volumes pulling upward and weights pushing downward. The center of the seesaw is the diver's center of gravity, about the middle of the stomach. The goal is to place the weights and gas spaces in such a manner as to achieve horizontal trim (Figure 9.4).

Figure 9.4: Diver trim can be compared to a seesaw, with different forces affecting the ultimate balance.

In rebreather designs, counterlung placement is fixed or only slightly adjustable. Weight placement may be altered to suit the diver's needs, within limits. Normally in OC scuba all of the weights are carried on the weight belt. This facilitates removal to create buoyancy in the event of an emergency. With a rebreather, it may be preferable to remove some weight from the belt and carry it elsewhere. Some divers prefer to attach weights to the upper part of the shoulder straps, to counteract counterlung lift (Figure 9.5). This is especially useful with back-mounted counterlungs that are high over the shoulders. Others prefer to mount the weight directly inside the rebreather adjacent to the counterlung. Regardless, in the event of an emergency, you must be able to jettison enough weight to achieve significant buoyancy at the depths you will be diving.

Mouthpiece Clearing

Just as with OC scuba, we will need to remove the rebreather mouthpiece from our mouth under water—when testing or switching

to the emergency bailout bottle, for example. To change from the rebreather to OC system, first exhale most of the gas from your lungs, leaving sufficient volume to clear your open-circuit regulator second stage. Close the rebreather mouthpiece and then remove it from your mouth. Put the OC regulator second stage in your mouth, clear it and resume breathing. If you are using a mass flow system and expect to be off the SCR for a significant time, you may want to close the gas supply valve to prevent gas loss.

Figure 9.5: Placing weights on your rebreather shoulder straps may improve trim by off-setting counterlung bouyancy. Courtesy of Aleph Alighieri.

To return to rebreather use, reverse the steps. First turn on the SCR gas supply, if it was closed earlier. If using a mixed gas CCR, confirm the gas in the breathing loop is safe to breathe by looking at your oxygen sensor display. Then place the rebreather mouthpiece in your mouth,holding sufficient gas in your lungs to clear it. If your rebreather mouthpiece has a purge valve, you can clearit using that device. If not, clear the mouthpiece by displacement: look up and exhale, expelling gas from your lungs to displace the water from the mouthpiece. Then seal your mouth on it, open it, and resume breathing. Before opening the mouthpiece, it is good to expel most of the gas from your lungs into the water, to avoid exhalinggas with a high inert gas fraction into the breathing loop.

Mouthpiece Recovery

Like OC regulators, many rebreathers have their breathing hoses attached behind the diver's head. Because the hoses are buoyant, when the mouthpiece is removed from the mouth, it invariably floats up. Often, it floats above and behind the diver's head. There are two ways to recover a lost mouthpiece. The first is to reach behind the head with one or both hands. Grasp the breathing hoses where they fasten to the counterlung or shell, then pull the breathing hoses back over your head, gently working your hand towards the mouthpiece. This method may be awkward for some divers, especially if wearing a restrictive exposure suit or heavy diving mitts. Many divers may find a second option easier. Move to a vertical position in the water, then simply lean backwards while looking up. Once you lean back, the mouthpiece may be found floating immediately above your eyes and mouth. It is easy to recover it in this position with a minimum of arm contortions. This method may not work with all units or gear configurations. Try both techniques to see which works best with your rebreather, then practice until it is second nature.

Communication

Rebreather divers use the same hand and light signals as OC divers. As a rebreather diver, though, you may need a few additional hand signals to communicate information like "change set point" or "loop PO2", special hand signals are shown in Figures 9.6 to 9.8. There are three command signals typically used by divers (Figure 9.6). These are signals that for safety reasons require a response from all members of the team. The first is a question, "Are you OK?" It is answered either with an "OK" sign, or a "Not OK" signal. If not OK, then a second hand signal is used to indicate the nature of the problem. The second command signal is, "Hold!" All divers in the team respond with the same signal, indicating they received and understood the command. Finally, the "terminate dive" signal demands the immediate termination of the dive, with an immediate ascent to the surface as rapidly as safely permitted. The

same signal is repeated by all divers in the group before initiating ascent. A variety of hand signals specialized for rebreather divers is provided in Figure 9.7. The "bubble check" signal may be used as a command ("check me for bubbles"), as a question (is my rebreather bubbling or leaking?), or as a statement ("your rebreather is leaking"). If used in the last manner, the severity of the leak may be indicated by the distance you separate your fingers while giving the signal, or by the rapidity with which you repeat the signal. Any signal may be preceded by the "question" signal, to indicate that you are asking a question. Questions dealing with rebreather status, such as "SPG check" or "PO2 check" may be answered with the number signals presented in Figure 9.8. The number 1 to 5 are presented with fingers upright in vertical position and the palm facing outward. To show the numbers 6 to 9, hold your fingers horizontally with the palm facing in toward your body. The number zero looks like an "OK" sign, but with all fingers curled in together. When communicating PO2, give a two signal response. The first signal is always zero or one, followed by the decimal value of the PO2. Thus, if the PO2 in your loop is 0.9 atm, you would give the signal for zero, followed by the signal for nine. Your SPG pressure is usually given as a two or three digit sequence. If your SPG shows psi, you give two numbers indicating the thousands and hundred places of the contents (for example 3200 psi would be shown as the number three followed by the number two). If using bar, show a three number sequence to show the contents. If you are using a CCR, you will have two SPG readings to communicate. Always give the oxygen cylinder pressure first, followed by the diluent cylinder pressure. Hand signals suggesting set point management for mixed gas CCRs include "change to high set point" and "change to low set point." Generally these are acknowledged by giving an "OK" sign, and performing the necessary action.

Figure 9.6: Command Signals – These hand signals require a response from all divers in the team.

Figure 9.7: Hand signals for rebreather divers.

Figure 9.8: Hand signals for numbers.

Some specialized signals communicate problems with the rebreather. These include signals for "cannot breathe," "flooded loop," or "water in hoses." Suggested responses to these problems may be covered by "switch to OC," or "switch back to your rebreather." These are also depicted in Figure 9.7. You may find that you have needs for other specialized signals. Do not be afraid to develop your own signals as needed. If you do so, please contact me and let me know, so they may be incorporated in future editions.

Rebreather users have the advantage of being able to communicate audibly though. Rebreathers have two attributes lacking in OC equipment: The mouthpiece has a volume chamber, and the noise produced by the unit is low. This allows you to talk under water! You and your buddy can actually speak and comprehend each other while under water. Stick to simple words (one syllable preferred), and enunciate slowly and carefully.

Figure 9.9: Check your oxygen sensor display frequently.

Gauge Monitoring

As in OC diving, the gas supply must be monitored. We are accustomed to using our submersible pressure gauge (SPG). As experienced divers, we have learned how quickly the pressure drops, depending on our work rates and depths. In some cases, we know that we can sometimes go 30 or 40 minutes without checking the SPG. This is not the way we were trained, but through experience have learned that this is adequate for the type of diving we were doing. This is not adequate for rebreather use! With rebreathers, the rates at which pressure in the gas supply cylinder drops will differ significantly. The intuitive feel we have will

not be reliable, and must be relearned. With some rebreathers, the drop will be much more constant through time compared to OC. With others, it will be more closely tied to work rates. In either case, you should monitor your SPG every few minutes until you have sufficient experience to internalize the way the unit will work with your diving. As with OC scuba, you should surface with an adequate gas reserve in the supply cylinder. While this varies by unit, a minimum of 500 psi (35 bar) is reasonable. Some rebreathers have a PO2 gauge to monitor. The oxygen sensor (or sensors) is placed in the breathing loop so you know what you are breathing. In various designs the placement may vary, but the purpose of the sensor is to tell you how much oxygen you are breathing, in atm. This allows you to ensure that the PO2 is within the appropriate range. It some systems it may also be used to allow you to maintain a minimum PO2, allowing you to more accurately measure your inert gas loading. The display for PO2 is extremely valuable, as it removes much of the guesswork from using rebreathers. If your unit has it, monitor it very frequently, as it provides the only objective indication of what you are actually breathing. Generally, look at it every 30 seconds to every few minutes. Monitor it more frequently when you are changing depths, are close to the surface, or are working hard. This gives a good sense of the rebreather's PO2 maintenance over time. It also provides a concrete indication during ascents and descents, the most critical times of PO2 change. Remember that the reading will lag behind the actual PO2 content by five to 30 seconds, depending on the sensors and sensor location in the breathing loop.

Counterlung Gas Addition

Almost all rebreathers are designed to automatically add gas to maintain breathing loop volume. However, the rate of addition may be slower than the need. This usually occurs in rapid descents. If you are getting insufficient volume to take a full breath, you can manually add gas to the breathing loop. On most systems, manual works much more rapidly than the automatic, so be careful to not

over-inflate the breathing loop. While this is not generally dangerous, it does result in needless loss of gas and drop in system efficiency.

Nose Breathing

People are, as a rule, natural nose breathers. They inhale and exhale through the nose. During OC scuba training, you learned to inhale through your mouth. For some, this skill was difficult to master. Most of us also learned to exhale through our mouth. Some divers, however, exhale through their nose. In OC diving, this is acceptable. It helps keep the mask clear and increases the comfort level of some individuals. But this technique is inappropriate with rebreathers.

Pick a gauge, any gauge!

An experienced OC diver was trying the "latest, greatest" rebreather during an introductory dive experience. After a few minutes of cursory instruction, she entered the water and began her grand adventure. Descending gradually to 15 fsw (5 msw), she kept close watch on her gauges. After a few minutes, she felt that things "just weren't right." She started to switch to her bailout bottle and passed out. Safety divers brought her to the surface and back to shore. She woke up just a few moments after the rebreather mouthpiece was removed from her mouth so she could begin breathing surface air. What happened? It was determined that she was monitoring the submersible pressure gauges of the constant PO2 rebreather, and not the oxygen sensor gauge. Thus, even though she was continuing to breathe, her PO2 had dropped to a level incapable of sustaining consciousness. If your rebreather has oxygen sensors, you must monitor them on a frequent basis.

Figure 9.10: Nose exhalation is acceptable with open-circuit equipment, but very inefficient with rebreathers.

Exhaling through the nose on every breath results in the loss of a significant part of your breathing loop gas volume. Typically in rebreathers the gas is circulated four or more times before being dumped out of the breathing loop. Nose exhalations reduce this to one time. Effectively, this reduces the efficiency of the system to that of OC scuba. You lose the biggest advantage of the rebreather. If you are a nose exhaler, you will need to unlearn this to use rebreathers effectively.

Reflux

Reflux, burping, and regurgitation are normal human functions, but none are particularly enjoyable when underwater. If you reflux or burp while on OC, the gastric juices or gases you exhale from your mouth are immediately expelled from the regulator through the exhaust valve. With rebreather diving, anything you exhale from your mouth enters the breathing loop, to be recycled over and over again. This is not recommended with residual gastric contents! If you burp while using a rebreather, instead of exhaling from your mouth, immediately exhale from your nose. If you feel the need to regurgitate, then switch to OC bailout and ascend as soon as

reasonably possible. This will help keep your loop clean, and you a much happier diver!

Mask Clearing

Mask clearing is another exercise in which we can lose significant gas. During an OC dive, an unskilled diver may use two or three breaths to clear his mask. In rebreather diving, we want to clear it with a single, or a partial breath, if possible. Remember that everything exhaled through the nose is lost to the breathing loop, reducing unit efficiency and dive duration. The gas loss can be substantial. Let's assume Marvin has a large volume mask and takes two breaths to clear it. With an average lung size, this would be about 6 L of gas per breath, or 12 L total, about a half of a cubic foot. If he is at a depth of 130 fsw (40 msw, or 5 ata) when he clears his mask, the released volume would amount to 60 L (2.1 cf) of gas. This could be as much as 10% of the entire gas supply! You would not want to do that too many times during a dive! Use a mask that fits well, and learn to clear it efficiently. Switching to a low volume mask is a good idea with rebreathers. The lower volume will minimize the amount of gas needed for clearing. Practice clearing water from the bottom of the mask, stopping when bubbles first begin to escape. Many divers using rebreathers prefer a mask with a purge valve.

Bubble Checks

Masks are not the only places divers are accustomed to seeing bubbles originate. In OC equipment, small leaks from cylinder orings, valves, or second stages are not uncommon. Since all of these system components are pressurized with gas at a level greater than the ambient pressure (either at the cylinder pressure, or at an intermediate pressure about 145 psi (10 bar) above ambient), the chances of water leaking into the system are minimal to nonexistent. Divers have the opportunity after the dive to have equipment servicing done to correct the problem. This is not true of rebreather divers.

The primary component of all rebreathers, the breathing loop, is at ambient pressure. If this were not the case, we would not be able to breathe efficiently. Any bubbles observed coming from this subsystem of the rebreather probably signify another problem— water is leaking into the breathing loop! This could compromise the absorbent, effectiveness of the counterlungs, or impair function of the oxygen sensors. Thus, any sign of bubbles is cause for immediate termination of the dive, so the problem can be corrected prior to diving again on the unit.

Figure 9.11: At the beginning of every dive, and periodically afterwards, check your buddy's rebreather for bubbles. Courtesy of Elaine Jobin.

With most rebreathers, it is difficult or impossible for the diver to effectively determine if their system is leaking. Therefore, one of the responsibilities of the buddy is to perform regular bubble checks on the rebreather. The buddy visually examines the rebreather, paying particular attention to all components of the breathing loop, confirming that there are no bubbles coming from any part of the system not designed to emit them (for example, it is normal for overpressure valves on mass flow SCRs to bubble periodically). The first bubble check should be done immediately after entering the water, and done periodically thereafter. If any action has occurred

that might result in a problem, for example snagging a breathing hose, an immediate bubble check should be requested by the rebreather diver.

Figure 9.12: A loop flush done during ascent on constant mass flow SCR or when ascending on a mixed gas CCR in SCR mode will help prevent hypoxia.

Work Rates

With OC scuba, we generally do not plan heavy work during a dive. If we are using a rich EANx mixture, we are concerned with the effects of CO2 retention on oxygen toxicity. When we work harder, we breathe faster and deeper, and the regulator gives us what we need. With rebreathers, we must pay more attention to our work rates. Some designs are less problematic than others. For example, RMV-keyed designs supply gas commensurate with our effort. However, the more commonly used mass flow SCRs present a potential problem with heavy work.

Mass flow SCRs are generally set up with a maximum work rate in mind. As we saw in Chapter 6, the PO2 in the breathing loop varies based on the work rate. If we work harder than anticipated, then it is possible to drop the PO2 to an unacceptable level. If you are diving a mass flow system, and find yourself working hard, you should slow down and bring your effort under control. With mixed gas CCRs, PO2 levels should be checked more often when working hard. If the oxygen addition system fails, then under heavy work conditions PO2 can drop to hypoxic levels in just a few minutes. The only way to determine this is by monitoring the oxygen sensor display.

Ascent

Ascent is another time of high potential hazard with most rebreathers. During ascents, the pressure drops. The more rapid the ascent, the more rapid the pressure drop. If you are using a unit that delivers a relatively fixed PO2, a rapid ascent can drop the PO2 to an inadequate level. As an example: Marvin is using a system delivering 0.5 atm PO2 at 100 fsw (30 msw). If he ascends directly to the surface, his PO2 will drop to 0.12 atm, at which level unconsciousness may occur. If he is working hard during the ascent (say, by carrying an anchor he found on the bottom), the drop in PO2 may be even greater. Do two things while ascending using rebreathers. Firstly, ascend slowly. This allows the rebreather to maintain the PO2 in proper range during ascent. It is also beneficial from an inert gas standpoint. Just as in OC scuba, slow ascents allow for better off-gassing of inert gas from the body, resulting in lower risk of decompression illness. The second requirement is to flush the gas in the breathing loop. This is done only if you are using an SCR, SCR mode, or oxygen CCR. Manually flush the gas from the loop, replacing it with fresh supply gas. This brings the FO2 to a level close to that in the supply cylinder. Then you can ascend, with a good estimate of your initial gas content. On deeper dives, this procedure may be performed a

second time during the ascent. This technique, called a "Lifesaver Change" by some, is critical in systems in which a PO2 sensor is not installed. Because of the hazards associated with low PO2 during ascent with mass-flow controlled SCRs, some instructors prefer that the ascent not take place on the rebreather. They advocate that the ascent be conducted using the OC gas supply, to eliminate any possibility of going hypoxic during the ascent. While this is a valid technique, its use mandates a larger OC gas supply than would otherwise be required.

Figure 9.13: Learning dive techniques specific to rebreathers will allow you to enjoy a wide variety of environments. Courtesy of Aleph Alighieri.

1. Using rebreathers is more hazardous near the surface because: a. There are fewer oxygen molecules in the breathing loop, increasing the likelihood of hypoxia. b. Some rebreathers add too much EANx at the surface, increasing the likelihood of hypoxia. c. Snorkels do not work well with rebreathers, increasing the likelihood of hypoxia. d. All of the above are true.

2. All of the following are parts of proper mouthpiece protocol, except: a. Close the mouthpiece before you remove it from your mouth. b. Listen for gurgling indicating water in the mouthpiece or hoses. c. Ensure you hold an open mouthpiece above the water while talking at the surface. d. All of the above are true.

3. Descents from the surface should be:

a. Rapid, to quickly descend below the hypoxia danger zone. b. Slow, to allow the rebreather to compensate for counterlung volume changes. c. Slow, so that you do not allow constant mass flow SCRs to fill the breathing loop with 100 percent supply gas, thus avoiding problems with CNS oxygen toxicity. d. Slow, to allow constant mass flow SCRs to compensate the gas mixture as depth changes.

4. Ascents should be: a. Rapid, to quickly ascend through the hypoxia danger zone. b. Rapid, to minimize any additional inert gas uptake. c. Slow, allowing time to change to the bailout cylinder during the ascent. d. Done only after performing a loop flush in mass flow controlled SCRs.

5. Practices for rebreathers and OC diving differ in that: a. Nose exhaling is acceptable in OC, but not with rebreathers. b. Rebreather divers must manually close mouthpieces before removal underwater, while OC divers do not. c. OC divers can rapidly adjust buoyancy by changing lung volume, while rebreather divers must use the BC.

d. Rebreather divers must be wary of using their apparatus on the surface, due to hypoxia concerns. e. All of the above are true.

Match the following problem indications and rebreather malfunctions:

6. __ Increased WOB 7. __ Sudden loss of buoyancy 8. __ Gurgling

a. Poor placement

weight

b. Significant water in counterlung

9. __ Metallic taste

c. Wet absorbent scrubber

10. __ Counterlung fully bottoming, gas added every breath

d. Water in hoses

11. __ Inability to maintain a horizontal body position 12. __ Overpressure valve venting gas every breath

in

e. Caustic cocktail f. Exhaling nose

through

g. Mouthpiece left open on removal

Post-Dive Procedures Overview

Post dive maintenance of scuba equipment is extremely important. Properly caring for equipment after every dive will extend its usable life, preserve its value, minimize the need for repairs by manufacturer-authorized technicians, and, most importantly, minimize the chance of failure during a dive. For standard OC equipment, post dive care mostly consists of proper rinsing, drying, and storage. Rebreather care is considerably more detailed. With OC equipment, very little of the gear is exposed to biological contamination. The only parts exposed to exhaled breath are the regulator second stage and the buoyancy compensator (if orally inflated). With rebreathers, the entire breathing loop is exposed, and must be thoroughly cleaned and disinfected to prevent disease. This chapter will detail proper maintenance steps, especially as they differ from those for OC equipment.

Objectives After reading this chapter, you will be able to: 1. Detail the steps necessary to clean and disinfect rebreathers. 2. Describe post-dive scrubber canister care. 3. Explain the importance of logging your rebreather dives.

Cleaning and Disinfecting

After exiting the water, take off the rebreather and place it in a safe environment. During cleaning, be particularly careful to avoid twisting or sharply bending the breathing hoses and any cabling. Prevent the hoses, breathing bag, water trap, buoyancy compensator, or other delicate parts from rubbing against hard objects. Also keep these away from sharp objects that might puncture them. If you have not already done so, close all gas supply cylinders to the rebreather. Ensure that the mouthpiece is in the closed position. Clean the exterior of the unit with clean, fresh water. If the rebreather has a removable cover or fairing, remove it and rinse all visible components. Do not introduce water into any part of the rebreather that is supposed to remain dry, such as the breathing loop, highpressure cylinder, or electronics. Remove high-pressure cylinders, regulators, oxygen sensors, and electronics. Place them in a dry area away from the washing station. If the unit is not designed to do this, ensure that water will not enter components that might be damaged by wetting. In a spray bottle, bucket, or other suitable clean receptacle, mix a disinfectant solution as described in the disinfectant instructions. Commonly used cleaning agents are iodine based, like Betadyne® or Wescodyne®, or peroxymonosulfate, like VirkonS® and RelyOn®, broad spectrum anti-bacterial, anti-fungal, anti-viral compounds. Wescodyne® should be diluted using one part Wescodyne® to 213 parts water (about 3 and 1/2 teaspoons per gallon). The resultant solution should be iced tea colored. VirkonS® and RelyOn® are sold in tablet or powder form, and should be mixed 10 grams per liter of water.

Figure 10.1: Disinfection will kill any germs living in your hoses.

Remove the breathing hoses from the rebreather. If the mouthpiece is designed to be easily removable, remove that as well. If not, open the mouthpiece. Empty any water or saliva from the mouthpiece and hoses. Then fully immerse the hoses and mouthpiece in the disinfectant solution or spray disinfectant onto all inner surfaces. Let sit about 10 minutes. Do not leave parts in disinfecting solutions longer, as the solution may damage some rebreather parts. Remove or isolate the scrubber canister. Care for this unit will be described in the next section. Empty any water from the breathing bag and water traps. If these are removable, remove them and soak in the disinfectant solution or spray all inner surfaces. If not, either pour a small quantity of disinfectant into each, or wipe all accessible surfaces with a pad soaked in disinfectant. If foam pieces are used to trap water, remove and soak them in the disinfectant solution. Buoyancy compensators should be rinsed inside and out with fresh water, and then emptied. Some of the water should be emptied

through the oral inflator and some through the overpressure valve to rinse both of these. Leave the BC inflated to dry. After the disinfectant solution soak, rinse all components with fresh water. Pieces should be hung or placed to dry in a cool, shaded area. Minimize extended exposures to ultraviolet light, such as direct sunlight. Ultraviolet light will degrade the materials used to construct rebreathers. The drying area should be clean, and not subject to wind which might carry dirt, pollen or microorganisms onto the components. When interior parts are put down, it should be onto a clean surface, or on a clean towel. Allow all parts to dry thoroughly.

Figure 10.2: Compress hoses repeatedly to clear water.

The corrugations in the breathing hoses trap water. Before hanging to dry, remove as much of this water as possible. To do so, hold the hoses so that an open end faces down. Beginning at the top, compress the hose corrugations, working down the hose. (Figure 10.2). Repeat this several times. Alternatively, gently pull the hose ends so the corrugations flatten. Hold the hose ends, not the

flange, to prevent the hoses from slipping off their mounts and possibly injuring you or damaging them. These maneuvers will allow much of the trapped water to drain. Leave the mouthpiece in the open position while drying.

Figure 10.3: Blot foam pads between clean, dry toweling to remove moisture. Photo by Scott Cassellx

Clean toweling may be used to soak up excess water from the counterlungs and water traps before they air dry. Foam water traps may be compressed to remove absorbed water, but do not twist or wring out foam pieces (Figure 10.3). This will break down the foam, reducing its efficiency. If you are diving in a very humid area, try to leave components to dry in an air-conditioned area. A fan or hair drier on air or low heat setting may be used to speed drying. Inspect the rest of the rebreather. Dry any water or moisture. Look for residual absorbent dust that might have migrated through the system, paying special attention to the bypass valve on mass-

flow controlled units. Remove any absorbent dust found. Once dismantled, examine all components for any signs of deterioration, debris, or dirt, and clean or repair as necessary.

Scrubber Canister

Post-dive care of the scrubber canister depends on its state of use. The manufacturer rates the scrubber in the canister for a specified duration. This duration may vary based on the water temperature while diving. If the rated duration has not been reached during the dive, then the absorbent may be saved for use on a subsequent dive.

Figure 10.4: Scrubber canister bagged and marked.

If the absorbent is to be saved, wipe any water from the exterior of the canister. The absorbent must next be isolated from the ambient environment. Some rebreathers have canisters that should be sealed within the unit. With these systems, drain any water from the canister water trap, and then seal the inlet and outlet ports with end caps or plastic wrap. If the rebreather design calls for the entire canister to be removed for storage, place the entire canister in a plastic bag, and tie or

otherwise seal the bag. Place this bag in a second bag, and seal that one as well. Mark the bag with the user's name, absorbent grade, date of use, and cumulative time the canister was used, including pre-dive surface checks and calibrations. (Figure 10.4) This allows tracking of use, so you do not use the absorbent beyond manufacturer's specifications. A canister may be stored for as long as two weeks in this manner. Note that this practice and time frame is a rule of thumb, and has never been rigorously verified. Note also that the biggest issue with absorbent is moisture content. This may change during storage. Some manufacturers recommend partially consumed absorbent be discarded, and that you always begin a new day of diving with a fresh absorbent fill. If the absorbent has been completely depleted, or used for a significant portion of the specified time, then you must discard it. Open the canister and empty it. Depleted absorbent may be thrown away with normal refuse. Scrub the canister in clean fresh water with a bottlebrush dedicated for this use. Remove as much absorbent dust and residual material as possible. Pay special attention to sealing surfaces, gaskets, o-rings, sieves, and other specialized surfaces. Do not wash the canister in water that will contact with other parts of the rebreather. After washing, allow the canister to dry completely. While washing the canister, avoid contact with residual absorbent. Absorbent, even though it is depleted, will still contain alkaline material. This can cause chemical burns in some individuals, especially if the skin or absorbent is moist. Delicate skin and membranes, such as the eyes, tongue, and mucus membranes, are especially sensitive to absorbent dust. Rubbing the eyes with fingers containing some dust has resulted in eye irritation in some individuals. Wash your hands thoroughly after cleaning the canister.

Electronics

Small amounts of water can damage rebreather electronic components. The amount of damage increases with the time the two are in contact. Water in rebreathers comes from two sources – flooding and condensation. Any electronics in rebreathers must be examined as soon as possible after the dive to ensure there was no flooding, or to correct any flooding damage that may have occurred before it worsens. Condensation moisture, while less corrosive than salt water flooding, should also be dried.

Figure 10.5: Check battery voltages before and after diving your rebreather.

All user accessible electronics compartments should be opened and examined for water. Any water found should be immediately dried. If the flooding was significant, the compartment should be rinsed with alcohol to facilitate drying.

Battery compartments should be opened and examined. If flooded, the battery should be removed and discarded. If dry, it should be tested to verify proper output voltage (Figure 10.5). The battery may be reconnected if diving operations are to continue, or left disconnected if diving is finished for the day. Once electronic and battery compartments have been examined and are dry, they should be reassembled. Oxygen sensors should be removed from the system, if so designed. Replace the gas at the sensor with air, and check that it reads 0.21 atm. Store the sensors away from heat, in a dry area free of CO2. The sensors and the accompanying meters should be packed carefully, minimizing shocks and risk of damage caused by placing heavy objects on them.

Figure 10.6: Log your rebreather dives to provide proof of experience.

Reassembly

Once all components are completely dry, the rebreather may be reassembled. As you are putting it together, examine all seals and sealing surfaces for any dirt or damage. Use appropriate lubricant on any surfaces that require it. Assemble the components loosely. This will minimize memory set in the sealing surfaces, and make the next pre-dive checks easier.

Bailout Systems

Any bailout scuba system must be washed after the dive, just as you would any scuba unit. Rinse the exterior with fresh water. Pass water through the regulator second stage mouthpiece while the system is still pressurized with gas from the high-pressure cylinder. Then disassemble and lay the parts out to dry in a cool, shaded area. When dry, store in a clean, dry location out of direct sunlight.

Post-Dive Barotrauma

If the rebreather you have been using provided a high FO2 breathing mix, then you must consider that you have the potential for middle ear oxygen absorption syndrome and take preventive steps. Equalize your ears using any standard manual technique (Valsalva maneuver, e.g.) immediately after surfacing, and every 30 minutes after the dive for several hours. If your ears customarily equalize with no undue effort, you may not need to follow this schedule. However, if you normally have difficulty equalizing, you may need to manually equalize more frequently and for a longer period.

Post Dive Maintenance

Rebreathers are mechanical devices. You use them. They break. It is the nature of things. The important thing is that you fix them

immediately. It is not as easy to find replacement parts for rebreathers as it is OC equipment. It may take some time to correct problems. If you leave even minor problems to fix later, eventually you will end up entering the water with an inoperable or partly operable rebreather. Rebreathers are life support equipment. You need to keep yours serviceable.

Figure 10.7: Proper post-dive maintenance of your regreather is vital to its continued functioning.

Occasionally, you may use spares to correct unexpected problems that may arise. When you do so, remember to replace them in your repair and service kit. That will keep you diving another day when the same spare is needed.

Log Your Dive

As the saying goes, no job is done until the paperwork is finished. The same is true of diving. Dive logs provide important verification of

experience. This is as true for rebreather systems as for open-circuit scuba. Your dive log provides a history of your diving. It allows you to tally cumulative time on different diving systems, which is an important part of the type-rating process. If you opt to dive with a new diving operation, want to rent a rebreather from a dive resort, or even to dive with a new partner, your logbook may be requested to verify recent experience on a specific rebreather. If you are unable to provide this written record, many operations will not rent you rebreathers or let you use one without a checkout dive first. Logging your dives is an important part of the learning process. Your rebreather education does not end with your certification course. You will find that you learn new lessons on almost every dive for at least the first several dozen after your training course. Writing down "lessons learned" after dives will help you remember them. The process involves more of your senses and increases the repeated consideration of the points learned. It will also help you recall the lessons after lapses in rebreather diving activity.

Figure 10.8: Placing paper towels in your counterlung openings will prevent insects from taking up residence.

Finally, logging your dives will add to your enjoyment of diving. Reading your logbook years after the fact will remind you of good times, provide the grist for many sea stories, and leave you laughing about experiences you would have otherwise forgotten.

Summary

Proper post-dive procedure is vital to rebreather diving. Without it, your equipment is more likely to malfunction, you are at greater risk of health problems, and your future ability to participate in rebreather diving is impaired. To assist you, a post-dive checklist is in Table 10.1. Specific manufacturers' post-dive maintenance

recommendations for many rebreathers are included in the appendices at the end of this text. Table 10.1: Post-Dive Checklist Rinse exterior of rebreather. Mix disinfectant. Clean mouthpiece and hoses in disinfectant. Clean breathing bag and water traps with disinfectant. Rinse all components and set out to dry. Rinse, empty, and dry BC. Empty or bag and mark canister. Examine electronics, test, and disconnect batteries. Verify oxygen sensor function, and store appropriately. Reassemble loosely. Wash and dry bailout system. Log your dive!

Figure 10.9: Calculating your dive tables and oxygen clocks is part of your dive logging activity.

1. Foam water traps should be dried after disinfecting by: a. Wringing them out, and letting them air dry. b. Blotting them between clean towels, and letting them air dry. c. Throwing them in a clothes drier on "low" setting. d. Letting them air dry. e. Foam water traps should not be dried after disinfecting.

2. Wescodyne® used to disinfect rebreather components should be: a. Used full strength. b. Diluted 50:50. c. Diluted 1:213. d. Diluted to 10%.

3. Rebreather hoses should be soaked in appropriately diluted disinfectant: a. Until they no longer smell of contaminants. b. For one minute.

c. For 10 minutes. d. For one hour.

4. If a filled scrubber canister is saved for use on a subsequent dive, you should mark the outside with: a. The grain size of the absorbent. b. The time the scrubber was used on the last dive. c. The expiration date of the absorbent. d. All of the above.

5. Rebreather dives should be logged because: a. Dive stores or resorts may not rent rebreathers without proof of recent experience. b. Logbook use is an important part of the learning process, reinforcing lessons learned. c. Information in the logbook will help plan subsequent dives. d. All of the above.

Long-Term Maintenance Overview

Immediate post-dive maintenance covers about 80% of rebreather maintenance requirements. The other 20% is considered long-term maintenance –as critical to keeping your unit in working order as post-dive care. Long-term maintenance is designated by schedules. Unlike OC scuba, rebreathers require regular monthly, annual, and longer-term upkeep. This chapter presents an overview of longterm maintenance required for typical rebreathers. However, different manufacturers may have different requirements, service schedules, or servicing standards. In all cases, you should follow the service protocols established by your rebreather manufacturer. In addition, some rebreathers may incorporate components or sub-assemblies not considered in this text. This may include specialized sensors, motors, valves, or other elements. These pieces will have their own long-term maintenance schedules. You must follow them. Addressing all possible variations is outside the scope of this text, and the reader is referred to their user and service manuals.

Objectives

After reading this chapter, you will be able to: 1. List four monthly rebreather maintenance activities.

2. Explain the importance of annual rebreather servicing. 3. List three servicing activities that need to be performed at time frames longer than one year.

4. Describe proper rebreather storage.

Why More Frequent Maintenance?

OC scuba gear is very robust. It can take a lot of abuse and still function properly. When it fails, you generally know immediately— either you get no gas when you inhale or it free-flows. This is not true with rebreathers. Rebreathers can fail in a multitude of ways that are not immediately apparent. Since they are generally more complex, they usually have problems more often. Monthly and annual maintenance tasks are designed to minimize the chances of operational failure during rebreather dives (Table 11.1). It is necessary because of system complexity, and because system failures are frequently masked. Some of the tasks done monthly on some systems should be performed prior to every dive on others. Differences in timing depend on equipment reliability, component materials and durability, and ease of performing the task.

Monthly Maintenance

If used, all rebreather mouthpiece hoses should have a monthly interior scrub. Despite the post-dive disinfectant washes and drying, some water often remains in the hose corrugations. This water can allow growth of fungi or bacteria. These may not be immediately unhealthy, but can be extremely unpalatable. To remove any such growths, the hoses can be scrubbed with an ordinary bottlebrush dedicated to the purpose. A disinfectant solution as described in the previous chapter may be used for this purpose.

Figure 11.1: Use a bottlebrush to scrub your hose interiors every month.

A mass flow controller inspection should be done at least monthly, if not performed every diving day. Adequate flow through the controller is essential to maintain proper PO2. If dirt, debris, or salt crystals block part of the sonic valve, the reduced flow may be inadequate to support life. Disassemble the valve and examine all gas inlets. Confirm that there is no dirt or salt buildup on the threads, walls, or sinter filters. If there is residue, either clean it away or take the unit to an authorized repair facility to have the mass flow controller serviced.

Table 11.1: Generalized Rebreather Maintenace Schedule— Always follow manufacturers' recommendations!

Some monthly and annual maintenance items need not be performed if the system has not been used since the last checks. Others need to be performed regardless of the amount of use (or non-use). This table differentiates these

items. Blue items need be done only if the rebreather was used since the maintenance activity was last done.

Figure 11.2: Clean canister and absorbent dust filters monthly.

As explained earlier, proper interstage pressure is also vital if the mass flow controller is to perform correctly. Again, if checking the interstage pressure is not part of your daily pre-dive checklist, it should be verified at least monthly if the SCR is being used. Absorbent baskets or canisters, housings, and other components may build up CaCO3 (calcium carbonate), the end product of the CO2 absorption reaction (seashells are made of this). This will not soak off in water. To remove such residue, soak the pieces in weak acid, such as white vinegar (about 5% acetic acid). Short immersions (about 5 minutes for typical accretions) will dissolve and remove such build-ups. Some rebreather parts get dirty, with particulate matter accumulating in places where rinses will not remove it. Disassembly of the impacted parts is the only solution. Mouthpieces with rotating barrels are one common place this occurs. The barrels get difficult to turn on or off, and grit in the mouthpiece may score the surfaces, leading to uncorrectable leaks. When grit accumulation occurs, disassemble the mouthpiece, remove and clean o-rings and barrel surfaces, and reassemble. This type of problem is also common with

some magnetic reed switches, cabling connectors, and other rebreather parts exposed to sand and debris. Finally, after periods in which the rebreather has been used, or if it has been improperly stored, the counterlung should be washed and disinfected. Again, in some units this is done after every day's diving. In some rebreathers, access to the counterlung is very awkward, time-consuming, or both. In these designs, monthly counterlung scrubbing generally replaces daily cleanings purely for considerations of operational ease.

Annual Maintenance

An authorized service facility must perform factory maintenance. This may be required annually or at some other interval set by the manufacturer. If the rebreather is used much more than normal, authorized servicing should be performed more often than recommended. Service requirements vary by rebreather, and there are a variety of tasks that may be completed. Whether for diluent and oxygen, or for the single nitrox cylinder, all regulators in the rebreather should be serviced. Mass flow controllers also require servicing and verification of proper flow through all orifices built into the unit. Other valves in the rebreather require servicing as well. Rebreathers may include bypass valves, automatic and manual gas addition valves, or gas switching valves. Typically, valves are checked for proper operation. Gaskets, seats and o-rings are inspected, and tolerances adjusted as necessary.

Figure 11.3: It is best to check interstage pressures in mass flow controlled SCRs before every dive.

All rebreather system o-rings must be inspected and replaced as necessary. Oxygen-compatible o-ring lubricant like Tribolube® should be used in those parts of the rebreather that might be exposed to high pressure oxygen and oxygen fractions greater than 40%. Normal silicone grease may be used for waterproof electronics connectors, battery housings, etc. With SCRs, all parts of the highpressure gas supply system, including gas cylinders, regulators, gas addition valves, and mass flow controllers, should be oxygen cleaned. With oxygen and mixed gas CCRs, the oxygen regulator, oxygen SPG, and high- and intermediate pressure hoses should be oxygen cleaned. Some manufacturers specify oxygen cleaning the system annually, or on some other schedule. Others recommend that oxygen cleaning is required only if contamination is suspected.

Figure 11.4: Regulators used in rebreathers should be serviced annually.

Certified inspectors should visually inspect gas supply cylinders to ensure that no water has compromised them. If evidence of water

or rusting is found, then they should be properly cleaned before being placed back in use. Aluminum cylinders should be checked for thread or neck cracks.

Figure 11.5: Oxygen sensors should be replaced annually, or per manufacturers' recommendation.

Any electronics in the rebreather should be checked for proper function. Pressure transducers and other components may require annual recalibration. Oxygen sensors should be replaced with manufacturer-approved sensors as required by their maintenance schedule, usually annually. Mouthpiece valves must be lubricated to ensure smooth function. Mouthpiece mushroom valves should be examined to see if edges are curling or beginning to crack. If so, replacement is necessary. Also scrutinize breathing hoses for cracking, tears, or other evidence of aging.

Longer-Term Maintenance

Some items in the rebreather have longer routine service intervals. Every two years replace all o-rings and mushroom valves. This helps prevent system leaks caused by o-ring flattening, which occur when o-rings are kept compressed.

OC divers are familiar with cylinder hydrostatic testing, which is required in the United States every five years. Other countries have different regulations for high-pressure cylinders, such as Australia. The Australian government requires annual hydrostatic tests on cylinders used in scuba diving. Rebreather gas cylinders must conform to the same regulations as OC scuba cylinders.

Figure 11.6: Gas supply cylinders should be visually inspected annually. Courtesy of Scott Cassell.

Some rebreather gas supply storage is not in high-pressure cylinders, but in high-pressure flasks. These might have different service intervals. For example, the flasks in one rebreather must be tested every six years, using a non-destructive method other than hydrostatic.

Figure 11.7: Using improper lubricants, poor handling or storage practices, or installing inappropriate components may cause fires or explosions. Courtesy of Rex Rolston

Components that should be replaced every five years include breathing hoses, scrubber canister gaskets, all breathing loop gaskets, and some intermediate pressure tubing. High pressure hoses and some intermediate pressure hoses should be replaced every ten years, or sooner if inspection indicates the need. Finally, some manufacturers specify their own servicing at intervals. For example, Dräger requires that their Atlantis and Dolphin SCRs be serviced every six years by a Dräger-authorized facility.

Storage

Many maintenance problems can be avoided by storing your rebreather properly. Correct storage will also lead to lower maintenance costs and fewer aborted dives.

Before storing your rebreather, ensure that it is thoroughly clean and dry. If parts are not completely dry, do not store the unit until they are. Storing your rebreather while wet or damp can lead to bacterial growth in the breathing loop, mildew, and component degradation. All parts of the rebreather should be loosely attached. If components are stored fitted tightly together, as they would be during a dive, many parts may be damaged. O-rings and gaskets may permanently deform, reducing or eliminating their effectiveness. Some rubber parts may weaken or crack under sustained contact pressures at strained unions. Metal and electrical connectors may partially electrolyze, making them difficult or impossible to separate without damage. Like standard OC gear, buoyancy compensators should be stored partially inflated. This reduces the chance of wearing holes in the bladders, or having sharp salt crystals puncture the bladders from the inside. High-pressure cylinders should be stored with some minimal pressure, less than 300psi (20 bar). This is especially critical for steel nitrox and oxygen cylinders, in which the high oxygen fractions can lead to rapid oxidation if the cylinders are stored full.

Figure 11.8: Avoid crushing or kinking your hoses and cables, especially during transport.

Remove all batteries. Excessive discharge from batteries in storage can lead to leaks. Leaking alkaline electrolyte from cells can cause extensive damage to rebreather components, necessitating expensive repairs. Likewise, you may wish to short oxygen sensors, both to minimize the chances of leaks, and to preserve sensor life. If the rebreather will not be used for a month or more, then you may wish to store the sensors in an inert gas environment or cap them. Rebreathers should be stored where they will not have items placed on them, or kept in a sturdy container that will prevent damage from other objects. Use a container that is large enough to hold the unit without undue pressure from the top or sides. Hardsided cases, such as Storm Cases® or Pelican® cases, are commonly used. Pay special attention to the breathing hoses, electrical cabling, oxygen sensor cable, submersible pressure gauge hoses and any other display hoses. Store these so that they cannot kink. Kinks left for an extended period will lead to permanent damage and may compel replacement. Since some cables may cost as much as $900, the ounce of prevention is definitely worthwhile!

Figure 11.9: Hard-sided waterproof cases make excellent storage containers for rebreathers.

Courtesy of Elaine Jobin

Absorbent must be securely sealed. Storage should be in sturdy, airtight containers. If the scrubber is purchased in plastic or paper bags, then place these bags in a storage box sufficiently rigid to protect them from crushing by other objects. Do not store absorbents in areas subject to vibration or other movement. This will prevent damage leading to excessive dust in the material. Absorbent should never be stored for long periods in the scrubber canister. Also, it should always be kept in a sealed container, out of direct sunlight and other hot areas. Heat can drive the water in the absorbent out of the material, making it unusable.

Figure 11.10: Store your absorbent in a clean, cool location.

Do not store aluminum cylinders near absorbent. This will help minimize the possibility of absorbent coming in contact with cylinders, which can cause pitting or corrosion problems. Finally, store the entire unit someplace cool, dry, and well ventilated. Do not leave your rebreather in a car trunk, or structure that may be heated by the sun. Also avoid exposure to chemicals,

such as found in a cleaning supplies closet, garage, paint locker or compressor shed. Following these guidelines will help you enjoy many years of diving pleasure from your rebreather!

Figure 11.11: Do not store absorbents with other chemicals.

1. If stored for long time periods, rebreathers should be: a. Assembled as though ready to be used. b. Stored with the canister emptied of absorbent. c. Stored with rechargeable batteries depleted and removed from the unit. d. All of the above.

2. Rebreather service is more critical and must be done more frequently than open-circuit gear because: a. Rebreathers are less robust. b. Rebreathers are more complex. c. Rebreather failures may not be immediately apparent, yet may cause serious harm. d. All of the above.

3. Absorbent should be stored: a. In airtight containers. b. In a cool, dry location. c. In the rebreather canister.

d. All of the above. e. a and b.

Match the following maintenance items and time frames: 4.

___

Disinfect hoses, mouthpiece

5. ___ Disinfect counterlung 6. ___ Hydrostatic test of cylinders 7. ___ Service regulators 8. ___ Visual inspection of cylinders 9. ___ Oxygen clean system 10. ___ Inspect mass flow controllers for corrosion, dirt, and debris 11. ___ Non-destructive testing of flasks 12. ___ Scrub hose interiors with brush 13. ___ Replace oxygen sensors 14. ___ Visually inspect mushroom valves in mouthpiece

a. After every dive day b. Monthly c. Annually d. Every 2-6 years e. Never

Emergency Procedures Overview

Nobody plans on having an emergency underwater. No one wants one. Not a diver we know enjoys them. Yet, they happen, and being able to deal with them defines the difference between a successful and a dead diver. As we have read, there are many problems unique to rebreather diving. This chapter will cover the problems you are likely to experience, in the unfortunate event you experience any at all. There are many more less common problems that could occur. The basic skills presented here will allow you to cope with most, if not all of them.

Objectives After reading this chapter, you will be able to: 1. List the two most common reasons for emergency events while diving. 2. Describe emergency procedures to follow in the event of loss of gas supply, CO2 buildup, system flooding, loss of buoyancy, hypoxia, oxygen toxicity, and caustic cocktail. 3. Explain how to verify the accuracy of an oxygen sensor while under water. 4. Explain how to perform a bailout to an OC system. 5. Describe first aid for caustic cocktail and allergic reactions.

With rare exceptions, all emergencies are avoidable. Most adverse situations in rebreather use occur because of "pilot error" or poor maintenance. Be trained by a qualified rebreather instructor; dive within the limits of your training and equipment; be current on the rebreather you use; avoid poor environmental conditions; and make the decision not to dive when there is a known problem regardless of its magnitude. Following these rules will eliminate most "pilot error" problems. Maintenance problems can be avoided by making all of the pre- and post-dive checks suggested in this book, following all regular servicing schedules for your rebreather, and complying with its manufacturer's guidelines. Remember, only you can take these steps to prevent an emergency on your dives!

Philosophies of Emergencies

"There are no finished projects, only deadlines!" This chapter is like that. Emergency procedures can be addressed for pages, chapters, entire books. But this book has a deadline. Underwater emergencies are like that. You may have many options to consider, alternatives to try, but you have a very limited time to make a decision. The more choices you have, the more time you need to consider them, but you have a deadline for contemplating or trying options. You have a deadline: you must cope with the situation before you drown.

Figure 12.1: "When in doubt, bail out!" Courtesy of Elaine Jobin

Knowledge of too many options may impair your ability to cope with an emergency. As a new rebreather diver, you should stick to a single initial response. The emergency philosophy for all new rebreather users should be, "When in doubt, bail out!" You should develop the conditioning and muscle memory to respond to real or potential emergencies in one way—by changing immediately to OC bailout. You should always opt to change to a known gas supply that you know will support life. You should always carry sufficient OC gas to ascend safely to the surface. You cannot always rely on your mental acuity to make rational decisions when diving a rebreather. It is possible for oxygen content in the breathing loop to fall to levels that may impair your cognitive function. Sometimes, merely changing to an OC gas supply for a few minutes may radically improve your mental processing. You may then evaluate information completely differently and more appropriately. These are often called "sanity breaths." As you gain experience and knowledge in rebreather diving, you may expect your responses to situations to change as well. When you learned to dive on OC scuba, your first octopus regulator freeflow was probably cause to surface to address the problem. As an experienced OC diver, you probably now have a range of solutions

to the same issue. These might include turning the free-flowing unit upside down, placing a thumb over the mouthpiece opening, or breathing from the unit. You might even turn off the gas supply to the regulator, and turn it back on again to terminate the issue. These responses were not available to you as a new diver, nor would you likely have had time or the mental comfort to handle the situation using these methods at that time. As you grow in experience and knowledge with closed-circuit diving, you will gradually become more capable of identifying, assessing, and dealing with emergency situations. The way you handle a problem today may not be the way you do so a year from now. The information in this chapter is meant to help you down this path. Read again the first sentence in this section. Note there are many options for handling a much broader variety of scenarios. But deadlines interfered, and herein I limited consideration to scenarios appropriate to intermediate rebreather divers. Do not expect to be able to handle unexpected situations using all of this data, though. It may takes hundreds of dives before you can comfortably utilize this information in real situations. Continue to learn, continue to gain experience but in an emergency, remember the first rule of rebreather survival: "When in doubt, bail out!"

Loss of Gas Supply

Most losses of gas supply are preventable. Most occur because divers forget to monitor their submersible pressure gauges, or decide to stretch the dive "just a minute" to take that last lobster or photograph. Don't let this happen! Running out of gas on a rebreather is not fun! Some equipment failures may result in the loss of supply gas – for example, an o-ring failure. Debris lodging in mass flow controller may do the same. In some units even a battery failure may shut off gas flow.

Figure 12.2: Check your SPG to avoid running out of gas. Courtesy of Ramon Llaneza

One problem with rebreather diving is that loss of supply gas may not be immediately apparent. With mass flow controlled SCR units, there should be continuous injection of gas into the breathing loop. If this does not occur, you still have gas circulating in the system, but with little or no oxygen. This, of course, may lead to unconsciousness due to hypoxia. While you cannot hear the injection of gas into the loop in some cases, you can hear and your buddy can possibly see the venting of excess gas from the loop. A change in periodicity, or cessation, indicates a possible problem. Other rebreathers only add oxygen or other fresh gas to the breathing loop periodically. In many cases, a solenoid injects that gas. It may make an audible click as it operates. Normally, the time between injections (indicated by the audible clicks) is fairly even. If the time span becomes sporadic, or stretches to very long intervals, your rebreather is probably not functioning properly. Generally, problems do not occur suddenly, but creep up insidiously. This is why vigilance is important. It is important to learn how your rebreather operates, and the telltale signals that it is operating properly — or not. This becomes the warning bell that provides your first indication that you have a problem. A problem caught at this stage is much easier to handle. You have more time, more options, and are less likely to be in a task-loaded, immediately life-threatening situation.

Should you find yourself not receiving fresh gas, immediately switch to your bailout bottle, as described later in this chapter. Signal your buddy that you have a problem. If the problem is correctable, you can spend a minute or so correcting it while using your OC equipment. If you can define the problem, you can possibly correct it and then switch back to your rebreather to continue the dive. If not, abort the dive by ascending to the surface using your bailout bottle.

Hypoxia

As mentioned in Chapter 5, hypoxia is particularly insidious. Lack of oxygen affects your cognitive abilities, and you may be unable to recognize that there is a problem. This is why monitoring your oxygen sensor(s) frequently is so important. It is also why I strongly recommend that if an oxygen sensor is an option for your rebreather, you immediately purchase one and use it. Without an oxygen sensor, the first sign noted in hypoxia is often unconsciousness.

Figure 12.3: Administer oxygen for hypoxia, decompression sickness and embolism.

If you do experience signs or symptoms of hypoxia, immediately flush your breathing loop with fresh supply gas, or add oxygen to the loop. With some rebreathers, you can dothis with a manual injection valve. With others, like the Dräger Atlantis® or Dolphin®, exhale through your nose to dump the current breathing loop gas, and let the bypass valve add fresh gas from the supply cylinder. If this does not immediately happen, or correct the problem, or if another problem such as loss of supplygas prevents you from following this solution, switch to your bailout cylinder and ascend to the surface. If your first awareness of hypoxia is your dive buddy losingconsciousness, you must get both of you to the surface as rapidly as possible. If possible, keep your buddy's mouthpiece in their mouth. Some mouthpieces are equipped with retaining straps that help with this. This will maintain a dry airway. Add fresh gas to the breathing loop manually. If you cannot do this, just get your buddy to the surface! Any delays will escalate the problem. If it is not possible to flush the system with oxygen-rich gas, immediately remove the mouthpiece after you surface. Get your buddy breathing surface air, which has adequate oxygen to maintain life. Transport to a boat or shore, and put them on 100% oxygen. Since the unconsciousness or ascent may have caused secondary problems, such as drowning or air embolism, assess the diver. Check for breathing, pulse, rales, other pain, mental awareness, mood changes, or physical problems like skin numbness or paraplegia. If any of these symptoms are noted, treat for air embolism in a recompression chamber as discussed in the next section. In any event, qualified medical personnel should evaluate the diver after any loss of consciousness.

Decompression Illness

Decompression illness (DCI) includes both decompression sickness and arterial gas embolism. Often, it is difficult or impossible to differentiate between these in the field. Since the first aid for them is identical, we no longer try to diagnose the actual malady as a dive partner, but just follow the first aid procedures.

If DCI is suspected, put the diver in a prone position. If breathing or circulation is not present, begin mouth-to-mouth or cardiopulmonary resuscitation as required. Immediately begin administration of 100 percent oxygen, or as near to that as you have available. Remember that if you do not have surface oxygen, your CCR may have an oxygen supply. If you are using a SCR, your nitrox gas supply is better to breathe than surface air. DCI can only be treated in a recompression chamber, arrange transport to a chamber. Treat for shock while waiting and during transport. Water may be given, if requested by the victim.

Figure 12.4: A recompression chamber must be used to treat decompression illness. Transport any diver with suspected DCI to a chamber as quickly as possible.

Scrubber Failure

Scrubber failure or breakthrough will cause a rapid buildup of carbon dioxide in the breathing loop. If this occurs, you may

experience nausea, headache, fatigue, or confusion. If these symptoms occur, flush the breathing loop as described previously in this chapter. If the problem is CO2 buildup, the symptoms will quickly disappear. You can terminate the dive using your rebreather. Continue breathing from it, but exhaling out your nose. This essentially operates your rebreather in OC mode. Remember that you do not have a large gas supply, so the system may not be usable in this mode for long. If you do not have enough gas to ascend to the surface, switch to your bailout cylinder and abort the dive. It is also possible that there will be no symptoms prior to unconsciousness. In this case, the incident reaction should be as with hypoxia, getting the stricken buddy quickly to the surface and then breathing an oxygenrich gas.

Oxygen Toxicity

CNS oxygen toxicity is another malady that requires rapid corrective action. If you sense any early symptoms of CNS oxygen toxicity, (Figure 5.5, page 100) reduce the PO2 as rapidly as possible. The quickest way is to ascend. If you are diving a rebreather that has a diluent, you can manually add diluent to the breathing loop. If neither of these is possible, switch to your bailout cylinder. If you note toxicity signs in your buddy, signal them to ascend or switch to their bailout cylinder. It is possible that the first sign in a buddy will be an oxygen convulsion. If this occurs, attempt to keep the mouthpiece in place. If the mouthpiece has fallen out, do not replace it. Bring the diver to the surface immediately. Do not worry about which stage of the seizure the afflicted diver is in. The probability of an arterial gas embolism is very low, even if the diver is in the tonic (contraction) phase. Once at the surface, assess the victim for breathing and circulation. It is probable that water was inspired during the convulsions. Treat for shock, and transport to a medical facility for evaluation for near drowning and possible arterial gas embolism.

After surfacing, be alert for the off-O2 effect. This may occur up to several minutes after reducing the PO2. Treat symptoms as required. Pulmonary oxygen toxicity does not have the same immediacy as the other situations we have been discussing. Since pulmonary toxicity develops gradually, there is time to correct the situation (see pages 104-05). Typically, at the onset of symptoms the dive is terminated. If you are unable to ascend directly to the surface for any reason (overhead obstruction, decompression obligation, etc.), then you should reduce your PO2 as much as possible. If you cannot alter your PO2 in the rebreather, then you may wish to consider using your bailout air supply to reduce your PO2, depending on the circumstances. If the problem has been severe, hyperbaric and pulmonary physicians should be consulted before diving again.

Flooding

Flooding typically occurs early in the dive. It results from failure to keep the mouthpiece closed or sealed in the mouth, improper rebreather assembly, or compromise of the hoses or counterlungs. In rare cases, floods may result from catastrophic connector or o-ring failures, or puncture of the counterlung or hoses during the dive. Most floods can be prevented by performing proper negative and positive pressure checks while preparing the rebreather, and by doing a bubble check at the start of every dive. Flooding may manifest in several ways. Usually you will hear gurgling while inhaling or exhaling. You may also notice an increase in breathing effort, buoyancy loss, or a problem with CO2 buildup. Because some of these indications may be subtle, it is important that you can sense the normal sounds and sensations when your rebreather is operating correctly. Then you will recognize something abnormal. In the worst cases, you may inhale water or an absorbent solution (caustic cocktail). The degree of flooding and the type of equipment you are diving will dictate the best way to deal with flooding. In salt water, you can often differentiate between flooding and liquid originating from condensation or saliva buildup. If liquid is in the breathing hose,

lightly taste it. If it is salty, it most likely indicates flooding. If it tastes fresh, it is probably condensation from CO2 removal action, or a buildup of saliva in the hoses. If it tastes metallic, you may have a caustic cocktail solution. This will be addressed later. If you are in fresh water, then this "taste test" is less informative, since the salt signal is missing.

Figure 12.5: Your buddy may be invaluable in assisting you to identify and correct problems. Courtesy of Elaine Jobin

Figure 12.6: Body position can trap water from a partial flood in a benign location, like the counterlung, or allow it to spread to places like the scrubber or oxygen sensors, with severe consequences.

In general, at the first sign of flooding, the dive should be terminated. Signal your buddy and ascend directly to the surface. During this process, maintain a body orientation that will trap any water in the system in a benign location. Avoid any movements or rolls that could introduce water into the scrubber, and then into the inhalation breathing hose. If a decompression stop is required, you may be able to complete it using the rebreather. However, you may experience secondary problems with the system if you do. If the canister is even partially flooded, you may find it very difficult to breathe. In fact, many divers find the effort so great that their cheeks puff out with effort, leading to the expression "hamster cheeks." Canister flooding may also cause secondary CO2 buildup, due to inadequate scrubber operation. If the counterlung is flooded, then you may not be able to draw a full breath without making the bypass or automatic addition valve

add gas to the breathing loop. In extreme cases, you may barely begin an inhalation before the valve fires. You may also find a notable loss of buoyancy. Breathing slowly and shallowly will conserve supply gas in some systems, and minimize your chance of inhaling water. Add air to your buoyancy compensator to counter any buoyancy loss caused by flooding. Some rebreathers have a means to empty excess water from the counterlungs or canister while under water (Figure 12.7). If the system you are using allows you to do this, then you may be able to displace the water and continue the dive, depending on the original cause of water ingress.

Figure 12.7: The Halcyon SCR has a hand pump used to expel water from the breathing loop. Courtesy of George Irvine

Caustic Cocktail

Difficulties with absorbent solution, known as "caustic cocktail" or ACG Syndrome, range from minor to extremely severe. Problems may result from external contact, but the more serious damage comes from internal exposure, which includes ingestion and inhalation.

External contact often results from carelessness during rebreather post-dive maintenance. While emptying the scrubber, liquids from the canister are inadvertently splashed onto some part of the body. Wash the affected area with water, rinsing several times. If serious chemical burns develop or secondary allergic reactions are noted, seek medical care immediately. If any solution comes into contact with the eyes, irrigate immediately with water, and seek medical care. Internal exposure is potentially life threatening. The magnitude of the problem depends on of the type of absorbent used, the amount of water involved, the amount of time water is in contact with the absorbent, and, most particularly, the amount of dust in the absorbent and the rebreather. With the right (or wrong) conditions, the resulting solution can be very caustic.

Figure 12.8: Do not ingest or inhale caustic solution.

The best cure for caustic cocktail is prevention. Be alert for warning signs of possible flooding. If noted, deal with the problem, or terminate the dive. Failing prevention, limit your exposure. If during a

dive you notice any metallic taste (this indicates absorbent), itching, burning, or a "soapy" liquid in your mouth, terminate your dive. Close your mouthpiece, and expel any chemical-laden liquid from your mouth. Rinse your mouth with ambient water, and switch to your bailout cylinder. DO NOT INGEST OR INHALE ANY CAUSTIC SOLUTION! If you do inadvertently swallow some solution, drink lots of water and seek a physician's aid. Do not induce vomiting. Older texts have advocated drinking a weak acetic acid solution, such as vinegar. This is no longer recommended. Caustic cocktail ingestion may induce a vomiting reflex. If it does, attempt to minimize the vomiting, and maintain an open airway. The worst event is inhalation of caustic solution. This is almost always avoidable. If inhalation does occur, the secondary damage can be very severe. First aid consists of having the patient drink lots of water to dilute and flush any solution in contact with the upper esophagus. Administer oxygen, as well. Inhalation of caustic cocktail solutions has been reported to cause vomiting, coughing blood, and epiglottal spasms. If the event occurs at depth, any of these may result in an air embolism during ascent. First aid should include assessment (and treatment, if necessary) for it. After surfacing, seek medical attention, since symptoms can become progressively worse with time, and may require life support. Caustic cocktail inhalation may lead to chemical pneumonitis (fluid in the lungs caused by chemical burns), Adult Respiratory Distress Syndrome (ARDS), and esophageal swelling. Any of these reactions may not be seen until several hours after the event. All are life threatening and must be treated by a physician. Treatment includes intubation, placing the victim on a respirator, and general supportive care. ARDS is a particularly complicated problem, as damaged and destroyed lung cells leak fluid into the lungs, and the body actually invokes an immune response to the material in the lungs. Even for physicians in an intensive care unit, this is a challenging problem to cure. Prevention cannot be emphasized too strongly. Minimize dust content in your absorbent, perform all pre-dive checks, and conduct

a bubble check at the start of your dive. If any early warning signs of flooding are noted, abort the dive. And finally, do not, under any circumstances, inhale the caustic absorbent solution.

Electronics Failure

Electronics on rebreathers (that use them) may fail in a variety of ways. The two most common involve flooding or battery failure. If electronics controlling automatic gas addition fail, most systems have a way to manually add the appropriate gas. This may rely on use of a second oxygen sensor display, that reads the sensors directly. In some rebreathers, electronics control all gas addition. There is no manual override. Others may not have a backup oxygen sensor display. With these units you must change to an OC bailout supply or initiate a SCR use mode and abort the dive if you experience electronics failure. If the failed electronics controlled a dive computer providing inert gas loading status, you have two options: use backup dive tables of the appropriate type, or abort the dive. Whenever you rely on an electronic computer to provide decompression information, carry an analog backup (plastic tables, for example) or a second, independent electronic computer.

Oxygen Sensor Failure

If you suspect your oxygen sensor may be giving incorrect readings, you can check it while under water. You should know the composition of your gas supply (either primary supply or diluent), having analyzed it before the dive. Ascend to a depth at which that mixture may be used without fear of oxygen toxicity. Flush the breathing loop by inhaling through your mouth and exhaling through your nose until there is no further gas in the loop. Fill the loop from the known gas supply. Immediately repeat this procedure two more times. Your breathing loop should now be filled with that gas of known composition.

Next, determine the absolute pressure at your depth. For example, if you are at 66 fsw (20 msw), the ambient pressure is three ata. Multiply the ambient pressure by the fraction of oxygen in the supply gas. This figure should match what your oxygen sensor is reading. If it does, your sensor is operating properly. If not, then you should not rely on that sensor. You may have to abort the dive.

Buoyancy Loss

Unexplained buoyancy loss may be due to two failures – either the buoyancy compensator or the breathing loop (or both) losing integrity. Assess the situation. Add air to the BC, and look for leaks. If it is the BC, it may only leak in some body orientations. If that is the case, change to a "no-leak" position and end the dive. If such a position cannot be found, then jettison your weights, if necessary, and ascend to the surface. While going up, vent your lungs continuously. Flaring may help manage ascent rate. If the BC does not appear to be the problem, then it is most likely gas is escaping from your breathing loop. Look for other indications of rebreather flooding, like gurgling or increased WOB. Close the counterlung exhaust valve if it is accessible. Since this situation may develop into a caustic cocktail incident, the best course is to terminate the dive. The sections on flooding and caustic cocktail in this chapter provide more details.

An Unsavory Cocktail

Two divers were cruising the bottom in 20 fsw (6 msw), one using a SCR, the other OC equipment. The SCR diver was doing a test dive, never having used this particular type of SCR before, while the OC diver was supervising. The dive progressed uneventfully for 15 minutes, when the SCR diver decided to see how various body positions would affect breathing resistance. Eventually he tried an inverted position in the water. His next inhalation was of an extremely caustic solution! Immediately closing and spitting out the mouthpiece, he attempted to switch to

his bailout cylinder. As he did so, he realized he could not inhale, as the muscles in his throat had gone into spasm. He then began an emergency swimming ascent. Rising up a few feet, he recognized that the muscle spasm was also preventing him from exhaling. Not wishing to embolize, he sank back to the bottom. His buddy recognized there was a problem, but not its nature. He attempted to pass his octopus regulator to the SCR diver, who refused to take it. It was not until the SCR diver began flushing his mouth out with seawater, and the buddy saw a large cloud of white, absorbent-laden water spewed out that he identified the problem. After a minute or so of flushing, during which he held his breath, the SCR diver's muscles relaxed sufficiently to allow him to resume breathing. He switched to his bailout bottle and ended the dive. The affected diver spent the next hours in a state of severe nausea and with a very painful throat and trachea. The next several days he was unable to swallow comfortably, and the next two weeks he coughed up phlegm. Several factors contributed to this incident: (1) The hydrostatic pressure caused by the inverted position caused a loose or poorly machined connection between the absorbent canister and the water trap to pop loose, allowing water to flood the canister. (2) The absorbent was not manufactured specifically for diving applications, and had been subjected to significant mechanical agitation. Thus it had a high powder content, allowing the absorbent to dissolve very rapidly into solution. (3) The inverted position also allowed the cockrail to bypass the counterlung, which would normally have acted as a secondary water trap. Instead, it flowed directly into the inhalation hose and on into the diver's mouth. Many lessons can be learned from this: (1) Always perform a thorough pre-dive check of the system. (2) Only use absorbents made for use in rebreathers. (3) Remove as much powder as possible from absorbents before filling canisters. (4) Know how

body attitudes will impact potential mechanical failures in the rebreather.

Bailout

Several scenarios discussed in this chapter have advocated a switch to your bailout gas supply as one available option. In Chapter 9 you read how to switch from your rebreather mouthpiece to your OC bailout mouthpiece. In essence, the same procedure is used here, with minor variations. Earlier, you were told to exhale most of your breath into your counterlung before switching systems, saving only enough to clear your regulator. When you switched back to the rebreather, you again exhaled most of the gas from your lungs before opening the mouthpiece and breathing from the rebreather. These actions helped maintain stable buoyancy, and minimized alteration of the gas in the breathing loop. In an emergency, these guidelines do not hold true. If possible, follow them. Otherwise, take a full breath, close the rebreather mouthpiece, and switch to your OC system. Correct for any buoyancy changes with your buoyancy compensator. Remember, to prevent air embolism exhale while you have no mouthpiece in your mouth.

Figure 12.9: Practice emergency responses on a regular basis.

Once you have switched to the bailout system, stabilize yourself, on the bottom, or at your depth if the bottom is far below. It should not take more than a minute to do so. Then ascend to the surface, breathing normally from your bailout system regulator. You may need to manually vent residual gas from the breathing loop during ascent either by pulling the dump mechanism on the overpressure valve or briefly opening the mouthpiece to control ascent rate and boyancy. If necessary, conduct any safety stops or required decompression stops during the ascent.

SCR Mode

As a mixed gas CCR user you may rely on your diluent gas cylinder for OC bailout. These cylinders are usually small, and provide a very limited gas supply. In some emergencies, however, you can extend that supply by using it in semi-closed rebreather mode (SCR mode).

Emergency scenarios in which SCR mode is an appropriate response include those in which the breathing loop is not flooded and the absorbent is working properly. Examples include sensor variations, loss of oxygen supply, electronics failure, or solenoid failure in systems lacking manual oxygen addition valves. If you are deeper than 20 fsw (6 msw), first turn off your oxygen supply and purge the lines. This will prevent the rebreather from adding oxygen based on inaccurate sensor readings. Then purge the breathing loop, and replace the gas with fresh diluent. Inhale and exhale three times. On the third exhalation, exhale through your nose, venting the gas from the breathing loop to the ambient environment. Replace the lost gas with diluent or nitrox supply gas. Continue this three-breath cycle as you ascend, but modify it near the surface. The breathing gas you are breathing will be hypoxic by the second inhalation in the cycle. This is acceptable while at depth, but becomes dangerous near the surface. As you near the surface, flush the breathing loop with fresh gas, or switch to OC mode. If you are using a mixed gas CCR, you have another option. Once you reach a depth of 20 fsw (6 msw) and have stabilized your buoyancy, turn off your diluent supply and turn on your oxygen supply. Flush the loop with pure oxygen, and continue to use the system in oxygen CCR mode. This will allow you to conduct your safety stop without worrying about depleting your diluent supply completely. Of course, this part of the procedure only works if you have oxygen remaining.

Allergic Reactions

The most common allergic reactions will be to the iodine or other cleaning agent, absorbent, or the rubber in the rebreather. If the symptoms are minor (skin redness, hives, or itching), the only treatment needed is to remove contact with the offending material. Qualified medical personnel may recommend or administer Benadryl®.

Severe case symptoms may escalate to shortness of breath, difficulty breathing, and a feeling of the throat closing or the tongue swelling. These cases may require the administration of Benadryl® and steroids by a physician. Such medical attention should be obtained as rapidly as possible. Very severe cases may lead to respiratory and cardiac arrest. Competent providers may administer epinephrine, and the emergency medical system should be activated. Immediate medical care is mandatory. Hospitalization may be required. Physicians or qualified paramedics may have to perform a tracheotomy to maintain respiration. If you suspect that you may be allergic to any of the substances with which you will come into contact diving rebreathers, you should consult a physician before beginning your training.

1. The two most common reasons for emergency events while diving rebreathers are: a. Lack of training and poor maintenance. b. Lack of training and hypoxia. c. Hypoxia and CNS oxygen toxicity. d. Poor maintenance and pilot error.

2. The one action that will help with most rebreather emergencies is: a. Buddy breathing with your partner. b. Immediate ascent to the surface. c. Switch to your bailout cylinder. d. Flush the breathing loop.

3. Response to caustic cocktail includes: a. Flushing the mouth with fresh water, and transporting to secondary medical care. b. Drinking weak vinegar, and transporting to secondary medical care.

c. Changing the absorbent in the scrubber before diving the rebreather again. d. All of the above.

4. If signs of breakthrough are noted during the dive, you should: a. Flush the breathing loop with fresh supply gas, and terminate the dive. b. Relax, and continue the dive at a lower work level. c. Switch to your bailout cylinder before continuing the dive. d. Ascend to half of your depth to lower the PCO2 before continuing the dive.

5. An oxygen sensor reading may be verified under water by: a. Flushing the breathing loop with fresh supply gas, and terminating the dive. b. Ascending to a depth where it is safe to breathe a known diluent or supply gas, flushing the breathing loop, and checking that your oxygen sensor reads the correct FO2. c. Ascending to a depth where it is safe to breathe a known diluent or supply gas, flushing the breathing loop, read your oxygen sensor, and calculate the expected PO2 based on depth and supply gas composition. d. It cannot be verified under water.

6. If you experience signs or symptoms of hypoxia, you should: a. Ascend immediately to a shallower depth. b. Flush the breathing loop with fresh supply gas or add oxygen. c. Surface and treat for air embolism. d. All of the above.

7. If your dive buddy has a severe allergic reaction while using a rebreather, you should: a. Eliminate contact with the allergenic substance or material, and continue to monitor. b. Flush the breathing loop with fresh supply gas or add oxygen to continue the dive. c. Administer Benadryl®, steroids, and be prepared to intubate. d. Eliminate contact with the allergenic substance or material, and seek medical attention.

8. In the event your rebreather becomes completely unusable under water, you should: a. Keep the mouthpiece in your mouth and conduct an emergency swimming ascent. b. Immediately spit the mouthpiece out and switch to your bailout cylinder, then ascend.

c. Flush the breathing loop with fresh supply gas, or add oxygen, and surface. d. Close the mouthpiece and switch to your bailout cylinder, then ascend.

9. If you experience symptoms of acute oxygen toxicity, you should: a. Flush the breathing loop with fresh supply gas or add oxygen, and surface. b. Immediately ascend. c. Reduce your breathing rate and work harder, to metabolize excess oxygen. d. Any of the above.

10. Most rebreather emergencies can be avoided by: a. Diving within the limits of your training. b. Performing all required pre-dive checks. c. Avoiding poor environmental conditions. d. Following the manufacturer's service schedules. e. All of the above.

Travel Overview

Most of us do not live on the shore of a diveable body of water. So, to use our rebreathers, we must travel. This may mean jumping in the car and driving a few minutes to the beach, or packing up and flying 12 hours to a resort. Regardless, we have to get there somehow. Unhappily, travel is one of the most extreme activities to which you will expose your rebreather. Most damage to rebreathers occurs during transport. Many of us now travel to exotic international destinations to dive. Dive travel is burgeoning because we want to see virgin reefs, outlandish fish and exotic cultures (not to mention being able to dive in warm, clear water instead of the cold mud we have at home). International travel with rebreathers has its own challenges, beyond "normal" international travel. This chapter will provide suggestions to reduce damage risk and make travel with your rebreather easier.

Objectives After reading this chapter, you will be able to: 1. Describe three actions each to make traveling with a rebreather easier by automobile, boat, and plane. 2. List ten items to include in an emergency spares kit for rebreathers. 3. State three special concerns regarding international travel.

General Travel Hints

Before traveling, prepare your rebreather. Make sure the system is clean and dry before packing. Disconnect any batteries to prevent them from being accidentally drained. Fasten any meters and displays so that if the rebreather is picked up, these will not fall to the end of their cabling or the ground and be damaged. Pad sensors and gauges well to minimize mechanical shocks. Set mechanical dual battery indicator/oxygen sensor meters to an oxygen sensor position, to prevent damage to the meter needle due to jarring. If possible, travel with the absorbent canister empty. Buy travel insurance that covers loss and damage.

Travel by Automobile

At some time, your rebreather will most likely see the inside of an automobile trunk or boot. When it does, you would like it to come out looking and working as well as it did going in. The first step in this direction is to place the rebreather gently into a position that does not kink hoses or pinch the counterlung. Use a travel case if available. If not, block the rebreather in place so that it will not tumble around. Place padding around the soft parts so hard objects, like tire irons, cannot shift and puncture counterlungs and hoses. Think about the environment in your trunk or the inside of your vehicle. Have you stored oily rags there? This would compromise oxygen clean parts of the rebreather. What about your (almost) empty emergency gas can? How hot is it where you park? Minimize contact with all solvents and potentially damaging chemicals. Remember that heat will drive water from the absorbent, rendering it useless, and will also damage oxygen sensors. Park in the shade, and keep your car cool. Finally, keep an expensive item like your rebreather covered and out of view. Neither you (nor your insurance company) would like to see it making its travels without you!

Figure 13.1: Packing your equipment properly will prevent damage during travel.

Figure 13.2: A small portable compressor can easily support a group of rebreather divers while traveling to remote locations. Courtesy of Cameron Etezadi

Travel by Boat

People often are prone to seasickness on rolling boat decks. So too are rebreathers. Only the symptoms differ. Divers get nauseous

and wish they would die. Rebreathers fall from hatches or decks, puncturing hoses, breaking hard pieces, and causing damage to other fragile components. The end result is the same, though: divers get nauseous and wish they would die! Secure your rebreather in a safe place. The boat may be stable at the dock, but once under way on rough seas, things are anything but still. If you keep your rebreather low, gravity cannot take over and "help" it down to the deck. Secure it so it will not roll or shift. People also have a tendency to roll around in rough seas. They intend to move in one direction, and find themselves unexpectedly (and sometimes violently) thrown in another. Put your rebreather where it will not provide the landing pad for their unanticipated flights.

Figure 13.3: Prevent heat damage to your rebreather's components – keep them covered and out of the sun.

We often exit the water cold and shivering after a dive. We sit in the sun to warm up, preparing ourselves for the next dive. This is good for us, but not for our rebreathers. Between dives, keep them out of the sun or covered to prevent heat damage to the absorbent and sensors. Boats are rarely dry anywhere above decks. When filling the scrubber canister on the back of a boat, be very careful to avoid

getting moisture into the canister. People often walk by, dripping water from their wet suits. Wind may be carrying spray from the bow down the length of the vessel. In rough seas, waves slosh on board. Find a sheltered, protected area to refill your scrubber canister. Optimally, you should fill it before you board the vessel, and minimize the number of times you must fill it while aboard. Finally, when assembling the system, stay clear of gas or diesel fumes from the boat engines. They are commonly blown across the lower deck. If you smell them as you assemble your unit, you may find fumes in the system recirculate with you for the entire dive. You would feel pretty poor by the end of your dive, after breathing diesel odors for 30 minutes! If available, upper decks are generally drier, cleaner, and less prone to fumes than lower decks.

How Would You Like Your Sensors Cooked?

An expedition left the United States for Guam. They were going to search for artifacts in 130-260 fsw (40-80 msw). They decided to use mixed-gas, closed-circuit rebreathers. Upon landing in Guam, they first cleared their equipment through customs. Moving it to the tarmac outside, they built a large pile of cases under the tropical sun. They then went to arrange their on-island transport. The merciless sun beat down. Several hours later, they had their trucks, loaded their gear, and drove off. The next day, they calibrated the CCRs and conducted their first orientation dives. During the dives, the divers were unable to keep the units at the proper PO2. Upon surfacing, they complained that the oxygen meters seemed to be providing spurious readings. Opening the units, the maintenance technician found that all of the oxygen sensors had dried-out membranes, and thus were not operating properly. Leaving the CCRs sitting in the sun at the

airport had fried the sensors, necessitating their replacement. This was accomplished ($900 later), and the expedition continued their dives without further mishap. Would you like your sensors sunny-side up?

Figure 13.4: Air travel requires careful packaging of your rebreather and absorbent.

Travel by Plane

Airline companies have a reputation for their ability to test luggage quality. They use a protocol called "test to destruction." Who doesn't have a story about the suitcasethat came back with holes in the side, or cases being returned with missing handles, etc.? Pack your rebreather securely in a hard-sided case. Pad it well. The author has found that cases manufactured by Underwater Kinetics and Pelican Products stand up to abuse and protect rebreathers they contain. Pack your rebreather with the assumption that the airline employees will select your bag for exceptionally vigorous "testing."

Breathing hoses should be removed from the rebreather beforeshipping. They will be better protected if you can pack them in another hard container inside the shipping case.Carrying a spare hose segment is a good idea, as hose damage is fairly common. Shipping high pressure gas cylinders is one of the most problematic aspects of traveling with your rebreather. Government security regulations mandate that all cylinders must be empty. They must also have the valves removed, and be left open for inspection. If a dust cap or piece of tape is left covering the cylinder opening, the cylinders are subject to confiscation. Airlines may impose more stringent requirements. Making appropriate arrangements with theappropriate airline representatives prior to the flight to get approval for shipping may minimize this problem. Rather than traveling with your personal cylinders, it may be simpler to obtain rental cylinders at your final destination. Many resort locations now offer 2L and 3L rental cylinders specifically for rebreather divers. Thesesame resorts can usually help you with oxygen and absorbent supplies.

Figure 13.5: There is nothing more pleasurable than using your rebreather in some warm, exotic dive location. Courtesy of Aquanauts Grenada

Some absorbents, not all, are classified as hazardous materials, and may not be shipped on passenger aircraft. Classification is based on the actual chemical composition of the compound. For example, anything with more than four percent sodium hydroxide is classified as a hazardous material. Several manufacturers compound their absorbent specifically to meet airline requirements. Sofnolime® and Sodasorb® are both formulated to avoid classification as a hazardous material. A far better solution is to check with the manufacturer of your absorbent prior to flying, and obtain from them a written confirmation that their product meets airline hazardous materials specifications. This statement may save you several hours of arguing. Material Safety Data Sheets (MSDSs) for several common absorbents and the pertinent section of FAA regulations covering hazardous materials are included in Appendix Q to assist you in your travels. In practice I have found that I encounter fewer security issues if I travel with solid state absorbents like Micropore's ExtendAir® cartridge than if I carry standard granular absorbents. Batteries are another item now subject to increased scrutiny. In particular, the quantity of lithium batteries that may be carried is restricted. If lithium batteries are installed in the item in which they are to be used, then they are not regulated. Loose batteries, such as spare or replacement batteries, must fall within prescribed lithium content limits. In some instances, security agents screening equipment may mandate that any installed batteries of any type be removed from the item to prevent accidental shorting during transport.

Figure 13.6: Material Safety Data Sheet.

Travel in a Post-9/11 World

Since the terrorist events of September 11, 2001 (9/11), traveling with many items on commercial airliners has become much more restrictive. Any item that could be construed as a weapon, or could be even possibly used as a weapon, may now be subject to either carry-on or checked baggage restrictions. Most of these new regulations have been established by the United States Transportation Security Administration (TSA), which has oversight for traveler safety in the United States. Many of their regulations have been adopted by security screening agencies in other parts of the world as well. The problem that we face as rebreather divers is that most TSA screeners are unfamiliar with their own regulations when it comes to unusual items like compressed gas cylinders, carbon dioxide absorbents, or rebreather components like BOVs or full face masks. Any item is subject to confiscation at the discretion of the individual

TSA agent screening your luggage. It helps if you carry a copy of the relevant TSA regulations or guidelines covering items you may be carrying as part of your rebreather equipment, but this is not a complete solution. The guidelines issued by central TSA management are just that... guidelines. Any of these guidelines may be interpreted differently by different screeners. As an example, some screeners may look at an empty compressed gas cylinder, and allow it as carry-on luggage if the valve is removed and the cylinder left open as required. Another may look at the "Oxygen" label on the cylinder, and disallow that cylinder as containing a hazardous material while allowing you to carry an empty diluent cylinder. A third might confiscate both cylinders, as they could potentially be used as bludgeons. All of these scenarios have happened to me during travel on commercial airlines. Producing a copy of the relevant regulations may help in some cases, but not always. If not, I have found it beneficial to ask for a supervisor to consider the items I am carrying. Often a TSA agent who is a diver themselves, or is more familiar with diving equipment, will allow items that those unfamiliar with diving will not. If the supervisor is unwilling to allow the gear, then asking him to consult another supervisor may prove beneficial. Maintaining a positive, cooperative, agreeable attitude will more likely lead to a positive decision than adopting an aggressive or combative demeanor. If you are carrying the items as part of your personally accompanied items in the cabin of the aircraft and they are disallowed, you may be permitted the opportunity to check the items instead. The issue here is that you have no direct contact with the agent screening your bags. All checked bags are subject to x-ray screening, and may then be opened and examined by hand. This is usually what has happened with my dive equipment.

Figure 13.7: Carrying cylinders in your hand luggage and treating TSA agents with courtesy and respect seems to yield the highest probability that your cylinders will arrive at your final destination with you.

Figure 13.8: Traveling with a small oxygen booster will allow you to get full fills at nearly any resort worldwide.

If the agent examining your checked luggage is unfamiliar with regulations, you do not have the opportunity to educate or reason with them. Items such as cylinders may be confiscated without your knowledge, with your finding out only when you reach your final destination. To mitigate this possibility, I enclose a letter to any TSA agent who might open my baggage explaining the purpose of the equipment, the regulations involved, and asking them to call me on my cell phone if they have any questions. A sample of a letter is included in Appendix Q. Even with these measures, I have had cylinders confiscated. While researching this book, and in an effort to solve my own travel problems, I have attempted to go to TSA management at both the regional and national levels. I asked for clarifications to their regulations, and a letter stating that if these regulations or guidelines

were followed, that the items in question would be permitted as baggage. After weeks of interaction, I ultimately received the reply that individual supervisors at any site have ultimate authority to allow or disallow any item, regardless of regulations! Thus, you must deal with the local supervisor, as they have final authority in any case, and will be supported in their decisions by the regional management. The current situation, while both dismaying and capricious, is reality. Whenever possible, I try to pack items that might be confiscated from my checked baggage as carry-ons, so that I have the opportunity to discuss issues with the TSA agents if there is a problem. An even better solution is to arrange for the items to be shipped to your final destination using a shipping service like the United Parcel Service (UPS) or Federal Express, or to obtain an equivalent rental or purchased item at your final destination. Of course, neither of these options may be possible. The final words I can offer you as a traveler are "Good luck!"

While Away From Home

Many destinations will not have normal support items for rebreather diving. Carry a spares kit containing those user serviceable parts you might require. Your kit might include the items in Table 13.1, page 323. Include any other items that might be required for your particular model of rebreather. Along with the spares, you may need tools. Many times I have arrived at a "well-equipped" dive shop to find they did not have the size wrench I needed to make a minor adjustment. Note what you use on a frequent basis to pre- and post-dive your unit. Then assemble a kit containing the wrenches, screwdrivers, hex keys, pliers, etc. that you might need. Also include any instruments, like a voltmeter, that you use. Put all this into a special dry box, and dedicate it to your rebreather (Table 13.2). This way you always have what you need available. Confirm that you can get the breathing gas or diluent you need at your destination. You may need EANx with higher oxygen content than available. In some states or countries, you may need a medical prescription to get 100% oxygen. Carry any special filling adapters or

whips necessary to fill your rebreather gas supply cylinders. You shouuld also carry the instruments needed to analyze your breathing gas; either two oxygen analyzers, or one analyzer and an oxygen sensor on your rebreather. Table 13.1: Spares Kit Items Table 13.2: Tool Kit Items Tackle or dry box Oxygen compatible o-ring grease Disinfectant O-rings used in your rebreather, especially odd sizes Spare breathing hoses Spare dust trap pads Special fasteners or clips Hose adapters Batteries Oxygen sensors Gaskets Spare switch Special electrical connectors Screws, nuts, bolts Solder Electrical Wire Other items as needed for your rebreather

Slot screwdriver (2 sizes) Phillips head screwdriver (2 sizes) Crescent wrench (2 sizes) Allen key set (metric or imperial, as required) Pliers Needle nose pliers O-ring pick Multitool (like Leatherman®) Scuba tool Diagonal cutters Voltmeter Soldering iron Cylinder filling adapters Oxygen meters Flow meter Interstage pressure gauge Special tools, like hose tools or rebreather specific box wrenches

Figure 13.9 Carry spares, tool, and gas adapter kits when traveling with your rebreather.

It's an old adage: "You meet many interesting people when you travel." Expect to be one of those "interesting people." Rebreathers are not common at many dive destinations. Other divers are likely to ask you many questions about that "strange contraption" on your back. Be patient – they are your dive buddies of tomorrow. Also, you can tactfully help them learn how to avoid damaging your rebreather as they share boat space with it, and you! Another helpful hint: take a small hair drier capable of operating with no heat. It can be used to blow through counterlungs and breathing hoses, drying them inside much faster than by air-hanging. This can be a real benefit if you will be packing to travel soon after the dive, and will reduce the chance that any "beasties" will take up residence in your rebreather.

International Travel

If you travel to other countries to dive (and who doesn't hope to?), you have a few other items to consider. Different countries use different "standard" fittings and connectors. An oxygen adapter from the United States will not fit a cylinder from Australia. An air cylinder valve from Germany will not mate to a fill station in California. Carry whatever adapters you will need to get your cylinders filled. Likewise,

carry electrical adapters to allow you to use any tools or battery chargers that you need while overseas.

Figure 13.10: Special regulations may apply to your rebreather in foreign countries. Obtain the proper documents and customs forms.

Figure 13.11: Lay your rebreather on a blanket or mat when diving from a beach or other sandy area.

Recreational rebreathers evolved from military systems. In fact, some recreational systems are more advanced in design than their military counterparts. Some countries may consider rebreathers to be restricted in the same manner as machine guns, bazookas, and land mines. Both import and export may be restricted, requiring special permission and documentation to enter or leave the country. Before travel, check with representatives from all countries you will be leaving or entering to determine if any special regulations apply. Finally, carry a bill of sale or receipt that shows you have purchased the equipment in your home country. This can prevent a zealous customs agent from trying to assess an import or export tax on your expensive rebreather. Some countries have registration documents that are issued by the customs agency that prove that you did not acquire the equipment abroad. These must be obtained before you leave on your travels. You may have to register your gear with customs on entry and then show you are taking it away again (not selling it) to avoid paying import tax on it. Paying taxes hurts enough – let's not do it more than once!

Figure 13.12 Rebreathers may allow you increased bottom time and a more enjoyable vacation. Courtesy of Wakatobi Dive Resort

Common Sense

Most of the items mentioned in this chapter you might consider nothing more than "common sense." You're right! Unfortunately, many of these "common sense" items are commonly ignored, forgotten, or considered "uncommon sense" by those that matter— divers. If you find new situations that are not covered in this chapter (I once had to transport my dive gear via camel), common sense will prevent many problems and keep both you and your rebreather happy and diving. By the way, if you do travel via camel, make sure all breathing hose ports are blocked, as camel fleas in the loop would be hard to breathe! So become an uncommon diver, and practice common sense.

Figure 13.12: When traveling by camel, be sure breathing hose ports are blocked to prevent fleas from entering breathing loop.

1. When traveling by automobile, you should: a. Park in the sun, so that the absorbent will be sufficiently warm to function. b. Move any oily rags to the opposite side of the trunk, away from the rebreather. c. Block the rebreather to minimize movement while traveling. d. B and C above.

2. On a dive boat, you should: a. Stow your rebreather on the deck, rather than a bench. b. Pre-dive your rebreather in an area clear of fumes. c. Keep the rebreather covered or out of direct sunlight. d. All of the above.

3. When traveling by airplane, you should: a. Empty gas cylinders and remove valves before traveling. b. Pack absorbent in a rigid container. c. Pack the rebreather in a sturdy shipping container. d. All of the above.

4. If traveling internationally, you should: a. Carry gas cylinder fill adapters. b. Register your rebreather with the appropriate customs authority before leaving the country. c. Check before leaving to see if any special import/export regulations apply. d. Carry a Bill of Sale proving ownership and location of purchase. e. All of the above.

5. List 10 items to carry in your rebreather spares kit: _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________

Where Do You Go From Here? Overview

Now that we have reached the end of the road, there is only one more lesson to pass on to you: this is not the end of the road! This has only provided an introduction. After the lectures, pool work, and open-water dives you have done (or will do) with your instructor, you have a measure of knowledge and competency that will enable you to be certified to use one model of rebreather. Your C-card is a license to continue learning, not a certificate that says you have learned all there is to know.

Objectives After reading this chapter, you will be able to: 1. List six ways to keep your knowledge of rebreather development current. 2. Explain what actions to take after a hiatus from rebreather diving. 3. State a minimum amount of rebreather experience to have logged before participating in technical diving activities with your system.

Keep Diving

It is important now that you get out and practice the skills you have learned. Go diving. Read more about your rebreather and other

rebreathers. Go diving. Read about others' experiences with rebreathers, and learn from their mistakes. Go diving. Attend educational programs where you can learn more about rebreathers. Go diving. Swap sea stories with other rebreather divers, and learn from their trials and tribulations. Go diving. Join rebreather list servers on the Internet. And go diving! If you think you detected a subtle theme in the foregoing paragraph, you did. Frequent diving experience using your rebreather is the key to remembering what you have learned in your course. It is the only way to develop the muscle memory that will help you when something goes wrong (as some day it will!). If you do not continue rebreather diving, you will soon forget most of this material. If you must take an extended break from your rebreather diving, then it is best that you complete a refresher program with a qualified rebreather instructor before you jump back in the water. Depending on how long a break, the refresher program might last from a few hours to a few days.

Figure 14.1: You may find additional rebreather training beneficial. Courtesy of Elaine Jobin

Figure 14.2: Each course builds on the introductory course, which readies you for more complex dives. Left photo courtesy of Dennis Ratcliffe Right photo courtesy of Myfanwy Rowlands

Figure 14.3: Do not attempt technical dives with a rebreather until you have logged at least 50 to 100 hours in standard openwater environments. Right photo courtesty of National Park Service and Brett Seymour

Figure 14.4: Qualified instructors can help you refresh your skills in a specialized program.

Your entry-level SCR or CCR Diver course has introduced you to the basic skills and knowledge you need to use your rebreather. Upon certification, you are considered competent to engage in openwater rebreather diving activities without supervision, provided the diving activities and the areas dived approximate those of training. One of the common problems new rebreather divers face is that they are so excited about their new skills, they immediately want to jump in and participate in the same types of activities they do while diving OC scuba. This natural tendency can get you in trouble. You might be a very experienced cave or wreck diver, decompression diver, or underwater photographer using OC equipment, but not rebreathers. You need to spend some time in benign environmental conditions, with little task loading, getting used to your rebreather and internalizing the skills you have recently learned. Do not start "pushing the envelope" until you have at least 25-50 additional hours

using your rebreather. Better still, get additional advanced rebreather training from a qualified instructor to prepare you for your ultimate goals.

Future Training

Your basic rebreather class is a ticket to other training as well. From there you may elect to get type rated to use other rebreathers. The material learned in this text and your class will significantly reduce the time necessary to do so. You may cross-certify to use either SCRs or CCRs. This text has provided much of the knowledge base necessary to do so. As mentioned in the first chapter, there is no single type of scuba equipment that is "best" for every circumstance, and perhaps you will discover another type of rebreather will better fit your needs.

Figure 14.5: By the time this young girl is old enough to dive, rebreathers will be smaller, lighter, and much easier to use. Courtesy of Elaine Jobin

If you have been certified as an entry-level SCR or CCR diver, you may wish to pursue advanced SCR or CCR training, which will introduce you to the principles of decompression diving using a rebreather. The procedures build on those you learned in your rebreather course, and on those taught in Decompression Techniques, Technical Nitrox, or Extended Range Diving specialty courses. The advanced rebreather class covers theory, special

procedures, equipment considerations and rigging, and other information. Some rebreathers may be used with helium-based breathing gases, which also requires additional advanced training.

Mastering Rebreathers, Volume 2

The purpose of this text is to discuss introductory rebreather principles, and introduce the readers to rebreathers, basic physiology and physics, and their use for recreational diving environments. What if you want to go beyond standard recreational limits?

Figure 14.6: New rebreathers and new modifications to current rebreathers come out frequently. Stay abreast of new developments by reading magazines and talking to other rebreather divers. Coutesy of Pascal Eeckhoudt

Mastering Rebreathers, Volume 2 has been written with this objective in mind. It continues where this text leaves off. Volume 2 covers the use of rebreathers for decompression diving, overhead environment diving, including caves and wrecks, trimix and other helium based diluent gas diving, and ice diving. Other topics include gas management, bailout options, advanced emergency procedures, and accident analysis.

Volume 2 focuses primarily on the use of mixed gas closed circuit rebreathers, as they provide the greatest functionality and flexibility for these types of diving. As with this text, Volume 2 is not meant as a stand-alone instructional program. It is meant to be read as part of an advanced training program under the guidance of a qualified rebreather instructor. If you are interested in pursuing the types of diving listed, it will provide an invaluable resource as you undertake such training.

Staying Current

We live in exciting times! For the first time in the history of recreational diving, mixed gas rebreathers are becoming widely available. The first mass-produced unit, the Dräger Atlantis®, was introduced in 1996, and as of the time of this writing it is estimated that more than 6,000 units have been sold worldwide. Ambient Pressure Diving introduced the Inspiration® in 1998, the first mixed gas CCR mass-produced for the recreational market. Since that time, AP Diving has sold about 7,000 CCRs of different models and configurations.

Figure 14.7: Mastering Rebreathers, Volume 2 By: Jeff Bozanic.

Figure 14.8: AP Diving's Inspiration® and Evolution® are the most available mixed gas CCRs in the recreational diving community.

More than 30 other manufacturers have announced, produced, or sold more than 50 models of rebreathers in various quantities. As with any newly mass-produced technology, widespread distribution results in identification of problems, areas that can be improved, and new ideas to make the use of that technology lower risk and easier. We can expect this to occur with all rebreathers now on the market, as well as those that may be introduced over the next several years. Just as automobiles were not introduced in 1906 with power steering, fuel injection, and disc brakes, our rebreathers today lack innovations that rebreather users of the next century will take for granted. In fact, we probably would not even recognize most of the technology that will be used in diving equipment a century from now! Because we are just starting out on the exciting path of rebreather development, innovations are coming rapidly. As an example, in the first two years the Dräger Atlantis® was out, they

made a host of major and minor design and production changes to their units. In fact, the changes were so significant, the unit was renamed the Dolphin®. Some changes were immediately apparent, others were invisible to the user. They improved the counterlung by constructing it of a heavier, sturdier material, added drain plugs, and changed from metal to plastic connectors (correcting an abrasion problem that wore holes in the counterlung). The scrubber canister was redesigned to prevent warping and over-tightening. Additional sonic valves have been made available for the mass flow controller. Dosage filters have been made of a new material, to prevent clogging of the filter. Perhaps most importantly, an oxygen sensor has been made available. And this is only a partial list of the improvements! Similar changes are coming to CCRs. CO2 sensors, automatic diluent injection to compensate for PO2 spikes, ADBOVs, single-side hose configurations, smaller and lighter units, improved gas control software, integrated inert gas monitoring algorithms, video liquid crystal displays (LCDs) integrated in masks to display PO2 and decompression status, solid state scrubbers, reusable absorbent media, disposable counterlungs and breathing hoses, and selfcleaning or anti-bacterial materials are all potential improvements on the horizon. Some have been announced and are in production development now. It is a time of rapid change. As of this moment, you are up-to-date in using your rebreather for recreational diving. Tomorrow you will be a little out of date. By the day after tomorrow, you'll be notably behind. By next week or next month, you may be completely out of date. The only solution is to stay abreast of the new developments occurring with rebreathers in general, and yours in particular. As new information and rebreathers are being introduced regularly, it is impossible for a textbook like this to remain current. Use other resources to find updated information. Read diving magazine articles detailing rebreathers; surf the web for new information; visit your local dive store and chat about what is occurring in the rebreather market. These activities and others will help you keep your

knowledge current. Remember that this is your responsibility as a certified rebreather diver!

Figure 14.9: Consumer dive trade shows are good places to learn about new rebreathers and improvements to old ones. Courtesy of Elaine Jobin

The Adventure Continues...

Congratulations on your accomplishment! I envy you your new adventure, setting out to master the use of your rebreather. You will find that while you are still learning, you will be having a grand time cuddling up to unwary fish, spending twice as long in the water with less gear, and enjoying the challenge. What more could any diver ask? As you set out on your own, keep your lessons in mind. Dive within the limit of your knowledge and ability. Stay safe. Be responsible for yourself. And, most importantly...

Have Fun Diving!

Contents of Appendices A. Dive Tables US Navy–Air, 2008 US Navy–Constant PO2, 2008

US Navy–Air, 1999 US Navy–Constant PO2, 1999 Bühlmann–Air Bühlmann–Constant PO2 B. AP Diving Inspiration/Evolution C. B & E Manufacturing Nautilus D. Dive Rite O2ptima CCR E. Dräger Atlantis/Dolphin DrägerRay OXYguage LAR V F. Halcyon RB80 G. Hollis Gear Prism 2 H. Hydrospace Engineering Neptune NX Neptune LP I. Innerspace Systems

Megalodon J. Jetsam Technology Classic KISS Sport KISS K. MDEA Frog MK-1 L. Poseidon Discovery Mk 6 M. Rebreathers Australia Abyss Stingray N. rEvo Rebreathers rEvo II CCR rEvo II Hybrid CCR O. Titan Dive Gear Titan CCR P. VR Technology Sentinel Q. Documentation/Regulations Federal Aviation Administration (FAA) Letter Code of Federal Regulations (CFR) TSA Information Flying With Compressed Gas Cylinders Sample Letter to TSA Screening Agents Material Safety Data Sheets R. References S. Glossary T. Index

U.S. Navy--Air

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break is recommended if oxygen decompression exceeds 20 minutes

NOAA and REPEX Oxygen Exposure Limits: PO2 (atm): 1.6 1.5 1.4 1.3 1.2 1.1 1.0 Depth (fsw): 218 202 187 171 155 139 124 Depth (msw): 66.4 61.6 57 52.1 47.2 42.4 37.8 O2 Limit (Single dive, min): 45 120 150 180 210 240 300 O2 Limit (24 hr total, min): 150 180 180 210 240 270 300 %CNS per min: 2.22 0.83 0.67 0.56 0.48 0.42 0.33 OTU per minute: 1.92 1.78 1.63 1.48 1.32 1.16 1.00 Table 2 Symbol Key: 1:06

Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives.

Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 30 fsw (9.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to

compute the equivalent dive time. Decompress using the 30 fsw (9.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

U.S. Navy Dive Tables -- Air

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 0.7 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower than 16 fsw (4.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes 79 This depth profile exceeds the MODo of 77 fsw (23.3 msw)

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 570 Maximum per 24 hours (min) 570 %CNS per minute OTU per minute:

0.18 0.47

Table 2 Symbol Key: 6:07 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 39 fsw (12.1 msw) repetitive

dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 39 fsw (12.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 0.7 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 0.8 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower than 19 fsw (5.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes 98 This depth profile exceeds the MODo of 92 fsw (28 msw)

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 450 Maximum per 24 hours (min) 450 %CNS per minute OTU per minute:

0.22 0.65

Table 2 Symbol Key: 3:22 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 43 fsw (13.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to

compute the equivalent dive time. Decompress using the 43 fsw (13.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 0.8 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 0.9 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower than 22 fsw (6.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes 108 This depth profile exceeds the MODo of 108 fsw (32.8 msw)

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 360 Maximum per 24 hours (min) 360 %CNS per minute OTU per minute:

0.28 0.83

Table 2 Symbol Key: 3:26 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 46 fsw (14.1 msw) repetitive

dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 46 fsw (14.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 0.9 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 1.0 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower 26 fsw (7.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes 124 This depth profile exceeds the MODo of 124 fsw (37.6 msw)

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 300 Maximum per 24 hours (min) 300 %CNS per minute 0.33 OTU per minute: 1.00 Table 2 Symbol Key: 4:17 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 49 fsw (15.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to

compute the equivalent dive time. Decompress using the 49 fsw (15.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 1.0 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 1.1 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower 29 fsw (8.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 240 Maximum per 24 hours (min) 270 %CNS per minute 0.42 OTU per minute: 1.16 Table 2 Symbol Key: 5:05 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 53 fsw (16.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 53 fsw (16.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 1.1 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 1.2 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower 32 fsw (9.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 210 Maximum per 24 hours (min) 240 %CNS per minute 0.48 OTU per minute: 1.32 Table 2 Symbol Key: 6:11 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 56 fsw (17.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 56 fsw (17.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 1.2 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 1.3 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower 35 fsw (10.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 180 Maximum per 24 hours (min) 210 %CNS per minute 0.56 OTU per minute: 1.48 Table 2 Symbol Key: 7:20 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 59 fsw (18.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 59 fsw (18.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 1.3 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 1.4 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower 39 fsw (11.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 150 Maximum per 24 hours (min) 180 %CNS per minute 0.67 OTU per minute: 1.63 Table 2 Symbol Key: 7:23 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 62 fsw (19.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 62 fsw (19.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Constant PO2 Dive Tables -- PO2 = 1.4 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

PO2 = 1.5 atm These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008 Table 1 Symbol Key: Dives shallower 42 fsw (12.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 120 Maximum per 24 hours (min) 180 %CNS per minute 0.83 OTU per minute: 1.78 Table 2 Symbol Key: 9:10 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 66 fsw (20.1 msw) repetitive dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 66 fsw (20.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Warning: Diving using gas mixtures with oxygen partial pressures greater than 1.4 atm is not recommended.

Constant PO2 Dive Tables -- PO2 = 1.5 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes Warning: This table should be used only for contingency planning purposes or emergency situations.

PO2 = 1.6 atm

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S. Navy Diving Manual, Volume 2, Revision 6, 15 April 2008

Table 1 Symbol Key: Dives shallower 45 fsw (13.9 msw) have no nitrogen loading Highest repetitive group that can be achieved at this depth, regardless of bottom time A 5-minute air break or change to lower set point (SP) is recommended if oxygen decompression (SP maintained at 1.6 atm at 20 fsw / 6.1 msw) exceeds 20 minutes

NOAA and REPEX Oxygen Exposure Limits: Maximum Single Exposure (min) 45 Maximum per 24 hours (min) 150 %CNS per minute 2.22 OTU per minute: 1.92 Table 2 Symbol Key: 10:25 Dives following surface intervals longer than this are not repetitive dives. Use actual bottom times in the Air Decompression Tables (Table 1) to compute decompression for such dives. Table 3 Symbol Key: If no RNT is given, then the repetitive group does not change Read horizontally to the right to the 69 fsw (21.1 msw) repetitive

dive depth. Use the corresponding residual nitrogen times to compute the equivalent dive time. Decompress using the 69 fsw (21.1 msw) table (EAD 30 fsw / 9.1 msw).

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Warning: Diving using gas mixtures with oxygen partial pressures greater than 1.4 atm is not recommended.

Constant PO2 Dive Tables -- PO2 = 1.6 atm

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

Warning: This table should be used only for contingency planning purposes or emergency situations.

US Navy Dive Tables -- Air These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999. Table 1 Symbol Key: * Highest repetitive group that can be achieved at this depth regardless of bottom time.

NOAA and REPEX Oxygen Exposure Limits: PO2 (atm): 1.6 1.5 1.4 1.3 1.2 1.1 1.0 Depth (fsw): 218 202 187 171 155 139 124 Depth (msw): 66 61 57 52 47 42 37 O2 Limit (min): 45 120 150 180 210 240 300 O2 Limit (24 hrs): 150 180 180 210 240 270 300 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change. Read one column to the right, use that RNT and depth group for the next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL)

Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999. Constant PO2 Dive Tables—PO2 = 0.8 atm Table 1 Symbol Key: Dives shallower 19 fsw (6 msw) have no nitrogen loading * Highest repetitive group that can be achieved at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 450 Maximum per 24 hours 450 % CNS per minute 0.22 OTU per minute 0.65 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 0.9 atm Table 1 Symbol Key: Dives shallower 23 fsw (7 msw) have no nitrogen loading * Highest repetitive group that can be achieved at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 360 Maximum per 24 hours 360 % CNS per minute 0.28 OTU per minute 0.83 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 1.0 atm Table 1 Symbol Key: Dives shallower 26 fsw (8 msw) have no nitrogen loading * Highest repetitive group that can be achieved at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 300 Maximum per 24 hours 300 % CNS per minute 0.33 OTU per minute 1.00 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 1.1 atm Table 1 Symbol Key: Dives shallower 29 fsw (9 msw) have no nitrogen loading * Highest repetitive group that can be achieved at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 240 Maximum per 24 hours 270 % CNS per minute 0.42 OTU per minute 1.16 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 1.2 atm Table 1 Symbol Key: Dives shallower 33 fsw (10 msw) have no nitrogen loading Highest repetitive group that can be achieve at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 210 Maximum per 24 hours 240 % CNS per minute .48 OTU per minute 1.32 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 1.3 atm Table 1 Symbol Key: Dives shallower 36 fsw (11 msw) have no nitrogen loading Highest repetitive group that can beachieved at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 180 Maximum per 24 hours 210 % CNS per minute 0.56 OTU per minute 1.48 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 1.4 atm Table 1 Symbol Key: Dives shallower 36 fsw (12 msw) have no nitrogen loading Ä Highest repetitive group that can beachieved at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 150 Maximum per 24 hours 180 % CNS per minute 0.67 OTU per minute 1.63 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 1.5 atm Table 1 Symbol Key: Dives shallower 43 fsw (13 msw) have no nitrogen loading Ä Highest repetitive group that can be achieve at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 120 Maximum per 24 hours 180 % CNS per minute 0.83 OTU per minute 1.78 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

Warning: Diving using gas mixtures at an oxygen partial pressure greater than 1.4 atm is not recommended. This table

should be used only for contingency planning purposes or emergency situations.

These tables are based on the U.S. Navy Dive Tables, as printed in the U.S Navy Diving Manual—Volume 2, Revision 4, January 1999.

Constant PO2 Dive Tables—PO2 = 1.6 atm Table 1 Symbol Key: Dives shallower 46 fsw (14 msw) have no nitrogen loading Ä Highest repetitive group that can be achieved at this depth regardless of bottom time

NOAA Oxygen Exposure Limits Maximum Single Exposure 45 Maximum per 24 hours 150 % CNS per minute 2.22 OTU per minute 1.92 Table 3 Symbol Key: ** If no RNT is given, then the repetitive group does not change Read one column to right, use that RNT and depth group for next dive

Table 3: Repetitive Dive Timetable 00 Light Face Numbers are Residual Nitrogen Times (RNT) 00 Bold Face Numbers are Adjusted No-Decompression Limits (ANDL) Table 2: Surface Interval Time (SIT) Table Time Ranges in Hours:Minutes

Warning: Diving using gas mixtures at an oxygen partial pressure greater than 1.4 atm is not recommended. This table

should be used only for contingency planning purposes or emergency situations.

Bühlmann Algorithm Dive Tables--Air

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's

ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas containing an oxygen fraction of AT LEAST 0.21. Accelerated decompression data are for use with gas containing an oxygen fraction of AT LEAST 0.75 at the 20 fsw/6msw and 15 fsw/4.5msw stops. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not account for physical condition of the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA and REPEX Oxygen Exposure Limits: PO2 (atm) 1.6 1.5 1.4 1.3 1.2 1.1 1.0 Depth (fsw) 218 202 187 171 155 139 124 Depth (msw) 66 61 57 52 47 42 37 O2 Limit (Single dive, min) 45 120 150 180 210 240 300 O2 Limit (24 hr total, min) 150 10 180 210 240 270 300 %CNS per minute 2.22 0.83 0.67 0.56 0.48 0.42 0.33 OTU per minute 1.92 1.78 1.63 1.48 1.32 1.16 1.00 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min)

Bühlmann Algorithm Decompression Tables -- Air

Accelerated decompression data are for use with gas containing an oxygen fraction of AT LEAST 0.75 at the 20 fsw/6msw and 15 fsw/4.5msw stops.

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 0.7 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 0.7 ATM. Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not

account for physical condition of the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 450 Minutes Maximum per 24 hours 450 Minutes %CNS per minute 0.18 OTU per minute 0.47 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 16 fsw (5 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 0.7 atm

Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM.

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 0.8 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 0.8 ATM. Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not

account for physical condition of the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 450 Minutes Maximum per 24 hours 450 Minutes %CNS per minute 0.22 OTU per minute 0.65 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 19 fsw (6 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 0.8 atm

Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM.

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 0.9 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 0.9 ATM. Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not

account for physical condition of the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 360 Minutes Maximum per 24 hours 360 Minutes %CNS per minute 0.28 OTU per minute 0.83 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 23 fsw (7 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 0.9 atm

Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM.

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 1.0 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 1.0 ATM. Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not

account for physical condition of the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 300 Minutes Maximum per 24 hours 300 Minutes %CNS per minute 0.33 OTU per minute 1.00 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 26 fsw (8 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 1.0 atm

Accelerated decompression data are for use with gas at a constant PO2 of AT LEAST 1.1 ATM.

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 1.1 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 1.1 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not account for physical condition of

the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 240 Minutes Maximum per 24 hours 270 Minutes %CNS per minute 0.42 OTU per minute 1.16 Key: Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 29 fsw (9 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 1.1 atm

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 1.2 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 1.2 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not account for physical condition of

the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 210 Minutes Maximum per 24 hours 240 Minutes %CNS per minute 0.48 OTU per minute 1.32 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 33 fsw (10 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 1.2 atm

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 1.3 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 1.3 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not account for physical condition of

the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 180 Minutes Maximum per 24 hours 210 Minutes %CNS per minute 0.56 OTU per minute 1.48 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 36 fsw (11 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 1.3 atm

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 1.4 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 1.4 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not account for physical condition of

the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 150 Minutes Maximum per 24 hours 180 Minutes %CNS per minute 0.67 OTU per minute 1.63 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 39 fsw (12 msw) have no nitrogen loading

Bühlmann Algorithm Decompression Tables -- PO2 = 1.4 atm

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 1.5 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 1.5 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not account for physical condition of

the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 120 Minutes Maximum per 24 hours 180 Minutes %CNS per minute 0.83 OTU per minute 1.78 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 43 fsw (13 msw) have no nitrogen loading

Warning: Diving using gas mixtures with oxygen partial pressures greater than 1.4 atm is not recommended.

Bühlmann Algorithm Decompression Tables -- PO2 = 1.5 atm

Warning: This table should be used only for contingency planning purposes or emergency situations.

Bühlmann Algorithm Constant PO2 Dive Tables -- PO2 = 1.6 atm

These tables are based on Equivalent Air Depth (EAD) conversions of the IANTD Air Diving & Decompression Tables © 1995 IANTD, Inc./Repetitive Diver, Inc. These tables are based on Bühlmann's ZHL-16 Algorithm for 0-1000 feet (0-300m) above sea level. They were originally produced using Cybortronix DPA software. The Repetitive Dive Groups are not transferable to ANY other tables. A 3-minute safety stop is required for all dives. These tables are for use with gas at a constant partial pressure of oxygen (PO2) of AT LEAST 1.6 ATM. The 15 fsw/4.5msw stops MUST be taken at 15 fsw/4.5msw. These tables do not account for physical condition of

the diver, difficulty of the dive, water temperature, or other individual variables.

NOAA Oxygen Exposure Limits: Maximum Single Exposure 45 Minutes Maximum per 24 hours 150 Minutes %CNS per minute 2.22 OTU per minute 1.92 Key: Repetitive Group Letter Residual Nitrogen Time (min) Adjusted No-Decompression Time (min) Dives shallower than 46 fsw (14 msw) have no nitrogen loading

Warning: Diving using gas mixtures with oxygen partial pressures greater than 1.4 atm is not recommended.

Bühlmann Algorithm Decompression Tables -- PO2 = 1.6 atm

Warning: This table should be used only for contingency planning purposes or emergency situations.

AP Valves Inspiration/Evolution Pre-Dive Checklist

Check for signs of dirt, deterioration and damage to any part of the CCR at all stages.

DIVE NUMBER 123

1. 2. 3. 4. 5. 6. 7.

Inspect pneumatics hoses Inspect handsets, cables, electrical connections Check batteries Refill scrubber basket if necessary Install charged basket, lubricated o-ring and spacer ring Check lid o-ring and attach lid Analyze, check pressure, and install filled gas cylinders O2____% ______ psi/bar Diluent____% ______ psi/bar O2____% ______ psi/bar Diluent____% ______ psi/bar O2____% ______ psi/bar Diluent____% ______ psi/bar

8. Install canister (snorkel to right), blow oxygen hose clear and attach to solenoid inlet, attach hoses to counterlung T-pieces 9. Route electronics cables 10 Assemble counter lungs and install manual gas addition valves 11. Check hoses, mouthpiece, verify non-return valve operation, and install ensuring gas is to the right 12. Negative pressure test 13. Turn on cylinders, pressurize lines, turn off, look for

pressure drop 14. Turn on cylinders 15. Check interstage pressures: oxygen (7-7.5 bar) and diluent (8.5-9.5 bar) 16. Turn on electronics, confirm all OK 17. Calibrate—"Yes" before every dive if handset has been turned off 18. Verify operation of oxygen and diluent manual addition valves 19. Verify operation of automatic diluent addition valve 20. Positive pressure test 21. If not diving immediately, turn cylinders off, drain IP lines, flush breathing loop with air, and turn electronics off 22. Replace cover 23. Stow

AP Valves Inspiration/Evolution Immediate Pre-Dive Checklist

DIVE NUMBER 123

1. Verify all gas supplies on 2. Verify bailout supply (ies) and check bailout regulators 3. Check BC 4. Confirm ADV is operational 5. Verify handsets are on AND DO NOT SWITCH OFF 6. Check PO2 at least 0.7 atm, pre-breathe 7. Verify mouthpiece (open/closed)

8. Bubble check

AP Valves Inspiration/Evolution Post-Dive Checklist

Check for signs of dirt, deterioration and damage to any part of the CCR at all stages. For hygienic reasons a post-dive check must be completed between users. DIVE NUMBER 123

1. Switch unit off, inspect harness, unlatch cover 2. Rinse unit with fresh water 3. Check and remove gas cylinders, recharge if necessary O2 ______ psi/bar Diluent ______ psi/bar O2 ______ psi/bar Diluent ______ psi/bar O2 ______ psi/bar Diluent ______ psi/bar 4. Remove scrubber assembly 5. Check counterlungs for water ingress (drain) 6. Disinfect breathing hoses, mouthpiece, counterlungs (10 min soak time) 7. Open scrubber canister 8. Remove absorbent basket 9. If absorbent basket is still usable, seal basket and log absorbent use Total Time Used: _____hr: ______ minutes Total Time Used: _____hr: ______ minutes Total Time Used: _____hr: ______ minutes

10. Rinse scrubber housing and hang to dry 11. Rinse breathing hoses, mouthpiece, counterlungs thoroughly with potable water and hang to dry 12. Inspect displays, cables, electrical connections 13. Inspect pneumatics assemblies 14. Cover breathing hose ports with breathable mesh or vented plugs 15. Allow CCR to dry 16. Reassemble CCR 17. Secure straps 18. Stow unit

AP Valves Inspiration/Evolution Pre-Dive Procedures Because CCRs are in a continual state of modification and development, these procedures are provided as guidelines only. The manufacturer's procedures should be referred to and followed in all cases. In the event of any discrepancies, always follow the most current version of the manufacturer's instructions. At all stages the operator should be checking for signs of dirt, deterioration or damage to any part of the apparatus. Note that gas flow in the Inspiration flows in from over the left shoulder, and out over the right shoulder. There are several versions/models of AP rebreathers. These procedures cover the Classic Inspiration, Inspiration with Vision electronics, Evolution, and Evolution Plus. Some differences exist when working with the Vision electronics compared to the Classic electronics. When there are differences, the step will be listed twice, with the step number followed by a "c" for Classic and a "v" for Vision electronics (example, Step "3c" versus "3v"). Follow the procedures for the electronics you are using. Note that for Classic electronics,

button use is achieved by sliding the magnetic switches upward and releasing. Vision electronics button use is achieved by pressing the switch pads with the ball of a finger. STEP 1

PROCEDURE

Inspect Check all gas lines, manifold, tubing, pneumatics connections, regulators, addition valves, hoses mountings and SPGs for signs of looseness, dirt, corrosion or damage. Replace if necessary.

2c

Inspect handsets, cables, electrical connections

Inspect both handsets, cables, mountings, electronics pod, alarm, solenoid, sensors, battery box and connectors for correct alignment, looseness, dirt, corrosion and damage, clean or replace as necessary.

2v

Inspect handset, HUD, cables, electrical connections

Inspect the handset, HUD, electrical cable leading to handset and audio alarm, fiber optic cable leading to HUD, mountings, electronics pod, alarm, solenoid, sensors, battery box and connectors for correct alignment, looseness, dirt, corrosion and damage, clean or replace as necessary.

3c

Check Remove battery box lid and install two CR223 batteries batteries in battery box, contacts down. Place lid on battery box and install mounting screw. Turn on both handsets by first moving the black switch to the up position and then sliding the left button up. On the handset you turned on first (master handset), when it says "DIVE NOW?" slide the center button up. When it says "CHECK DILUENT" confirm with the center button. When it says "CALIBRATE?" say

"No" using the right button. When it says "OPEN O2 VALVE" confirm using the center button. All 3 sensors should be reading approximately 0.21 atm. The alarm should be beeping and the screen should be alternating between "MASTER 0.70" and "LOW OXYGEN." If you see a message that says "LOW BATTERY" the battery for that handset must be replaced before continuing. Wait for the second handset to display slave mode information. Turn off first handset by sliding black switch down. Slave handset should change to master handset display. Turn on second handset. The new master handset should be showing "LOW OXYGEN" warning. If it also alternates to "LOW BATTERY" warning then the battery for that handset needs to be replaced. After the second handset enters slave mode, turn off the current master handset. The slave handset should switch to master handset mode. Turn master handset off. 3v

Check Remove the electronics cover by unscrewing batteries center retaining nut and lifting off center tube. Remove battery box lid and install two CR223 batteries in battery box, contacts down. Place lid on battery box and install mounting screw. Turn on the electronics by depressing the left button for a minimum of two seconds. The handset will automatically scroll through a series of self-tests. When it gets to the battery checks, it will test each battery under load. If either battery indicator reads less than 2 squares after this check, replace the battery indicated. Turn electronics off by depressing the left and right buttons simultaneously, then confirming power down using the right button.

4

5

Refill See specific absorbent basket packing scrubber procedure following this section. basket if necessary Install charged basket, lubricated oring and spacer ring

Drop charged basket into housing, spring-side down. Clean and lightly lubricate lid and sidewall o-rings with appropriate lubricant. Place sidewall o-ring on top of the absorbent basket. Place spacer ring on top of sidewall oring.

6c

Check lid o- Install lid on scrubber housing aligning notch in ring and lid with snorkel on housing. While holding lid attach lid down, secure lid with four lid screws. Do not over tighten.

6v

Check lid o- If CCR is equipped with the optional ring and temperature stick, connect the temperature attach lid stick wiring connector. Insert excess wiring into the electronics cover to prevent wires being pinched when the lid is installed. Install lid on scrubber housing aligning notch in lid with snorkel on housing. While holding lid down, secure lid with three clips. Clips are spring loaded, so you may need to lift them slightly before rotating them 90° to secure them.

7

Analyze, check pressure, and install filled gas cylinders

Select appropriate diluent for intended dive profile. Analyze the gas in both the diluent and oxygen cylinders. Confirm both are full. Place cylinders in case, oxygen on the right, diluent on the left. Check o-rings are in place and

attach the regulators. Secure cylinders with Velcro bands. 8

Install canister (snorkel to right), blow oxygen hose clear and attach to solenoid inlet, attach hoses to counterlung T-pieces

Place canister stack in center of case with Velcro restraining band fbetween the snorkel and absorbent housing, and the snorkel laying on the right side against the backframe. Place breathing hoses into appropriate notches on top of case. Settle canister with snorkel nestled towards the bottom of the case next to the oxygen cylinder. Secure restraining band. Briefly turn on oxygen cylinder to blow solenoid oxygen line clear. Lubricate solenoid gas line oring and connect to solenoid fitting on top of lid using knurled collar. Do not over tighten. Route alarm cable through notch adjacent to left breathing hose notch and place alarm retaining ring on left counterlung T top. Clean and lubricate two o-rings on two breathing hoses coming from scrubber basket. Connect to top of T-fittings on each counterlung. Make barely finger tight.

9c

Route Place electronic hoses in case notches next to electronics breathing hose notches. One handset and cables alarm cable on left, one handset on right.

9v

Route Place electronic line attached to handset electronics through the case notch adjacent to left the cables breathing hose. Route the fiber optic cable leading to the HUD through the same notch if you will wear it on the left side of your mouthpiece, or through the right breathing hose notch if wearing it on the right side of your mouthpiece. If to the right, you may have to

temporarily slide the scrubber canister down to allow it to be inserted. 10

Assemble counter lungs and install manual gas addition valves.

Confirm assembly screw is tight pushing addition valve button open and tightening assembly screw with fingers. Lightly lubricate both orings and place o-rings in addition valve mounts on counterlungs. Install manual addition valves on counterlung by placing on top of o-ring and tightening knurled collar until snug. Position so quick disconnect nipple points towards addition line. Oxygen addition valve attaches to exhalation (right) counterlung. Diluent manual addition valve attaches to inhalation (left) counterlung. Do not attach manual addition valves to counterlung while connected to gas supply lines. Confirm harness (2") and counter lung (1") straps are connected. Connect gas lines to manual addition valves.

11

Check hoses, mouthpiece, verify nonreturn valve operation, and install ensuring gas is to the right

Inspect mouthpiece barrel, hoses, connectors and o-rings for dirt, cuts, nicks, deterioration or damage; replace if necessary. Operate mouthpiece several times to ensure free operation. Open mouthpiece. Place inhalation (left) hose end against palm and try to inhale. If it is possible to inhale, then the exhalation valve is either missing, defective or installed incorrectly, or there is a hole in the hose; replace/reinstall as necessary. Repeat this process with the exhalation hose end (right) and attempt to exhale, if it is possible to exhale then the inhalation valve is either missing, defective or installed incorrectly, or there is a hole in the hose; replace/reinstall as necessary. Confirm gas flow is in from left and out to right.

Close mouthpiece, check that o-rings on hose connections are in place, clean and lubricated, and attach to lower T's on counterlung. Ensure inhalation hose is connected to inhalation counterlung (left side). Make barely finger tight. With the hoses hanging down, check mouthpiece alignment. Bite tabs should be facing up. If not, loosen connectors and realign as necessary. Failure to check the mushroom valves can lead to CO2 buildup within the breathing loop causing hypercapnea or death. 12

13

14 15

Negative Place mouthpiece in mouth, open the pressure mouthpiece and inhale through the mouth and test out your nose until there is a slight vacuum. Close mouthpiece and remove from mouth. Wait 30 seconds. Open the mouthpiece. You should hear an inrush of air. If unsuccessful, check for loose or poor connections or a hole in the breathing loop. Turn on cylinders, pressurize lines, turn off, look for pressure drop

Turn on the oxygen cylinder valve. Pressurize the lines. Turn off the valve. Watch the SPG and look for pressure drop. If drop is seen, check the oxygen regulator DIN connector. Try again. If drop is still seen, check all other connectors and solenoid. Repeat with diluent cylinder.

Turn on Turn both cylinders on. cylinders Check Using an interstage pressure (IP) gauge, check interstage both oxygen and diluent IPs by disconnecting pressures: gas lines at the manual gas addition valves.

oxygen (77.5 bar) and diluent (8.59.5 bar)

Oxygen IP should be 100-110 psi (7-7.5 bar), diluent IP should be 125-140 psi (8.5-9.5 bar). Reconnect the gas lines to the manual addition valves.

16c

Turn on electronics, confirm all OK

Turn on both handsets by moving the black switch to the up position and sliding the left button up. On the handset you turned on first (master handset), when it says "DIVE NOW?" slide the center button up. When it says "CHECK DILUENT" confirm using the center button.

16v

Turn on electronics, confirm all OK

Turn on the electronics by depressing the left button for a minimum of two seconds. The handset will automatically scroll through a series of self-tests.

17c

Calibrate —"Yes" before every dive if handset has been turned off

If the handset was turned off for any reason, then start calibration ("YES") by sliding the left button up. The next screen asks for ambient pressure. If you know it, adjust up/down by sliding the left or right buttons as appropriate. Slide the center button up to confirm with default or true value. "OXYGEN PERCENTAGE" is then requested. Set to actual percentage of oxygen used (if different than 100%), or use 98% as a default. Slide the center switch up to confirm. Open the mouthpiece, and slide the center button up to confirm. Confirm "OPEN O2 VALVE" using the center button. The solenoid will begin adding oxygen until it says "CALIBRATING." You will now have all three sensors providing oxygen readings. Close the mouthpiece!

17v

Calibrate —"Yes" before every dive if handset has been turned off

If the handset was turned off for any reason, then start calibration ("YES") using the left button. "OXYGEN PERCENTAGE" is then requested. Set to actual percentage of oxygen used (if different than 100%), or use 98% as a default. Confirm with center switch. Open the mouthpiece, and confirm with center button. Confirm "OPEN O2 VALVE" using the center button. The solenoid will begin adding oxygen until it says "CALIBRATING." You will now have all three sensors providing oxygen readings. Close the mouthpiece!

18

Verify operation of oxygen and diluent manual addition valves

Depress the manual diluent addition valve (inhalation counterlung on the left side). You should hear gas being added. Depress the manual oxygen addition valve (exhalation counterlung on the right side). You should hear gas being added.

18

Verify operation of automatic diluent addition valve

Place the mouthpiece in your mouth. Open mouthpiece. Inhale through your mouth and out your nose until you hear and feel gas being added from the automatic diluent addition valve. Close the mouthpiece.

20

Positive Close the overpressure relief valve by screwing pressure it clockwise. Inflate the breathing loop using the test oxygen manual addition valve until the overpressure valve operates. Submerge the CCR in water, looking for bubbles. Pay particular attention to all breathing hose connectors. There should be no bubbles. If

bubbles found, remove CCR from water and correct problem. Reopen overpressure valve. 21

If not diving immediately, turn cylinders off, drain IP lines, flush breathing loop with air, and turn electronics off

If you will not be diving until the following day, turn the cylinder valves off, and drain the IP lines using the manual gas addition valves. Flush the breathing loop manually with air until PO2 drops below 0.5 atm. Then turn the electronics off by as previously explained.

22

Replace Replace the cover and fasten the case latches. cover On older Inspiration units, clip four case clips. On newer units and Evolutions, pull two black handles over case projections.

23

Stow Stow the unit securely, so it will not shift during transport. Verify that no SPG hoses, electronics cables, or BC are pinched or liable to contact hard surfaces. Secure handsets to prevent mechanical shocks and vibration.

AP Valves Inspiration/Evolution Immediate Pre-Dive Procedures

STEP

PROCEDURE

1

Verify all gas Verify gas contents on SPG. Confirm cylinders supplies on are on by injecting gas into the breathing loop using first the oxygen and then the diluent manual gas valves, simultaneously watching appropriate SPG. If SPG needle moves during manual valve operation, check cylinder valves to insure they are turned on.

2 Verify bailout supply (ies) and check bailout regulators

Breathe from the alternate second stage on the BC inflator to confirm function. If used, turn bailout cylinder ON. Check SPG for contents. Breathe from bailout regulator, while watching SPG. SPG should remain steady. If not, confirm cylinder valve is ON.

3

Check BC Verify BC LP inflator hose is attached. Add a small amount of air to BC with LP inflator.

4

Confirm ADV Insert mouthpiece in mouth and open. Inhale is operational from mouth, and exhale through nose. Repeat until ADV activates, adding diluent into the loop. Close mouthpiece and remove from mouth.]

5

Verify Confirm handset(s) and HUD are operational. handsets are DO NOT SWITCH HANDSET(S) OFF UNTIL on AND DO AFTER THE DIVE IS COMPLETED! NOT SWITCH OFF

6 Check PO2 at Place mouthpiece in mouth and open. Breathe least 0.7 atm, on loop while watching handset. Confirm that all pre-breathe three sensor readings vary with respirations. Raise PO2 to at least 0.7 atm.

7

Verify If entering water while using the rebreather, mouthpiece leave mouthpiece in mouth, open. Otherwise, (open/closed) close mouthpiece and remove from mouth.

8 Bubble check After entering water, submerge just below the surface and have buddy perform visual check of breathing loop and connectors for signs of leakage or bubbles. If found, correct before diving.

AP Valves Inspiration/Evolution Absorbent Basket Packing Procedures

You must follow proper packing procedures when filling the absorbent basket. Improper filling can lead to channeling, and CO2 buildup in the breathing loop. This can lead to diver injury or death.

STEP

PROCEDURE

1 Remove Unscrew spider retaining nut. Remove spider and spider upper scrim. and upper scrim 2

Install Insure lower scrim is dry. Place in the bottom of the lower scrubber basket, with the split edges spread evenly

scrim against the inner walls of the basket. 3

Fill Place the basket on a roll of tape or other suitable basket stand allowing the scrubber basket to sit flat and steady without resting directly on the thumb removal tab or thermistor cable. Pour CO2 absorbent granules into the center of the basket so that the grains push the scrim edges against the inner basket wall. Fill until about one third full. Fill from about 12 inches (30 cm) above the basket, to allow excess dust to blow off. Avoid skin contact. Ensure granules are evenly spread. Gently tap the basket and allow granules to settle (do not handle too roughly, thus preventing powdering of granules). Repeat the process by thirds until the granules reach about 5 mm (1/4") below the upper rim of the basket. Do not use the last inch of absorbent from the supply container, as most of the dust and powder caused during transport will have settled there.

4

Install Install the upper scrim by placing over the center shaft, spider and allowing the split edges to drape over the edge of the basket. Install the spider on the center shaft, ensuring that the rim sits within the basket wall. Screw on the spider retaining nut. Do not over-tighten. Screw should be finger tight only.

5

Settle Gently tap basket sidewalls. Rotate basket as you do grains so. Tighten spider retaining nut every 5-10 taps. Do not over-tighten. Screw should be finger tight only. If the spider bottoms out, reopen spider and continue from Step #3. Do not over pack the scrubber basket. This may cause channeling, increase work of breathing, and lead to early breakthrough and/or hypercapnea.

6

Seal If the filled basket is going to be left unused for >24 canister hours, seal it in doubled plastic bags. Mark the outer bag with number of hours of use, absorbent size, and the date. If the CCR will be used in the next 24 hours, you may install it in a dry unit with the breathing loop sealed to prevent any absorbent decay.

AP Valves Inspiration/Evolution Post-Dive Procedures

Check for signs of dirt, deterioration or damage to any part of the apparatus at all stages of the post-dive procedures. For hygienic reasons a post-dive check must be completed between users.

STEP

PROCEDURE

1c

Switch unit Turn black switches on both handsets to the off, unlatch down position. On older units, unclip four case cover clips by pushing red catches forward and releasing black clips. On newer units, pull two black handles off case projections.

1v

Switch unit Turn electronics off by depressing the left and off, unlatch right buttons simultaneously, then confirming cover power down using the right button. Release cover by pulling two black handles off case projections.

2

3

4

Rinse unit Ensure mouthpiece is closed. Thoroughly rinse with fresh the exterior with fresh water, including latches, water manual gas addition valves, cylinders, hoses, gas lines, regulators, ADV, BC, and canister housing. Check and remove gas cylinders, recharge if necessary

Check cylinder pressures and record. Ensure both cylinder valves are closed and purge lines using the manual gas addition valves. Check that SPGs read 0 psi/bar. Remove regulators and lift cylinders out, refill as needed. Cap the regulators.

Remove Unscrew breathing hose connectors at the top scrubber of the T's on the counterlungs. Remove the assembly audio alarm retaining ring. Free all electronics cables from retaining notches. Unfasten Velcro retaining band from scrubber housing. Unscrew oxygen line collar from the lid. Remove scrubber assembly from case.

5

Check counterlung for water ingress (drain)

Remove manual gas inflation valves from the bottom of the counterlungs by unscrewing the knurled collars. Drain any liquid from each counterlung. If quantities are excessive, check system for leaks that may have contributed to the problem.

6

Disinfect breathing hoses, mouthpiece, counterlungs (10 min soak time

Disconnect breathing hose couplings from scrubber assembly. Spray disinfectant solution into mouthpiece, breathing hose openings, counterlung T's (upper and lower), and the manual gas addition valve mounting ports. Let stand 10 minutes before rinsing.

7v

Open Unlock the lid by holding it down and removing scrubber the four retaining screws. Do not lose the canister screws. Remove the lid. Dry excess condensation from the inner area of the lid, and hang lid to dry..

7c

Open Unlock the lid by slightly lifting and rotating the scrubber three retaining nuts 90°. Remove the lid. canister Remove the electronics cover by unscrewing center retaining nut and lifting off center tube. Dry excess condensation from the inner area of the lid, and hang lid to dry.

8

9

10

Remove Lift out the spacer ring and sidewall o-ring. Lift absorbent out the scrubber basket. If using Vision basket electronics with temperature stick, disconnect the temperature stick wiring and install protective caps on each end. If absorbent basket is still usable, seal basket and log absorbent use

Dry the outside of the absorbent basket. If the charged basket is going to be reused, seal it in doubled plastic bags. Mark the outer bag with number of hours of use, absorbent size, and the date. If the CCR will be used in the next 24 hours, you may reinstall it in a dry unit with the breathing loop sealed to prevent any absorbent decay. Log the total number hours of use on the log sheet. If the basket will not be reused, remove the spider retaining nut, remove the spider and upper scrim, and dispose of the used absorbent. Remove the second scrim and allow both to thoroughly dry.

Rinse Rinse the scrubber canister housing and set to scrubber dry.

housing and allow to dry 11

Rinse breathing hoses, mouthpiece, counterlungs thoroughly with potable water and hang to dry

12

Inspect displays, cables, electrical connections

13

14

Rinse breathing hoses and mouthpiece by flushing water from the inhalation hose through the hose assembly. Hang to dry. Remove the manual inflation valves from each counterlung. Remove the sealing o-rings, and place on the gas supply lines, then reconnect LP supply lines to manual addition values to prevent loss. Rinse the counterlungs with fresh water by flushing from the tops and bottoms of the T's, and allowing to drain from the manual inflation valve ports at the bottom of each counterlung. Leave standing upright to dry. Manipulate each counterlung after drying to drain residual water from the manual inflation valve ports. Inspect both handsets (Classic electronics) or the handset and HUD (Vision), alarm, cables, and connectors for looseness, dirt, corrosion and damage, clean or replace as necessary.

Inspect Check all lines, pipes, tubing, connections, pneumatics regulators, addition valves, mountings and HP assemblies gauges for signs of looseness, dirt, corrosion or damage. Replace if necessary. Cover breathing hose ports with breathable

Plug the ports on the breathing hoses and counterlungs with breathable mesh (nylons) or vented plugs, to prevent ingress of insects or other organisms.

mesh or vented plugs 15 Allow CCR to If diving will continue immediately, reassemble dry the unit, ensuring all components are dry. Otherwise, leave the unit open in a secure place to thoroughly air dry. Leave the lid out, but cover the sensors with a dry paper towel. 16

Reassemble When thoroughly dry, loosely reassemble all CCR components. Unless using the unit within 24 hours, do not install the battery or a packed canister. If a packed canister is installed, clearly mark the outside of the unit with the total time of use. Leave the sealing o-rings out of the manual gas inflation valves. (They can be placed over the gas supply lines to prevent loss.)

17

Secure Ensure all straps, buckles and harness straps components are in good condition and untangled; secure as needed. Stow SPGs and displays to minimize strain and kinking of cables and hoses. The handset(s) should be clipped or otherwise securely fastened to the unit, to prevent damage from dangling.

18

Stow unit Store unit in a clean, dry location with moderate temperatures; or if diving will continue within 24 hours you may pre-dive as required.

B&E Manufacturing Nautilus Pre-Dive Checklist

Check for signs of dirt, deterioration, damage and lubrication to any part of the CCR at all stages

DIVE NUMBER 123

1. Battery status:

Voltage Primary / Secondary Primary / Secondary ____ / ____ ____ / ____ Primary / Secondary Primary / Secondary ____ / ____ ____ / ____ Primary / Secondary Primary / Secondary ____ / ____ ____ / ____ 2. Remove gas cylinders, crown, and hose assembly 3. Fill and analyze cylinders O2_____% Diluent_____% Bailout_____% O2_____% Diluent_____% Bailout_____% O2_____%

Date Installed

Diluent_____%

Bailout_____%

4. Remove bucket 5. Inspect O2 sensors and sensor wires for damage 6. Refill scrubber basket if necessary (see procedure below) Accumulated scrubber time _____ hrs _______ min Accumulated scrubber time _____ hrs _______ min Accumulated scrubber time _____ hrs _______ min 7. Insert filled scrubber basket and reinstall bucket 8. Check hoses, mouthpiece, verify non-return valve operation, and install ensuring gas is to the left

9. Install filled gas cylinders 10. Install crown and hose assembly (do not latch the crown) 11. Turn on primary and secondary electronics 12. Check display for battery warning lights, replace if necessary 13. Verify that battery box covers are closed and latched 14. Turn on oxygen supply and open DSV 15. Verify that battery box covers are closed and latched 16. Complete calibration (see calibration procedure detail) 17. Turn off oxygen supply, purge the O2 line and reduce the PO2 level by breathing down the loop, watch the PO2 fall

18. Turn off primary and secondary electronics 19. Select a setpoint; write it on the slate 20. REPLACE THE SETPOINT SELECTOR SWITCH COVER 21. Latch the crown 22. Turn on diluent valve, check power inflator & bailout open circuit regulator 23. Evacuate the loop to verify automatic diluent addition valve operation, turn diluent gas supply off, bleed diluent line ito the BCD and check for leaks 24. Inspect all hand tight fittings 25. Perform positive pressure test (1 min minimum test) 26. Perform negative pressure test (1 min minimum test) 27. Open O2 and diluent cylinder valves

28. Verify O2 and diluent manual addition valve operation 29. Record cylinder pressures O2_______psi/bar Diluent_______psi/bar Bailout______psi/bar O2 Diluent_______psi/bar Bailout______psi/bar _______psi/bar O2 Diluent_______psi/bar Bailout______psi/bar _______psi/bar 30. Close cylinder valves, wait 2 minutes, confirm pressures match above 31. Open O2 and diluent cylinder valves 32. Turn on primary and secondary electronics

33. Perform a 5 minute pre-breathing sequence, verify set point operation (0.7), warm up scrubber 34. Confirm mouthpiece closed

B&E Manufacturing Nautilus Immediate Pre-Dive Checklist

DIVE NUMBER 123

1. Verify all gas supplies on 2. Verify bailout supply (ies) and check bailout regulators 3. Check BC 4. Confirm ADV is operational 5. Verify electronics on 6. Check PO2 at least 0.7 atm 7. Verify mouthpiece (open/closed) 8. Check for gas leaks

B&E Manufacturing Nautilus Canister Packing Checklist

DIVE NUMBER 123

1. Unscrew canister cover and remove foam pad 2. Discard old absorbent, retain and clean both foam pads

3. Visually inspect the canister for wear (especially threads and mesh); clean or replace as needed 4. Place empty canister on clean dry towel, ensure lower foam pad is in place 5. Plug center core of canister 6. Fill to 1/3 full, tap canister gently to settle grains 7. Fill to 2/3 full, tap canister gently to settle grains 8. Fill to full, tap canister gently to settle grains 9. Remove central core plug, install upper foam pad, install canister cover 10. Tap canister circumference gently to settle grains, periodically tightening cover 11. Check canister fill by gently shaking canister and listening for noise; if noted remove cover and add some absorbent. Return to Step 10 above. 12. Install packed canister in unit, or bag and seal it if use will not occur for over 24 hours

B&E Manufacturing Nautilus Calibration Checklist

DIVE NUMBER 123

1. Turn on oxygen cylinder 2. Open mouthpiece 3. Turn on power switches for primary and secondary displays 4. Activate manual oxygen addition valve for a count of five seconds, release for five seconds while observing the secondary display. Repeat three times. Secondary display readings should have reached their highest level. If in doubt, repeat the oxygen addition cycle once more.

5. Press and hold both calibration switches for about five seconds or until the display indicates calibration has been accepted 6. Confirm calibration is accepted on the primary display (the green light will blink twice) 7. Confirm calibration is accepted on the secondary display (the green lights on one row will alternately blink twice)

B&E Manufacturing Nautilus Post-Dive Checklist

Check for signs of dirt, deterioration and damage to any part of the CCR at all stages. For hygienic reasons a post-dive check must be completed between users. DIVE NUMBER 123

1. Battery run time Run time this dive ____ hrs____ min Accumulated run time ____ hrs____ min Run time this dive ____ hrs____ min Accumulated run time ____ hrs____ min Run time this dive ____ hrs____ min Accumulated run time ____ hrs____ min 2. Rinse unit with fresh water, loosen crown and rinse underneath, flush console arm assembly 3. Turn unit off (primary and secondary) 4. Check and remove gas cylinders, recharge if necessary O2 _______psi/bar Diluent_______psi/bar O2 _______psi/bar Diluent_______psi/bar O2 _______psi/bar Diluent_______psi/bar

5. 6. 7. 8.

Check counterlungs for water ingress (drain) Remove crown and hose assembly Remove absorbent bucket If absorbent canister is still usable, seal canister and log absorbent use (hours:minutes), otherwise discard used absorbent Total Time Used: ______hr: _______ minutes Total Time Used: ______hr: _______ minutes Total Time Used: ______hr: _______ minutes 9. Inspect sensors and seals for damage; if excess moisture is present, allow the sensors to dry 10. Install cylinders and stand unit up 11. Disinfect disassembled breathing hoses, mouthpiece, and canister pads by soaking in disinfectant solution for 10 minutes in the absorbent bucket 12. Remove components, and pour half of solution into each counterlung, allow to sit for 10 minutes 13. Rinse all components with fresh water and allow to dry 14. Inspect harness, wings, console, high pressure lines, low pressure lines, and regulators for damage 15. Cover breathing hose ports with breathable mesh or vented plugs 16. Allow CCR to dry 17. Reassemble CCR, leaving bucket free of the bucket seals 18. If storage will be prolonged, remove batteries and store battery covers in battery box 19. Stow unit

Dive Rite O2ptima Pre-Dive Checklist

Check for signs of dirt, deterioration and damage to any part of the CCR at all stages DIVE NUMBER 123

1. 2. 3. 4.

Attach wings and harness Inspect pneumatics hoses Inspect handsets, cables, electrical connections, HUD Open scrubber canister, inspect o-rings, wiring, and O2 flow tube 5. Install ExtendAir® cartridge and close scrubber canister 6. Analyze, check pressure, and install filled gas cylinders O2____% ____ psi/bar Diluent____% ____psi/bar O2____% ____ psi/bar Diluent____% ____ psi/bar O2____% ____ psi/bar Diluent____% ____ psi/bar 7. Attach outer cover 8. Assemble and install counterlungs 9. Check hoses, mouthpiece, verify valve operation, and install 10. Confirm ADV shut-off valve is open, and attach ADV supply hose 11. Attach O2 and diluent manual addition supply and SPG hoses 12. Negative pressure test 13. Turn on cylinders, pressurize lines, turn off, look for pressure drop

14. Turn on cylinders 15. Install batteries in handsets, turn on 16. Calibrate if first dive of day (see below) 17. Verify solenoid operation 18. Verify operation of ADV, oxygen and diluent manual addition valves 19. Attach HUD 20. Positive pressure test (dip test) 21. If not diving immediately, turn handsets off, flush loop with air, turn cylinders off 22. Stow unit

Dive Rite O2ptima Calibration Checklist DIVE NUMBER 123

1. Confirm oxygen cylinder and handsets on 2a. If using calibration ports (else go to Calibration Step 2b) Remove breathing hoses from canister Attach calibration ports and oxygen supply line Turn oxygen on, flush, turn oxygen off 2b. If not using calibration ports Flush breathing loop with oxygen three (3) times, close mouthpiece 3. Scroll Primary handset to OPT, MV Display 4. Check readings, wait five minutes, confirm readings stable 5. Scroll Primary handset to OPT, Calibrate O2, Ready 6. Scroll Secondary handset to OPT, Calibrate O2, Ready

7. If using calibration ports, remove ports and reattach breathing hoses and oxygen LP line

Dive Rite O2ptima Immediate Pre-Dive Checklist DIVE NUMBER 123

1. Verify all gas supplies on 2. Verify bailout supply (ies) 3. Check BC 4. Verify both handsets are on 5. Check PO2 ≥ 0.5 atm, pre-breathe 6. Verify mouthpiece (open/closed) 7. Bubble check

Dive Rite O2ptima Post-Dive Checklist

Check for signs of dirt, deterioration and damage to any part of the CCR at all stages. For hygienic reasons a post-dive check must be completed between users. DIVE NUMBER 123

1. 2. 3. 4.

Unclip cover and rinse unit with fresh water Check harness and other rebreather parts Remove batteries from handsets Check and remove gas cylinders; refill if necessary O2____% ____ psi/bar Diluent____% ____ psi/bar O2____% ____ psi/bar Diluent____% ____ psi/bar

O2____% ____ psi/bar Diluent____% ____ psi/bar 5. Detach all supply hoses from ADV and manual addition valves 6. Detach ADV, hoses, and mouthpiece 7. Detach counterlungs and check for water ingress (drain) 8. Disinfect breathing hoses, mouthpiece, counterlungs, ADV 9. Remove ExtendAirR cartridge; if still usable, seal cartridge and log use Total Time Used: _____hr ____ min Total Time Used: _____hr ____ min Total Time Used: _____hr ____ min 10. Wipe inner surfaces of scrubber canister surfaces with disinfectant, allow to dry 11. Drain breathing hoses, mouthpiece, counterlungs and hang to dry 12. Inspect handsets and DIVA, cables, electrical connections 13. Rinse and drain BC (remove if desired) 14. Inspect pneumatics assemblies 15. Plug breathing hose ports 16. Allow CCR to dry 17. Reassemble CCR 18. Secure straps 19. Stow unit

Dive Rite O2ptima Pre-Dive Procedures

Because CCRs are in a continual state of modification and development, these procedures are provided as guidelines only. The manufacturer's procedures should be referred to and followed in all cases. In the event of any discrepancies, always follow the most current version of the manufacturer's instructions. At all stages the operator should be checking for signs of dirt, deterioration or damage to any part of the apparatus. Note that gas flow in the O2ptima flows in from over the left shoulder, and out over the right shoulder.

STEP

PROCEDURE

1

Attach wings Attach the wings and harness to the backplate. and harness Place the wings on the mounting screws. Then place the harness on the mounting screws. Place stabilizing plates and washers on screws, then fasten using wing nuts on each screw. Place the back pad over the mounting screws if used.

2

Inspect Check all hoses, connections, regulators, pneumatics addition valves, and HP gauges for signs of hoses looseness, dirt, corrosion or damage. Replace if necessary.

3

Inspect Inspect the handsets, cables, connectors, and handsets, HUD for looseness, dirt, corrosion and damage; cables, clean or replace as necessary.

electrical connections, HUD 4

Open scrubber canister, inspect orings, wiring, and O2 flow tube

Open the scrubber canister by rotating the end caps 90° and removing them. Inspect the sensors and wiring for damage or corrosion. Inspect all o-rings (end caps, ExtendAir® cartridge seals, and oxygen flow tube), cleaning and lubricating as needed. Ensure the oxygen flow tube is firmly attached to the solenoid tubing. Reinstall the canister housing to the left end cap (containing the sensors).

5

Install ExtendAir® cartridge and close scrubber canister

Check the outer edges of the ExtendAir® scrubber cartridge to make sure there are no nicks, cuts, or indentations. Correct any telescoping of the cartridge, by placing on a flat surface and firmly realigning the coils. Remove any center core plug. Insert the cartridge into the scrubber canister so that the oxygen flow tube slides through the core of the cartridge. Fasten the oxygen flow tube (wing nut, cotter pin, or o-ring) as necessary. Close the scrubber canister by attaching the right end cap.

6

Analyze, check pressure, and install filled gas cylinders

Select appropriate diluent for intended dive profile. Analyze the gas in both the diluent and oxygen cylinders. Confirm both are full. Install the cylinders with the valves pointed down (use the appropriate attachment for your model O2ptima) and secure, with the oxygen cylinder on the right and diluent cylinder on the left. Check o-rings are in place and attach the regulators.

7

Attach outer cover Assemble and install

If available, fasten the elastic restraining straps over the pressure hoses and cylinders using the Fastex® clip. Attach the outer cover using the Fastex® clips. On some models, the strap with the upper clip should be threaded through the loop on the side of the cover before being closed.

8

Counterlungs Attach the counterlungs to the harness using the Velcro bands on the back sides of the counterlungs. The exhalation counterlung (containing the overpressure valve) goes on the right side, and the inhalation counterlung belongs on the left side. Then attach the Fastex® clips on the top of the counterlungs to the corresponding clips on the outer cover. Inspect fittings and o-rings on counterlungs. Clean and lubricate the ADV, T-piece, and hose o-rings. Attach the ADV to the inhalation counterlung by inserting it and screwing down the red retaining ring. Note that all red threaded connectors are reverse threads. Attach the Tpiece to the exhalation counterlung using the black retaining ring. Then install the hoses between the scrubber canister and the ADV (red connectors) and the T-piece (black connectors). If used, confirm the lower counterlung restraining strap is connected to the Fastex® connector on the bottom of either of the counterlungs.

9

Check hoses, mouthpiece, verify valve operation and install

Inspect mouthpiece barrel, hoses, connectors and o-rings for dirt, cuts, nicks, deterioration or damage; replace if necessary. Operate mouthpiece several times to ensure free operation; repair or replace components if

necessary. Open mouthpiece and breathe through it. Seal the inhalation (red, left) side, and simultaneously try to inhale. If it is possible to inhale, then the exhalation valve is either missing, defective or installed incorrectly; replace/reinstall as necessary. Repeat this process with the exhalation hose (black, right) side by sealing opening and attempt to exhale, if it is possible to exhale then the inhalation valve is either missing, defective or installed incorrectly; replace/reinstall as necessary. Close mouthpiece. Clean and grease the breathing hose connector o-rings. Attach the inhalation hose to the bottom of the ADV and the exhalation hose to the bottom of the Tpiece. Assemble hand tight. Do not overtighten. The nuts merely hold the hose in place; they do not provide o-ring sealing pressure. 10

Confirm ADV shut-off valve is open, and attach ADV supply hose

11

Attach O2 and diluent manual addition supply and SPG hoses

The ADV LP diluent hose supply comes from the diluent regulator. Thread it through the retaining loop on the inhalation counterlung, then screw it on to the hose port on the ADV. Confirm ADV shutoff valve is in the open position. Thread the green oxygen LP supply hose through the retaining loop on the exhalation counterlung, then attach it to the manual oxygen valve on the exhalation counterlung using the QD fitting. Thread the black diluent LP supply hose through the retaining loop on the inhalation counterlung, then attach it to the manual diluent valve on the inhalation counterlung the QD fitting. Route the SPG hoses along the counterlungs, then secure

them with the Velcro bands at the bottom of each counterlung (oxygen SPG on right side, diluent SPG on left side). 12

13

14

Negative Open the mouthpiece and inhale through your pressure test mouth and exhale out your nose until there is a slight vacuum. Close mouthpiece and remove from mouth. Wait 30 seconds. Open the mouthpiece. You should hear an inrush of air. If unsuccessful, try immersing the mouthpiece in water and reattempt the test. If still unsuccessful, check for loose or poor connections or a hole in the breathing loop. Turn on cylinders, pressurize lines, turn off, look for pressure drop

Turn on the oxygen cylinder valve. Pressurize the gas lines. Turn off the valve. Watch the SPG and look for pressure drop. If drop is seen, check the oxygen regulator DIN connector. Try again. If drop is still seen, check all other connectors. Repeat with diluent cylinder.

Turn on Turn both cylinders on. cylinders

15

Install batteries in handsets, turn on

16

Calibrate if See calibration procedure below.

Unscrew the battery caps on the two handsets. Clean and grease o-rings on the caps. Install either lithium or alkaline AA cells with positive terminal inserted. Do not install previously used alkaline AA cells. Reinstall caps, confirming that they are fully tightened. Handsets should turn on as you install the caps. If not, press the left switch on the bottom of the handset.

first dive of the day 17

18

Verify Open mouthpiece. Press the manual diluent solenoid addition valve (MDV) while watching oxygen operation sensor readings on handset until you hear the solenoid adding oxygen just below the set point level. Close mouthpiece. Verify operation of ADV, oxygen and diluent manual addition valves

Place mouthpiece in mouth and open. Breathe in from the loop, and out through your nose until you hear or feel the ADV add gas. Close mouthpiece. Press the manual diluent addition valve (MDV) and oxygen manual addition valves to confirm proper operation and gas addition.

19

Attach HUD Attach DIVA (HUD) to mount on mouthpiece. You may wish to wrap the DIVA cable around the hose several times to prevent entanglement.

20

Positive Place mouthpiece in mouth and open. Orally pressure test inflate the breathing loop by inhaling through (dip test) your nose, and exhaling through your mouth. Repeat until the breathing loop is completely full. Close the mouthpiece. Submerge the CCR in water, looking for bubbles. There should be none. If found, remove CCR from water and correct problem.

21

If not diving immediately, turn handsets off,

If you will not be diving until the following day, open mouth piece, turn the cylinder valves off, and drain the IP lines using the manual gas addition valves. Flush the breathing loop

flush loop manually with air until PO2 drops below 0.3 with air, turn atm. Then turn the handsets off. Close the cylinders off mouthpiece. 22

Stow unit Stow the unit securely, so it will not shift during transport. Verify that no SPG hoses, electronics cables, or BC are pinched or liable to contact hard surfaces. Secure wrist displays to prevent mechanical shocks and vibration.

Dive Rite O2ptima Calibration Procedures

STEP 1

2a

Confirm oxygen cylinder and handsets on

PROCEDURE If you were interrupted in the preceding portion of the checklist, confirm the oxygen cylinder is still on, the handset is on, the HUD is on (LEDs flashing), and external battery is connected.

If using Complete following three steps. calibration ports (else go to Calibration Step 2b) Remove Remove the two breathing hoses from the top of

breathing the scrubber canister. Note that the red hoses from connector is a reverse thread. canister Attach calibration ports and oxygen supply line

Attach the calibration ports by screwing them onto the breathing hose connectors. Attach a LP oxygen supply line to the appropriate calibration port.

Turn oxygen on, flush, turn oxygen off

Open the oxygen cylinder, allowing oxygen to flow through and flush the scrubber canister. After fully flushing, turn off the oxygen cylinder valve.

2b If not using Complete following step. calibration ports Flush breathing loop with oxygen three (3) times, close mouthpiece

3

Place the mouthpiece in your mouth and open it. Inhale from your mouth and out your nose until the breathing loop is empty. Completely fill the loop with oxygen using the manual oxygen addition valve. Cycle the gas through the loop by taking several breaths. Repeat this cycle three times. While doing this, ensure that you do not inhale through your nose, nor that you empty the counterlungs to the point that the ADV adds gas. If so, begin this step from the beginning. After completely flushing the loop three times with oxygen, shut the mouthpiece and remove from your mouth.

Scroll Press the MENU button (left) on the primary Primary handset until you get to OP-Tions, MV Display.

handset to Then press the SELECT (right) button. OPT, MV Display 4

Check readings, wait five minutes, confirm readings stable

5

Scroll Primary handset to OPT, Calibrate O2, Ready

Press the MENU button (left) on the primary handset until you get to OPTions, Calibrate O2, Ready. Then press the SELECT (right) button. All three cells should read 0.98 –1.0 atm. The readings may be lower if you have the calibration gas setting selected for less than 98%

6

Scroll Secondary to OPT, Calibrate O2, Ready

Press the MENU button (left) on the secondary handset until you get to OPTions, Calibrate O2, Ready. Then press the SELECT (right) button. All three cells should read 0.98 –1.0 atm. The readings may be lower if you have the calibration gas setting selected for less than 98%.

7

If using calibration ports, remove ports and reattach breathing hoses and

If you are using the calibration ports, remove them from the scrubber canister. Reattach the breathing hoses to the scrubber canister. Remove the oxygen line from the calibration port and reconnect it to the rebreather. Turn on the oxygen cylinder.

Note the sensor output in millivolts (mV). After five minutes confirm that mV output for each sensor is between 40-65 mV and has remained stable.

oxygen LP line

Dive Rite O2ptima Immediate Pre-Dive Procedures

STEP

PROCEDURE

1

Verify all gas Verify gas contents on SPG. Confirm cylinders supplies on are on by injecting gas into the breathing loop using first the oxygen and then the diluent manual gas addition valves, simultaneously watching appropriate SPG. If SPG needle moves during manual valve operation, check cylinder valves to insure they are turned on.

2

Verify bailout Breathe using the open circuit second stage supply (ies) attached to the diluent cylinder to confirm proper operation. Turn bailout cylinder on. Check SPG for contents. Breathe from bailout regulator, while watching SPG. SPG should remain steady. If not, confirm cylinder valve is on.

3

Check BC Verify BC LP inflator hose is attached. Add a small amount of air to BC with LP inflator.

4

Verify both Confirm handsets are operational and DIVA is handsets are providing matching LED readings. on

5 Check PO2 at Place mouthpiece in mouth and open. Breathe least 0.5 atm, on loop while watching handset. Confirm that all pre-breathe three sensor readings vary with respirations. Raise PO2 to at least 0.5 atm. 6

Verify If entering water while using the rebreather, mouthpiece leave mouthpiece in mouth, open. Otherwise, (open/closed) close mouthpiece and remove from mouth.

7 Bubble check After entering water, submerge just below the surface and have buddy perform visual check of breathing loop and connectors for signs of leakage or bubbles. If found, correct before diving.

Dive Rite O2ptima Post-Dive Procedures Check for signs of dirt, deterioration or damage to any part of the apparatus at all stages of the post-dive procedures. For hygienic reasons a post-dive check must be completed between users.

1

STEP

PROCEDURE

Unclip cover and rinse unit with fresh water

Ensure mouthpiece is closed. Release four Fastex® clips to remove cover. Thoroughly rinse the exterior with fresh water, including the handsets, manual gas addition valves, cylinders, SPGs, hoses, and canister housing.

2

Check Check harness and all rebreather components harness and for signs of damage, fraying, or other other problems. Note and correct. rebreather parts

3

Remove batteries from handsets

Dry the handset exteriors. Unscrew the battery caps and remove the batteries from both handsets. If in an area liable to get wet, replace the battery caps. Remove them for long-term storage.

4

Check and remove gas cylinders; refill if necessary

Check cylinder pressures and record. Ensure both cylinder valves are closed and purge lines using the manual gas addition valves. Check that SPGs read 0 psi/bar. Remove regulators and lift cylinders out, refill as needed. Cap the regulators.

5

Detach all supply hoses from ADV and manual addition valves

Remove the diluent LP gas supply line from the ADV by unscrewing it from the hose fitting. Disconnect both the oxygen and diluent LP lines from the manual gas addition valves. Remove all gas lines from the retaining loops on the counterlungs, including the SPG lines.

6

7

Detach ADV, Disconnect breathing hose assembly by hoses, and unscrewing the two connectors at the top of the mouthpiece scrubber canister and the ADV and T-piece retaining rings. Note that red connectors and rings are reverse thread. Detach Detach the Fastex® clips at the top of each counterlungs counterlung. Remove the counterlungs from and check for the harness by separating the Velcro bands on

water ingress the backs of the counterlungs. Invert the (drain) counterlungs and allow any water to drain from the T-piece and ADV ports. Set the counterlungs aside for cleaning. 8

Disinfect breathing hoses, mouthpiece, counterlungs, ADV

Spray breathing hose, mouthpiece and ADV interiors with disinfectant solution. Spray disinfectant in the connector port of each counterlung, coating all inside surfaces. Let sit 10 minutes.

9

Remove ExtendAir® cartridge; if still usable, seal cartridge and log use

Open the scrubber canister by twisting the right end cap and taking it off. Remove the ExtendAir® cartridge retainer (wing nut, cotter pin, o-ring, or other). Slide the ExtendAir® cartridge out of the canister housing. If the ExtendAir® cartridge is still usable, replace it in its original container, using both inner and outer plastic bags. Mark the usage of the cartridge (hours:minutes) on the outside of the container, along with the date and your name. Mark the same in your dive log and post-dive checklist.

10

Wipe inner surfaces of scrubber canister surfaces with disinfectant, allow to dry

Spray disinfectant onto an absorbent towel. Wipe the inner surfaces of the scrubber canister with the towel. Do not touch or allow the towel to touch the sensor membranes. Allow the scrubber canister to air dry.

11

Drain Rinse the breathing hoses, ADV and breathing mouthpiece with fresh water and drain them.

hoses, Hang hoses from mouthpiece to dry. Hang mouthpiece, counterlungs with ports down to drain and dry. counterlungs and hang to dry 12

Inspect handsets and DIVA, cables, electrical connections

Inspect both handsets, DIVA (HUD), cables, and electrical connections for looseness, dirt, corrosion and damage; clean or replace as necessary.

13

Rinse and Flush the BC with fresh water through the drain BC manual inflator valve. Agitate the BC to rinse all inner surfaces. Drain either through the manual inflator valve or by using the overpressure valve dump cord.

14

Inspect Check all LP lines, connections, regulators, pneumatics addition valves, mountings and SPGs for signs assemblies of looseness, dirt, corrosion or damage. Replace if necessary.

15

Plug Plug the ports on the breathing hoses and breathing counterlungs with paper towels, to prevent hose ports ingress of insects or other organisms.

16

Allow CCR to If diving will continue immediately, reassemble dry the unit, ensuring all components are dry. Otherwise, leave the unit open in a secure place to thoroughly air dry. Leave the canister end cap off, but cover the sensors with a dry paper towel.

17

Reassemble When thoroughly dry, loosely reassemble all

CCR components. Unless using the unit within 24 hours, do not install the battery or a packed canister. If a packed canister is installed, clearly mark the outside of the unit with the total time of use. 18 Secure straps Ensure all straps, buckles and harness components are in good condition and untangled; secure as needed. Stow SPGs and displays to minimize strain and kinking of cables and hoses. The handsets should be clipped or otherwise securely fastened to the unit, to prevent damage from dangling. 19

Stow unit Store unit in a clean, dry location with moderate temperatures; or if diving will continue within 24 hours you may pre-dive as required.

Dräger Overview This part of the Procedures Guide Appendix is broken into four parts. Part 1 covers the Atlantis/Dolphin semi-closed rebreathers. This includes pre-dive, post-dive and canister packing procedures. Part 2 covers the DrägerRay semi-closed rebreather. It also includes pre-dive, post-dive and canister packing procedures for that unit. Part 3 covers procedures for the OXYgauge, which may be used on any of their semi-closed rebreathers. Part 4 provides pre-dive, postdive, and canister packing procedures for the LAR V oxygen closedcircuit rebreather. As discussed in an earlier portion of the textbook, adequate gas input in semi-closed rebreathers is important to prevent hypoxia. Dräger has engineered their units to provide 2.5 lpm oxygen input, which is sufficient oxygen for most people during most diving activities. All of the values in their manufacturer's guidelines that accompany those units, as well as the figures used in the tables in the following procedures, are based on that minimal flow rate. However, as discussed in Chapters 4 and 5 in this text, it is possible for a diver to consume more oxygen that 2.5 lpm under some circumstances for limited periods of time. An example might be if the diver was swimming very hard against a current, as may occur when returning to a boat after a dive. Additionally, large-bodied divers will usually metabolize more oxygen than smaller persons. For this reason, some experts recommend that SCR users plan their dives with a minimum oxygen supply of 3 lpm. This is the figure that is used in the text portions of the checklists below. If you have any questions about which figure is better for you personally to use, please discuss it with your instructor during your training. At no time, however, should you prepare your SCR for a dive with an oxygen supply rate of less than Dräger's recommended limit of 2.5 lpm! Because SCRs are in a continual state of modification and development, these procedures are provided as guidelines only. The manufacturer's procedures should be referred to and followed in all

cases. In the event of any discrepancies, always follow the most current version of the manufacturer's instructions.

Dräger Atlantis/Dolphin Pre-Dive Checklist Check for signs of dirt, deterioration, and damage to any part of the rebreather at all stages. DIVE NUMBER 123

1. Check cylinder pressure: ______, ______, _______ psi/bar 2. Check interstage (inlet) pressure: _______, _______, ________ psi/bar 3. Analyze supply gas: EAN ______, ______, _______

4. Calculate MOD: ______, ______, ______ fsw/msw (round shallow) 5. Determine which orifice to use based on supply gas (FsO2): 32% 40% 50% 60% (round down) 32% 40% 50% 60% (round down) 32% 40% 50% 60% (round down) 6. Select and setup the appropriate orifice on the mass flow controller 7. Cap other orifices 8. Check the flow rate FLOW METER DOSAGE TESTING DEVICE

____lpm or ____lpm or ____lpm or

_____sec _____sec _____sec

DRÄGER FLOW RATE CONSTRAINTS Supply Gas EAN60 EAN50 EAN40 EAN32

Flow Rate (Qs) (lpm) 5.7±10% 7.3±10% 10.4±10% 15.5±10%

Min Time (sec) 184 147 97 72

Max Time (sec) 250 196 135 90

9. Turn off gas supply 10. Confirm FiO2 FiO2 = ______, ______, ______ (If13v 17. Calibrate electronics if first dive of the day (follow calibration procedure below) 18. Verify operation of oxygen, diluent, and off-board manual addition valves. 19. Positive pressure test (dip test) 20. If not diving immediately, turn cylinders off, drain IP lines, flush breathing loop with air, and turn electronics off 21. Attach cover and stow

Titan Dive Gear Titan CCR Calibration Checklist DIVE NUMBER 123

1. Confirm oxygen cylinder and electronics (handset and HUD) on 2. Open mouthpiece. 3. Scroll to "Calibrate," begin calibration 4. Check mV output (40-65 mV) 5. Confirm cells read 0.98 –1.0 atm 6. Calibrate HUD 7. Confirm all three HUD LEDs flash orange once (red/green together) 8. Close mouthpiece

Titan Dive Gear Titan CCR Immediate Pre-Dive Checklist

DIVE NUMBER 123

1. Verify all gas supplies on 2. Verify bailout supply (ies) 3. Check BC 4. Verify both handset and HUD are on 5. Check PO2 at least 0.5 atm, pre-breathe 6. Verify mouthpiece (open/closed) 7. Bubble check

Titan Dive Gear Titan CCR Post-Dive Checklist Check for signs of dirt, deterioration and damage to any part of the CCR at all stages. For hygienic reasons a post-dive check must be completed between users. DIVE NUMBER 123

1. Unclip cover and rinse unit with fresh water 2. Check harness and other rebreather parts 3. Switch handset and HUD off

4. Check and remove gas cylinders; refill if necessary O2 _________ psi/bar Diluent _________ psi/bar O2 _________ psi/bar Diluent _________ psi/bar

O2 _________ psi/bar Diluent _________ psi/bar 5. Remove breathing hose assembly 6. Remove and recharge battery 7. Remove scrubber assembly 8. Check counterlungs for water ingress (drain) 9. Disinfect breathing hoses, mouthpiece, and counterlungs 10. Open scrubber basket, remove absorbent cartridge 11. If absorbent canister is still usable, seal canister and log absorbent use (hours: minutes) Total Time Used: _________________ minutes Total Time Used: _________________ minutes Total Time Used: _________________ minutes 12. Rinse scrubber housing and let dry 13. Rinse and drain breathing hoses and mouthpiece, hang breathing hoses to dry; rinse counterlungs 14. Rinse and drain BC 15. Inspect handset and HUD displays, cables, and electrical connectors 16. Inspect pneumatics assemblies, cap if not re-attaching gas cylinders 17. Plug breathing hose ports 18. Allow CCR to dry 19. Reassemble CCR 20. Secure straps 21. Stow unit

Titan Dive Gear Titan CCR

Pre-Dive Procedures

Because CCRs are in a continual state of modification and development, these procedures are provided as guidelines only. The manufacturer's procedures should be referred to and followed in all cases. In the event of any discrepancies, always follow the most current version of the manufacturer's instructions. At all stages the operator should be checking for signs of dirt, deterioration or damage to any part of the apparatus. Note that gas flow in the Titan flows in from over the left shoulder, and out over the right shoulder.

STEP 1

PROCEDURE

Inspect Check all hoses, connections, regulators, pneumatics addition valves, and HP gauges for signs of hoses looseness, dirt, corrosion or damage. Replace if necessary.

2

Inspect handset, cables, electrical connections, HUD

3

Install scrubber cartridge in basket

Inspect the handset, HUD, battery, cables, connectors, and HUD pod for correct alignment, looseness, dirt, corrosion and damage; clean or replace as necessary.

Remove bottom ring from inner scrubber basket. Verify scrubber seal is in the proper orientation with ridges to the outside edge. Check the outer edge of the scrubber cartridge (the side that will go against the scrubber seal) to make sure there are no nicks, cuts, or indentations. Correct any telescoping of the

cartridge, by placing on a flat surface and firmly realigning the coils. Confirm there is a plug in the center core. Drop cartridge with clean edge into the inner basket. Screw on the bottom ring. Insert inner housing into basket. Clean, grease and install the inner housing o-ring. 4

Attach lid to scrubber basket, attach, oxygen hose, and hose from manual gas addition block

Clean and grease two o-rings on lid. Place large ring around lid before pressing lid onto scrubber basket. Screw down the ring. Lay scrubber assembly on the backplate between counterlungs, with the HUD switch oriented away from the diver. Strap in place with Velcro band. Attach black gas feed hose from manual gas addition block onto black-colorcoded hose fitting by screwing it on. Screw on the green oxygen hose fitting onto the green color-coded hose fitting.

5

Check hoses, mouthpiece, verify valve operation and install

Inspect mouthpiece barrel, hoses, connectors and o-rings for dirt, cuts, nicks, deterioration or damage; replace if necessary. Operate mouthpiece several times to ensure free operation; repair or replace components if necessary. Open mouthpiece and breathe through it. Seal the two openings and the ADV connector on the inhalation (green, left) side, and simultaneously try to inhale. If it is possible to inhale, then the exhalation valve is either missing, defective or installed incorrectly or there is a hole in the hose; replace/reinstall as necessary. Repeat this process with the exhalation hose (orange, right) side by sealing the two openings and attempt to exhale, if it is possible to exhale then the inhalation valve is either missing, defective or installed incorrectly

or there is a hole in the hose; replace/reinstall as necessary. Close mouthpiece. Clean and grease the four sets of double o-rings on the breathing hose connectors. Attach the ADV to the inhalation (green, left) counterlung with the lid hose oriented towards the lid. Attach the lid hose to the outer lid hose adapter (green to green). Attach the exhalation T to the exhalation counterlung (orange, right) with lid hose oriented toward the lid. Attach the exhalation lid hose to the center lid adaptor (orange to orange). Assemble hand tight. Do not overtighten. The nuts merely hold the hose in place; they do not provide o-ring sealing pressure. 6

Attach ADV IP diluent supply hose to ADV. Confirm ADV shutoff valve is open.

7

Attach HUD You may wish to wrap the HUD cable around to mount on the hose several times to prevent entanglement. mouthpiece Snap in place with the LEDs facing towards the diver's face.

8

Analyze, check pressure, and install filled gas cylinders

The ADV IP diluent hose supply comes from the BC adaptor. Screw it on to the hose port on the ADV. Confirm ADV shutoff valve is in open position (slide towards the BC button).

Select appropriate diluent for intended dive profile. Analyze the gas in both the diluent and oxygen cylinders. Confirm both are full. Slide the cylinders into the restraining bands with the valves pointed down, and secure the Velcro straps, oxygen on the right, and diluent on the

left. Check o-rings are in place and attach the regulators. 9

Turn on cylinders, pressure hoses, turn off, look for pressure drop

Turn on the oxygen cylinder valve. Pressurize the hoses. Turn off the valve. Watch the SPG and look for pressure drop. If drop is seen, check the oxygen regulator DIN connector. Try again. If drop is still seen, check all other connectors. Repeat with diluent cylinder.

10

Turn on Turn both cylinders on. cylinders

11

Confirm Place mouthpiece in mouth and open. Breathe ADV in from the loop, and out through your nose until operation you hear or feel the ADV add gas. Close mouthpiece.

12

13

Negative pressure test and isolation valve check

Close the isolation valve by sliding it away from the BC button. Open the mouthpiece and inhale until there is a slight vacuum. Close mouthpiece and remove from mouth. Wait 30 seconds. Open the mouthpiece. You should hear an inrush of air. If unsuccessful, repeat test with the cylinders turned off and the intermediate pressure lines completely drained. If now successful, check for gas leakage into the breathing loop through the ADV, solenoid, or manual addition valves. If still unsuccessful, check for loose or poor connections or a hole in the breathing loop. Once test is successful, open the ADV isolation valve.

Check Using an interstage pressure (IP) gauge, check

interstage both oxygen and diluent IPs by disconnecting pressures hoses at the manual gas addition block. Both (8-10 should be 120-150 psi (8-10 bar). bar/120150psi) of O2 and diluent 14

Install Plug the 3-pin battery connector into the battery. external Move the locking ring down, and screw it onto battery the battery, locking it in place.

15

Turn on Turn on handset by pressing left handset switch, handset and followed by the right handset switch. Turn on HUD HUD by pressing once on the HUD switch on the top of the lid.

16

Check battery voltage; handset >3.4, external >13

17

18

Check the battery voltages by pressing the right handset switch 4 times. Confirm that they are above the minimum values: handset (int) greater than 3.4v, external (ext) greater than 13v.

Calibrate See calibration procedure below. electronics if first dive of the day Verify operation of oxygen, diluent, and off-board manual

Press oxygen (center, yellow on the manual gas addition block) manual addition valve and audibly confirm that gas is being added. Open mouthpiece. Press the diluent (red, upper) manual addition valve (MDV) while watching oxygen sensor readings on handset until you hear the solenoid adding oxygen just below the

addition set point level. Close mouthpiece. Press valves offboard (black, lower) manual addition valve, if used, and audibly confirm addition of gas. 19

Positive pressure test (dip test)

20

If not diving immediately, turn cylinders off, drain IP lines, flush breathing loop with air, and turn electronics off

Place mouthpiece in mouth and open. Orally inflate the breathing loop by inhaling through your nose, and exhaling through your mouth. Repeat until the breathing loop is completely full. Close the mouthpiece. Submerge the CCR in water, looking for bubbles. There should be none. If found, remove CCR from water and correct problem. If you will not be diving until the following day, open the mouthpiece, turn the cylinder valves off, and drain the IP lines using the gas addition block controls. Flush the breathing loop manually with air until PO2 drops below 0.3 atm. Then turn the handset off. To do this, press the MENU button once, followed by the SELECT button. If it will not go to TURN OFF option, dry the water contacts and try again. Turn the HUD off by tapping the HUD switch once. Finally, close the mouthpiece.

21 Attach cover Fold the cover back over the scrubber and stow assembly, and fasten. Use two Fastex® clips on each side of the cover. Stow the unit securely, so it will not shift during transport. Verify that no SPG hoses, electronics cables, or BC are pinched or liable to contact hard surfaces. Secure wrist displays to prevent mechanical shocks and vibration.

Titan Dive Gear Titan CCR Calibration Procedures STEP

PROCEDURE

1

Confirm oxygen cylinder and electronics (handset & HUD) on

2

Open Open by rotating mouthpiece lever into mouthpiece horizontal position.

3

Scroll to "Calibrate," begin calibration

If you were interrupted in the preceding portion of the checklist, confirm the oxygen cylinder is still on, the handset is on, the HUD is on (LEDs flashing), and external battery is connected.

Press the MENU button (left) two times, followed by the SELECT button (right) two times. Solenoid will add oxygen until oxygen levels are stable and cells calibrate.

4

Check mV While oxygen is being automatically added, output (40- confirm that mV output for each sensor is 65mV) between 40-65 mV and stable as the unit calibrates.

5

Confirm cells All three cells should read 0.98 –1.0 atm. The read 0.98 –1.0 readings may be lower if you have the atm calibration gas setting selected for less than 98%.

6

Calibrate HUD Tap the HUD button three times in rapid succession. At that point, all three LEDs should go solid red for approximately 4 seconds.

7

8

Confirm all three HUD LEDs flash orange once

Confirm all three HUD LEDs flash orange once (red/green together). If any LED flashes green, then red, that indicates failed calibration for that cell. Recalibrate, correct, or replace cell.

Close Rotate lever on mouthpiece to vertical position. mouthpiece

Titan Dive Gear Titan CCR Immediate Pre-Dive Procedures STEP

PROCEDURE

1

Verify all Verify gas contents on SPG. Confirm cylinders are gas on by injecting gas into the breathing loop using supplies on first the oxygen and then the diluent gas addition block buttons, simultaneously watching appropriate SPG. If SPG needle moves during manual valve operation, check cylinder valves to insure they are turned on.

2

Verify Turn bailout cylinder ON. Check SPG for bailout contents. Breathe from bailout regulator, while supply (ies) watching SPG. SPG should remain steady. If not, confirm cylinder valve is ON.

3 4

Check BC Verify BC LP inflator hose is attached. Add a small amount of air to BC with LP inflator. Verify both Confirm handset is operational and HUD is handset and providing matching LED readings.

HUD are on 5

6

7

Check PO2 at least 0.5 atm, prebreathe

Place mouthpiece in mouth and open. Breathe on loop while watching handset. Confirm that all three sensor readings vary with respirations. Raise PO2 to at least 0.5 atm. Continue to prebreath for three minutes. Ensure you have a mask on or pinch your nose shut during the pre-breathe procedure.

Verify If entering water while using the rebreather, leave mouthpiece mouthpiece in mouth, open. Otherwise, close (open/close) mouthpiece and remove from mouth. Bubble After entering water, submerge just below the check surface and have buddy perform visual check of breathing loop and connectors for signs of leakage or bubbles. If found, correct before diving.

Titan Dive Gear Titan CCR Post-Dive Procedures

Check for signs of dirt, deterioration or damage to any part of the apparatus at all stages of the post-dive procedures. For hygienic reasons a post-dive check must be completed between users. STEP 1

PROCEDURE

Unclip cover Ensure mouthpiece is closed. Thoroughly rinse and rinse the exterior with fresh water, including the

unit with manual gas addition block, cylinders, SPGs, fresh water hoses, and canister housing. 2

Check Check harness and all rebreather components harness and for signs of damage, fraying, or other problems. other Note and correct. rebreather parts

3

Switch Turn off handset by pressing the MENU button handset and (left) one time, and the SELECT (right) button HUD off one time. If "TURN OFF" does not show as an option, dry the wet switch contacts and try again. Turn off the HUD by pressing the HUD switch on the scrubber lid one time.

4

Check and remove gas cylinders; refill if necessary

5

Remove breathing hose assembly

Check cylinder pressures and record. Ensure both cylinder valves are closed and purge lines using the manual gas addition valves. Check that SPGs read 0 psi/bar. Remove regulators and lift cylinders out, refill as needed. Cap the regulators. Disconnect breathing hose couplings from the counterlungs and scrubber lid hose adapters. Remove LP hose from ADV. Unclip HUD from mount on mouthpiece.

6

Remove and Unscrew the external battery connector, and recharge remove the battery. Place some silicone grease battery in the connector receptacles. Recharge the battery as needed.

7

Remove Remove retaining strap. Disconnect all IP hose scrubber connectors. Remove the scrubber assembly.

assembly 8

Check With the unit upright, pull the dump cords at the counterlungs bottom of each counterlung. Any water in the for water counterlung will drain out. ingress (drain)

9

Disinfect breathing hoses, mouthpiece, and counterlungs

Spray breathing hose interior with disinfectant solution. Spray disinfectant in the upper hose connector port of each counterlung, coating all inside surfaces. Let sit 10 minutes, and then rinse with fresh water. Hang hoses from mouthpiece to dry. Leave counterlungs upright.

10

Open scrubber basket, remove absorbent cartridge

11

If absorbent canister is still usable, seal canister and log absorbent use (hours: minutes)

12

Rinse Rinse the basket, housing, cartridge retaining scrubber and lid rings, and set to dry. housing and let dry.

Unscrew the ring securing the lid to the absorbent housing. Dry the inside of the lid, and place to dry. Remove the inner absorbent cartridge, and unscrew the cartridge retaining ring. Remove the absorbent cartridge, and set aside. If the ExtendAir® cartridge is still usable, replace it in its original container, using both inner and outer plastic bags. Mark the usage of the cartridge (hours:minutes) on the outside of the container, along with the date and your name. Mark the same in your dive log and postdive checklist. Otherwise dispose of cartidge.

13

14

Rinse and drain breathing hoses and mouthpiece, hang breathing hoses to dry; rinse counterlungs

Rinse breathing hoses and mouthpiece by flushing each port or opening with fresh water beginning from the inhalation end and moving progressively to the exhalation end. Hang to dry. Rinse the counterlungs with fresh water by flushing from the top, and pulling the dump valve at the bottom of each counterlung. Leave standing upright to dry. Pull dump valve in each counterlung after drying to drain residual water.

Rinse and Flush the BC with fresh water through the drain BC manual inflator valve. Agitate the BC to rinse all inner surfaces. Drain either through the manual inflator valve or by using the overpressure valve dump cord.

15

Inspect handset and HUD displays, cables, and electrical connectors

Inspect wrist display, HUD, cables, and electrical connections for looseness, dirt, corrosion and damage, clean or replace as necessary.

16

Inspect pneumatics assemblies, cap if not reattaching gas cylinders

Check all lines, IP hoses, connections, regulators, addition valves, mountings and SPGs for signs of looseness, dirt, corrosion or damage. Replace if necessary. Cap regulators.

17

Plug Plug the ports on the breathing hoses and breathing counterlungs with paper towels, to prevent

hose ports ingress of insects or other organisms. 18 Allow CCR to If diving will continue immediately, reassemble dry the unit, ensuring all components are dry. Otherwise, leave the unit open in a secure place to thoroughly air dry. Leave the canister lid out, but cover the sensors with a dry paper towel. 19

Reassemble When thoroughly dry, loosely reassemble all CCR components. Unless using the unit within 24 hours, do not install the battery or a packed canister. If a packed canister is installed, clearly mark the outside of the unit with the total time of use.

20

Secure Ensure all straps, buckles and harness straps components are in good condition and untangled; secure as needed. Stow SPGs and displays to minimize strain and kinking of cables and hoses. The handset should be clipped or otherwise securely fastened to the unit, to prevent damage from dangling.

21

Stow unit Store unit in a clean, dry location with moderate temperatures; or if diving will continue within 24 hours you may pre-dive as required.

VR Technology Sentinel

VR Technology Sentinel

Full Pre-Dive Check Flowchart

Federal Aviation Administration Letter

U.S. Department of Transportation Federal Aviation Administration Dallas Fort Worth Civil Aviation Security Field Office 400 Fuller-Wiser Road Suite 224 Euless, Texas 76039-3877 Tel# (817) 354-2610 Fax# (817) 354-2639 To: S. Readey From. Luke Shelton Steam Machines, Inc. Special Agent, DFW Remarks: A depleted scuba tank cylinder does not meet the definition of a compressed gas, and as such is unregulated. A compressed gas must exceed 40 psi at room temperature and sea level to be considered a hazardous material. RE. 173.115(2)(b)(2) – Empty scuba tanks present no regulatory hazard.

Excerpted from the

Code of Federal Regulations Reference: Section 173.115 page 445

Research and Special Programs Administration, DOT 49 CFR Ch. 1 (10-1-97 Edition)

Subpart D – Definitions Classification, Packing Group Assignments and Exceptions for Hazardous Materials Other Than Class 1 and Class 7 Source: Amdt. 173-224, 55 FR 52634 Dec. 21, 1990, unless otherwise noted. Sec. 173.115 Class 2, Divisions 2.1, 2.2, and 2.3 – Definitions. (a) Division 2.1 (Flammable gas). For the purpose of this subchapter, a flammable gas (Division 2.1) means any material which is a gas at 20C (68 ° F) or less and 101.3 kPa (14.7 psi) of pressure (a material which has a boiling point of 20C (68° F) or less at 101.3 kPa (14.7 psi)) which – (1) Is ignitable at 101.3 kPa (14.7 psi) when in a mixture of 13 percent or less by volume with air; or (2) Has a flammable range at 101.3 kPa (14.7 psi) with air of at least 12 percent regardless of the lower limit. Except for aerosols, the limits specified in paragraphs (a)(1) and (a) (2) of this section shall be determined at 101.3 kPa (14.7 psi) of pressure and a temperature of 20C (68° F) in accordance with ASTM E681-85, Standard Test Method for Concentration Limits of Flammability of Chemicals or other equivalent method approved by the Associate Administrator for Hazardous Materials Safety. The flammability of aerosols is determined by the tests specified in Sec. 173.306(i) of this part. (b) Division 2.2 (non-flammable, nonpoisonous compressed gas– including compressed gas, liquefied gas, pressurized cryogenic gas, compressed gas in solution, asphyxiant gas and oxidizing gas). For the

purpose of this subchapter, a non-flammable, nonpoisonous compressed gas (Division 2.2) means any material (or mixture) which – (1) Exerts in the packaging an absolute pressure of 280 kPa (40.6 psia) or greater at 20C (68° F), and (2) Does not meet the definition of Division 2.1 or 2.3. (c) Division 2.3 (Gas poisonous by inhalation). For the purpose of this subchapter, a gas poisonous by inhalation (Division 2.3) means a material which is a gas at 20C (68° F) or less and a pressure of 101.3 kPa (14.7 psi) (a material which has a boiling point of 20C (68° F) or less at 101.3 kPa (14.7 psi)) and which – (1) Is known to be so toxic to humans as to pose a hazard to health during transportation, or (2) In the absence of adequate data on human toxicity, is presumed to be toxic to humans because when tested on laboratory animals it has an LC50 value of not more than 5000 ml/m3 (see Sec. 173.116(a) of this subpart for assignment of Hazard Zones A, B, C or D). LC50 values for mixtures may be determined using the formula in Sec. 173.133(b)(1)(i) of this subpart.

TSA Information The following information was taken directly from the Transportation Security Administration website. It may assist you in the security screening process.

Transporting Scuba Equipment

http://www.tsa.gov/travelers/airtravel/assistant/editorial_1190.shtm You may bring some scuba gear on-board an aircraft, but please follow the guidelines below. You may bring regulators, buoyancy compensators and masks, snorkels and fins as carry-on or checked baggage.

Knives and tools are prohibited from carry-on luggage. These items should be packed in checked luggage. Spear guns are prohibited from carry-on luggage. These items should be packed in checked luggage. Please sheath or securely wrap any sharp objects you pack in your checked luggage to prevent it from injuring baggage handlers and security officers. Knives and spear guns cannot be brought to a checkpoint. Pack these items in your checked baggage.

Compressed Gas Cylinders

http://www.tsa.gov/travelers/airtravel/assistant/compressed_gas.shtm Compressed gas cylinders are allowed in checked baggage or as a carry-on ONLY if the regulator valve is completely disconnected from the cylinder and the cylinder is no longer sealed (i.e. the cylinder has an open end). The cylinder must have an opening to allow for a visual inspection inside. Our Security Officers will NOT remove the seal or regulator valve from the cylinder at the checkpoint. If the cylinder is sealed (i.e. the regulator valve is still attached), the cylinder is prohibited and not permitted through the security checkpoint, regardless of the reading on the pressure gauge indicator. Our Security Officers must visibly ensure that the cylinder is completely empty and that there are no prohibited items inside. Passengers considering air travel with a compressed air or CO2 system would be advised to contact its manufacturer for guidance in locating a qualified technician, or to consider shipping the system to their destination via a parcel service. Note: This does not specifically address compressed oxygen cylinders. In speaking to TSA management, I was informed that compressed oxygen cylinders must be handled exactly as compressed air cylinders. Thus, they are allowed as either checked baggage or as a carry-on ONLY if the valve is completely removed from the cylinder and the cylinder is no longer sealed (i.e. the cylinder has an open end).

Photographic Equipment & Film

http://www.tsa.gov/travelers/airtravel/assistant/editorial_1248.shtm Photographic Equipment You may carry one (1) bag of photographic equipment in addition to one (1) carry-on and one (1) personal item through the screening checkpoint.

The additional bag must conform to your air carrier's carry-on restrictions for size and weight. Please confirm your air carrier's restrictions prior to arriving at the airport. Air carriers may or may not allow the additional carry-on item on their aircraft. Please check with your air carrier prior to arriving at the airport. Our screening equipment will not affect digital cameras and electronic image storage cards. Film The equipment used to screen checked baggage will damage undeveloped film. Pack your undeveloped film in your carry-on bag. High speed and specialty film should be hand inspected at the security checkpoint. To facilitate hand-inspection, remove your undeveloped film from the canister and pack in a clear plastic bag.

Flying With Compressed Gas Cylinders

Many TSA security screeners are unfamiliar with actual regulations regarding the transport of compressed gas cylinders or absorbent on passenger aircraft. As a result, on occasion some have forbidden cylinder or absorbent transport, and have required their removal from personal luggage. When traveling with rebreathers, it may be beneficial to help the security screeners by providing information they may not have. This may improve your chances of having your rebreather cylinders arrive at your destination with you. The following is an example of a note you might include with your cylinders in your checked luggage to accomplish this goal. I suggest affixing this to the sides of your cylinders and absorbent with tape before traveling. Jeffrey Bozanic, Ph.D. P.O. Box 3448 Huntington Beach, CA 92605-3448

To the TSA Agent who is opening this bag This baggage contains some small compressed gas cylinders. All of the cylinders in this luggage are empty...they are not pressurized. The valves have been removed and placed in another bag.

This includes all of the scuba tanks. The end of the cylinder has been left open to facilitate your inspection. While some of the cylinders are labeled for use with special gases like diluent, nitrox, or oxygen, they are all currently empty. There are no residual gases of any sort other than air in the cylinders. TSA regulations do not restrict the shipment of any empty cylinder aboard passenger or other aircraft. You may also have questions about the carbon dioxide absorbent (the while granular material) in my luggage. This is a filter media, used to remove carbon dioxide from my exhalations while scuba diving. It not a hazardous material, and also is not restricted by either TSA or the FAA. I have a MSDS for the filtration media should you like to review it. I can be reached within the airport on my cell phone at 714-xxx-xxxx if you have any questions, or I can be paged at the "my favorite airline" departure gates. Thank you for making our trips safer. Jeffrey Bozanic

Sofnolime® Molecular Products Limited

Material Safety Data Sheet Safety Data Ref: Date: Issue #:

SL1 28 08 94 1

The information in this safety data sheet is based on best knowledge available at the time and current national legislation. It provides guidance on

health, safety and environmental aspects of the product and should not be construed as any guarantee of technical performance or suitability for particular application. As specific conditions of use are outside the control of the supplier, the user is responsible for ensuring that the product is used in a safe way and the requirements of relevant legislation are complied with.

Note: The information in the original manufacturer's MSDS has been reformatted to match a standard format in this Appendix. This information may be outdated.

1 Identification Of Substance 2 Composition

Commercial Sofnolime® Name Chemical Soda Lime Name Components % w/w CAS Sodium 3% Hydroxide (NaOH) Calcium >75% Hydroxide Ca(OH)2

3

Hazards Identification 4 First Aid Measures

EINECS

131073-2

215185-5

130562-0

N/A

Supply Class CR34 N/A

Can cause burns to eyes and skin.

Inhalation Remove from exposure. Obtain medical attention if discomfort persists. Skin Drench with clean water. Obtain medical attention if skin becomes inflamed. Eyes Irrigate thoroughly with clean water. Obtain medical attention. Ingestion Wash out mouth thoroughly. Obtain medical attention. 5 Fire Fighting Extinguishing Water, foam, CO2 and powder are all Media

6

Accidental Release

7

Handling & Storage

8

Exposure Controls Personal Protection

9

Physical & Chemicals Properties

10

Stability & Reactivity

11 Toxicological Information

suitable. Fire & Material is non-combustible. Packaging Explosion may be combustible. Hazards Protective Breathing apparatus may be needed. Measures Personal Avoid inhaling dust. Avoid skin and eye Protection contact. Environmental No hazard. Protection Recovery Contain material. Sweep or vacuum up. Store in a cool, clean, dry environment. Avoid direct sunlight or temperatures in excess of 70C. Keep containers closed Occupational Component TWA/8h STE/10min Exposure NaOH N/A 2 mg/m3 Limits Ca(OH)2 N/A 5 mg/m3 Respiratory Nuisance dust mask recommended. Skin General purpose rubber gloves. Eyes Glasses to protect against dust. Hygiene Wash after skin contact. Appearance White or colored solids. Odor None. pH 12 – 14 Relative 0.9 g/cm3 Density Solubility in Slight. water Material is stable. Converts to calcium and sodium carbonates when exposed to air. Avoid contact with chloroform or trichloroethylene. Heat is generated when exposed to acids. Sodium LD50 = 500mg/kg rabbit Hydroxide

12

Ecological Information

13

Disposal Consideration

14

Transport Information

15

Regulatory Information

16

Other Information

Calcium LD50 = 7.3g/kg rat Hydroxide No risk of prolonged damage to animal or plant life. Converts to naturally occurring minerals. Incineration or landfill in accordance with local regulations. Materials not incinerated may be alkaline. Transport None Classification UN Number None Hazard Class N/A Packing N/A Group Supply corrosive Classification Risk Phrases R34 Causes burns Safety S2 Keep out of reach of children Phrases S25 Avoid contact with eyes In case of contact with eyes rinse immediately with plenty S26 of water and seek medical advice Wear suitable gloves and S37/39 eye protection The intended use of this product is as an absorbent for Carbon Dioxide and other acidic gases. It is suitable for use in anaesthetic equipment, hyperbaric systems, breathing apparatus, diving equipment, air purification systems and as a general industrial reagent. For further Molecular Products Ltd information Mill End contact: Thaxted Essex, CM6 2LT United Kingdom

Tel: 011.44.1371.830676 Fax: 011.44.1371.830998 Email: [email protected] 24 hr emergency tel: 011.44.0802.534621

Sodasorb® Dewey & Almy Chemical Division W.R. Grace & Company

Material Safety Data Sheet Prepared:

04-16-92

The data included herein are presented according to W.R. Grace & Company-Conn.'s practices current at the time of preparation hereof, are made available solely for the consideration, investigation and verification of the original recipients hereof and do not constitute a representation or warranty for which Grace assumes legal responsibility. It is the responsibility of a recipient of this data to remain currently informed on chemical hazard information, to design and update its own program and to comply with all national, federal, state, and local laws and regulations applicable to safety, occupational health, right-to-know and environmental protection.

Note: The information in the original manufacturer's MSDS has been reformatted to match a standard format in this Appendix. This information may be outdated.

1

Identification Of Substance

Commercial Sofnolime® Absorbent Name

General Alkaline carbon dioxide absorbent Chemical Description Note: These sheets cover all types of Sodasorb® : All mesh ranges: Fines, 4-8, 6-12 All grades: IND-A, F, H, HP, L, W; Reg-A, H, HC, L All moisture levels: 0-2% (L), 07% (F), 8-12% HC), 10-14% (A), 12-19% (Fines Ind H), 14-16% (H, HP), 14-19% (HP), 16-18% (W) 2

Composition

Components % w/w Calcium >73% Hydroxide (Ca(OH)2)

3

4

Hazards Identification

First Aid Measure

CAS

EINECS Supply Class

130562-0

Potassium < 5% 1310Hydroxide 58-3 (KOH) Sodium < 3% 1310Hydroxide 73-2 (NaOH) Dust can cause irritation and injury to the respiratory system if inhaled. Severe irritation upon contact with eyes. Irritation upon direct contact with skin. Harmful if swallowed. Prolonged or repeated overexposure can cause damage to eyes, skin, and mucous membranes. Carcinogenicity, mutagenicity, teratogenicity, and reproductive system effects are not indicated. Inhalation Remove to fresh air. Skin Wash affected area with water; if

5

6

irritation occurs and persists, get medical attention. Remove contaminated clothing. Eyes Immediately flush eyes with water for at least 15 minutes; get medical attention. Ingestion Dilute with weak vinegar solution or a 5% solution of ammonium chloride if these are available. If not, dilute with large quantities of water. Do not induce vomiting. Get medical attention. Fire Fighting Extinguishing Not available Media Flash Point Not flammable Hazardous Not available Combustion Products Upper Not available Flammability Limit Lower Not available Flammability Limit Auto Ignition Not available Temperature Explosion Not available Data – Impact Static Not available Discharge Sensitivity Special Not available Procedures Accidental Personal Avoid inhaling dust. Avoid skin and Release Protection eye contact. Keep all Sodasorb® that has been used with highly flammable anesthetics away from heat, sparks, and open flames, as residual amounts of these materials will be present.

7

Handling & Storage

8

Exposure Controls

Personal Protection

9

Physical & Chemicals Properties

Engineering If needed to stay below TLV. Controls Spill and Leak Scoop up and containerize in fiberProcedures board or lined metal containers. Rinse area, wash with soapy water and rinse. Recovery Contain material. Sweep or vacuum up. Keep from freezing. Do not use materials exposed to below freezing temperatures. Occupational Component TLV* Exposure Calcium 5 mg/m3 Limits Hydroxide Potassium 2 mg/m3 Hydroxide Sodium 2 mg/m3 Hydroxide Respiratory Use a particulate mask if necessary. Skin Wear gloves if necessary. Eyes Wear safety goggles if necessary. Hygiene Wash affected area with water. Physical State Solid Odor Not available Threshold Specific ~2 Gravity Density ~2 g/cm3 pH Not available Appearance White granules Odor None Freezing Point Not available Boiling Point Not available Vapor Not available Pressure

10

11

Vapor Density Not available Evaporation Not available Rate Solubility in Slightly soluble water Coefficient of Not available Water/Oil Distribution Volatiles Not available Stability & Conditions Product is stable, hazardous Reactivity Contributing to polymerization will not occur. Instability Incompatibility Not available Reactivity Will be neutralized by acids. Hazardous May react with chloroform slightly, Decomposition producing sodium formate, carbon Products monoxide, and phosgene. May react with trichloroethylene, producing dichloroacetylene, carbon monoxide, and phosgene. Toxicological Component LD50 LC50 Information Calcium 7340 mg/kg Not available Hydroxide (rat, oral) Potassium 365 mg/kg Hydroxide (rat, oral) Sodium 40 mg/kg Hydroxide (mouse, ipr.)

Ecological Information 13 Disposal Considerations

Not available Not available

12

14

Transport Information

Dispose of all products wastes and product rinses in accordance with all current local, provincial, and federal regulations. Material is not a hazardous waste as defined in the United States 40 CFR Section 261.3. See TDG Class

15 16

Regulatory Information Other Information

Emergency (617) 861-6600 Phone Number Prepared by Cheryl A. Malcom For further Process Development Department information (617) 861-6600 Dewey and Almy contact: Chemical Division W.R. Grace and Company-Conn. 55 Hayden Avenue Lexington, MA 02173

DiveSorb® Dräger Sicherheitstechnik GmbH

Material Safety Data Sheet Document: Version: Date of Issue: Supersedes: According to EC Directive:

MSDS 4594991 Rev 0, 09/98 09-04-98 Version 09/97 91/155/EEC

Reasonable care has been taken in the preparation of this information, but the manufacturer makes no warranty of merchantability or any other warranty, expressed or implied, with respect to this information. The manufacturer makes no representations and assumes no liability for any direct, incidental or consequential damages resulting from its use.

Note: The information in the original manufacturer's MSDS has been reformatted to match a standard format in this Appendix. This information may be outdated.

1

2

3

Identification Of Substance

Commercial DiveSorb® Name Part Number 6736680, 6736810, 6736950, 6737108, 6737199 Chemical Name Soda lime containing calcium hydroxide and alkali phosphates as well as 14-18% water Composition Components %w/w CAS EINECS Calcium hydroxide 74 - 80% 1305- 215-137-3 (Ca(OH)2) 62-0

Hazards Identification

Potential Health Effects

Alkali phosphate 6 - 8% N/A N/A Emergency This product is a non-combustible Overview solid. It is supplied in the form of white, odorless, hemispherical pellets. These pellets are severely irritating to eyes, skin and respiratory tract and may cause severe burns. Eyes Eye contact may cause corrosive damage with severe irritation, burns, and possible eye injury. Skin This product is severely irritating to the skin and may cause burns. Ingestion Ingestion of this product may cause nausea, vomiting and diarrhea. Inhalation Inhalation of dusts may cause severe respiratory irritation with coughing, shortness of breath, burns, and pulmonary edema. HMIS Ratings Health: 2 Fire: 0 Reactivity: 1

4

5

Hazard scale 0 = Minimal, 1 = Slight, 2 = Moderate, 3 = Serious, 4 = Severe, * = Chronic Hazard First Aid Inhalation If inhaled, immediately remove the Measures affected person to fresh air. If symptoms persist, get medical attention. Skin For skin contact flush with large amounts of water while removing contaminated clothing. If irritation persists, get medical attention. Discard any shoes or clothing items that cannot be decontaminated. Eyes Immediately flush eyes with water for at least 15 minutes, while holding eyelids open. Seek medical attention at once. Ingestion If material is ingested, immediately contact a physician or poison control center. Give 4 to 8 ounces of water or milk. DO NOT induce vomiting unless directed to do so by medical personnel. Fire Fighting Flash Point Not available Upper Not available Flammability Limit Lower Not available Flammability Limit Auto Ignition Not available Temperature Flammability Non-flammable Classification Rate of Burning Not available General Fire This material will not burn Hazards Hazardous Upon combustion, irritating or Combustion corrosive calcium oxide may be Products released

Extinguishing Media Protective Measures

6

Accidental Release

7

Handling & Storage

Use methods for the surrounding fire Wear full protective clothing, including helmet, self-contained positive pressure or pressure demand breathing apparatus, protective clothing and face mask NFPA Ratings Health: 2 Fire: 0 Reactivity: 1 Other: N/A Hazard Scale 0 = Minimal, 1 = Slight, 2 = Moderate, 3 = Serious, 4 = Severe Personal Do not inhale released dust. Use Protection dust mask with P2 filter. Take care to avoid eye contact, use safety goggles. Avoid skin contact. Containment Stop the flow of material, if this is Procedures without risk. Block any potential routes to water systems. Clean-Up Sweep up or gather material and Procedures place in appropriate container for disposal. Wash spill area thoroughly. Wear appropriate protective equipment during cleanup. Avoid the generation of dusts during cleanup. Evacuation Isolate area. Keep unnecessary Procedures personnel away. Special Follow all Local, State, Federal and Procedures Provincial regulations for disposal Handling Do not get this material in your Procedures eyes, on your skin, or on your clothing. Do not breathe dust from this material. Use this product with adequate ventilation. Wash thoroughly after handling. Storage Keep the container tightly closed Procedures and in a cool, well-ventilated place. Store away from acids. Store in

8

Exposure Controls

Personal Protection

9

Physical & Chemical Properties

10

Stability & Reactivity

original containers at temperatures ranging from minus 20C to 50C (-4° to 122° F). Occupational Component ACGIH OSHA NIOSH Exposure Ca(OH) 5 5 5 2 3 3 mg/m mg/m mg/m3 Limits/Engineering Use general ventilation and use Controls local exhaust, where possible, in confined or enclosed spaces Respiratory If ventilation is not sufficient to effectively prevent buildup of dust, appropriate NIOSH/OSHA respiratory protection must be provided Skin Use impervious gloves. Work clothing sufficient to prevent all skin contact should be worn, such as coveralls and long sleeves. Eyes Wear safety glasses with side shields General Use good industrial hygiene practices in handling this material Appearance White, hemispherical pellets Odor Odorless Physical State Solid pH 12.0 (in solution 1g/L @ 20C/68° F) Vapor Pressure Not applicable Vapor Density Not applicable Boiling Point Not applicable Melting Point Not applicable Specific Gravity Not applicable Relative Density 830 ± 100 g/l Solubility in water 1 g/l at 20C (68° F) Molecular Weight Mixture Chemical Stability Stable under normal conditions

11

Toxicological Information

Conditions to Avoid contact with acids. Do not Avoid use with chloroform or trichloroethylene. Incompatibility This product may react with light metals (aluminum) to form hydrogen gas Hazardous Upon combustion, irritating or Decomposition corrosive calcium oxide may be released Hazardous Will not occur Polymerization General Calcium hydroxide may produce eye, skin, respiratory system or gastrointestinal system irritation or burns. Solid particles or paste may react with moisture and protein in the eye and form clumps of moist compound, which are very difficult to remove. The clumps lodge in the cul-de-sac and act as reservoirs for the liberation of calcium hydroxide over time. May cause severe irritation, burns, excess tearing, conjunctiva edema, corneal edema, hemorrhage and opacification. Glaucoma has also been reported. Concentrated solutions of this product may be corrosive and cause severe or permanent damage to all tissue. Ingestion may cause nausea, vomiting, abdominal pain and diarrhea. Inhalation of dusts may cause severe irritation and possible burns. Calcium Oral LD50 = 7340 mg/kg rat Oral Hydroxide LD50 = 7300 mg/kg mouse Carcinogenicity None of this product's components are listed by ACGIH, IARC, OSHA, NIOSH or NTP

12

Ecological Information

13

Disposal Considerations

14

Transport Information

15

Regulatory Information

Because of the high pH of this product, it would be expected to produce significant ecotoxicity upon exposure to aquatic organisms and aquatic systems. Calcium hydroxide is harmful to aquatic life in very low concentrations. This product is not expected to accumulate in the food chain. General If discarded, waters may be classified as: D002 (Corrosive water). Waters must be tested using methods described in 40 CFR Part 261 to determine if it meets applicable definitions of hazardous wastes. No EPA waste numbers apply. Instructions Waste must be handled in accordance with all federal, state, provincial, and local regulations. Transport Not regulated as dangerous goods. Classifications U.S. DOT Not regulated as dangerous goods. Federal None of this product's components Regulations are listed under SARA Section 302 (40 CFR 355 Appendix A), SARA Section 313 (40 CFR 372.65), or CERCLA (40 CFR 302.4). This product is considered hazardous under 29 CFR 1910.1200 (Hazard Communication). State Regulations Calcium hydroxide appears on the hazardous substances lists of the following states: CA, FL, MA, MN, NJ, PA. Other state regulations may apply. Check individual state requirements. Inventory Calcium TSCA Yes hydroxide

16

Other Information

16

Other Information (continued)

DSL Yes EINECS Yes Component Calcium hydroxide is identified Analysis– WHMIS under the Canadian Hazardous IDL Products Act Ingredient Disclosure List [1% item 302 (991)] Contact Product Manager 412.787.8383 Manufacturer Dräger Sicherheitstechnik GmbH Revalstrasse 1 23560 Luebeck F R Germany Supplier Dräger Safety Inc. 101 Technology Drive Pittsburgh, PA 15275-1057 USA Tel: 412.787.8383 Fax: 412.787.2207 Emergency CHEMTREC 800.424.9300 Contact CHEMTREC emergency number is to be used in the event of chemical emergencies involving a spill, leak, fire, exposure, or accident involving chemicals. All non-emergency questions should be directed to customer service. Key/Legend EPA Environmental Protection Agency TSCA Toxic Substance Control Act ACGIH American Conference on Governmental Industrial Hygienists IARC International Agency for Research on Cancer NIOSH National Institute for Occupational Safety and Health NTP National Toxicology

OSHA NJTSR

Program Occupational Safety and Health Administration New Jersey Trade Secret Registry

Lithium Hydroxide® Molecular Products Limited

Material Safety Data Sheet Safety Data Ref: Date: Issue #:

LH1 19 06 98 3

The information in this safety data sheet is based on best knowledge available at the time and current national legislation. It provides guidance on health, safety and environmental aspects of the product and should not be construed as any guarantee of technical performance or suitability for particular application. As specific conditions of use are outside the control of the supplier, the user is responsible for ensuring that the product is used in a safe way and the requirements of relevant legislation are complied with.

Note: The information in the original manufacturer's MSDS has been reformatted to match a standard format in this Appendix. This information may be outdated.

1

Identification of Substance

2

Composition

3

Hazards Identification

4

First Aid Measures

5

Fire Fighting

6

Accidental Release

7

Handling & Storage Exposure

8

Commercial Name Chemical Name Components Lithium Hydroxide

Lithium Hydroxide Lithium Hydroxide (LiOH)] CAS EINECS Supply Class 1310-65-2 215-183-4 CR35

Can cause burns to eyes and skin. Dust causes severe irritation and damage to respiratory system. Inhalation Remove from exposure. Obtain immediate medical attention. Skin Drench with clean water. Obtain immediate medical attention if skin becomes inflamed. Eyes Irrigate thoroughly with clean water. Obtain immediate medical attention. Ingestion Wash out mouth thoroughly. Obtain immediate medical attention. Extinguishing Water, foam, CO2 powder are all Media suitable. Fire & Material is non-combustible. Packaging Explosion may be combustible. Hazards Protective Breathing apparatus will be needed. Measures Personal Avoid inhaling dust. Avoid skin and eye Protection contact. Wear appropriate protective equipment. Environmental Avoid using water to wash down until Protection most of solids cleared. Close drains where possible. Recovery Contain material. Sweep or vacuum up. Store in a cool, clean, dry environment. Keep containers closed. Occupational Component TWA/8h STE/15min

Controls

9

10 11 12 13 14

15

Exposure LiOH N/A 1 mg/m3 Limits Personal Respiratory Classified dust mask required. Protection Skin General purpose rubber gloves and protective overalls. Eyes Glasses to protect against dust. Hygiene Wash after skin contact. Physical & Appearance White solids Chemicals Odor None Properties pH 14 Relative 1.5-2.0 g/cm3 Density Solubility in Moderate water Stability & Material is stable. Converts to Lithium Reactivity Carbonate when exposed to air. Heat is generated when exposed to acids. Toxicological Lithium LD = 500mg/kg rabbit 50 Information Hydroxide Ecological Lithium Hydroxide is a strong alkali. Information Risk of permanent damage to animal, fish or plant life. Disposal Disposal by licensed contractor and Considerations carrier. Product is classified as special waste under UK regulations. Transport Transport Corrosive Information Classification UN Number 1739 Hazard Class 8 Packing 11 Group Regulatory Supply Very Corrosive Information Classification Risk Phrases R35 Causes severe burns Safety S22 Do not breathe dust Phrases

S25 S26

16

Other Information

Avoid contact with eyes In case of contact with eyes rinse immediately with plenty of water and seek medical advice S38 In case of insufficient ventilation wear suitable respiratory equipment S37/39 Wear suitable gloves and eye protection S45 In case of accident or if you feel unwell, seek medical advice immediately (show label where possible) The intended use of this product is as an absorbent for carbon dioxide and other low concentration acidic gases. It is suitable for use in air filtration equipment provided the product is held in an inert container with effective dust filters (HEPA). For further Molecular Products Ltd information Mill End contact: Thaxted Essex, CM6 2LT United Kingdom Tel: 011.44.1371.830676 Fax: 011.44.1371.830998 Email: [email protected] 24 hr emergency tel: 011.44.0802.534621

ExtendAir® CO2 Absorbent Micropore Incorporated

Material Safety Data Sheet Safety Data Ref: Date: Issue #:

21 05 2007 3

The information in this safety data sheet is based on best knowledge available at the time and current national legislation. It provides guidance on health, safety and environmental aspects of the product and should not be construed as any guarantee of technical performance or suitability for particular application. As specific conditions of use are outside the control of the supplier, the user is responsible for ensuring that the product is used in a safe way and the requirements of relevant legislation are complied with.

Note: The information in the original manufacturer's MSDS has been reformatted to match a standard format in this Appendix. This information may be outdated.

1 Identification of Substance

Commercial ExtendAir® CO2 Absorbent Name Chemical Calcium Hydroxide mixture

2 Composition

3

Hazards Identification

4

First Aid Measures

5 Fire Fighting

Name Product CL-XXX Number Components CAS

Approximate percent 85% minimum

Calcium 1305-62-0 Hydroxide Sodium 1310-73-2 3% Hydroxide Potassium 1310-58-3 2% Hydroxide Polyethylene 9002-88-4 10% maximum Immediate Irritant, can cause burns, may be concerns harmful if swallowed Potential Corrosive to eyes and skin on health effects contact, throat upon ingestion Overexposure Get medical attention for all cases of over-exposure Ingestion If conscious, drink water and get medical attention. Never give anything by mouth to an unconscious person. Eyes Immediately flush thoroughly with water for at least 15 minutes Skin Wash thoroughly with soap and water Flammable Not applicable Limits General Does not support combustion Hazard Extingushing Water spray, CO2, foams and Media powders are suitable

6

Accidental Release

7

Handling & Storage

8

Exposure Controls Personal Protection

Exposure Limits

9

Physical & Chemical Properties

Flash Point Not applicable Personal Wear suitable protective Protection equipment listed under exposure control/personal protection. Environmental Not a listed waste. Protection Recovery Take up for proper disposal as descibed under disposal Handling Do not get in eyes, on skin or clothing. Wash thoroughly after handling Storage Store in a dry area separate from incompatible materials Exposure Avoid skin and eye contact, do not ingest Skin Rubber gloves to prevent skin contact. Eyes Not applicable Respiratory Not applicable Hygiene Do not take internally Ingredient TWA 5 mg/m3 STEL CL Calcium Hydroxide Sodium 2 mg/m3 Hydroxide Potassium 2 mg/m3 Hydroxide Odor Odorless Appearance White solid pH 12 – 14 Percent 0 – 20% moisture Volatile

10

Stability & Reactivity

11

Toxicological Information

Vapor Not applicable Pressure Vapor Density Not applicable Boiling Point Not applicable Melting Point Not applicable Solubility in Slightly soluable Water Evaporation Not applicable Rate Conditions to Exposure to acids may generate Avoid heat Stability Stable under normal storage and temperature conditions Polymerization Will not occur Hazardous None Decomposition Products Incompatible Acids, metals, chlorinated or Materials nitrated organics Sodium LD50 = 500 mg/kg rabbit Hydroxide Calcium LD50 = 7.3 g/kg rat Hydroxide Toxicological None cited in Registry of Toxic Findings Effects of Substances (RTECS)

12

Ecological Information 13 Disposal Considerations

14

Material does not have an EPA waste number and is not a listed waste. Always assure compliance with all current local, state and federal regulations. Transport DOT Shipping None

Information

15

Regulatory Information

16

Other Information

Name: DOT ID None Number: TSCA This product is a mixture. The Inventory CAS numbers of all components are listed on the TSCA inventory. No classification or inventory under SARA EHS (302), SARA EHS TPQ (lbs), SARA 313, or Dominimis for SARA 313 is listed. Component CERCLA RQ OSHA Floor list (lbs) Calcium yes Hydroxide Sodium 1000 yes Hydroxide Potassium 1000 yes Hydroxide Comments This material is intended for use as a Carbon Dioxide absorbent. It is suitable for use in semiclosed and closed circuit breathing equipment. This MSDS has been created for informational purposes only and is based on technical data that Micropore Inc. believes to be true. We make no warranty, express or implied, for use of this information. Trademark All rights reserved. ExtendAir® is a registered trademark of Micropore Inc. Manufacturer Micropore Inc.

350F Pencader Drive Newark, DE 19702 Tele: 302-731-4100 Contact CHEMTREC Emergency telephone: 800-424-9300

Tribolube-71 Aerospace Lubricants, Inc.

Material Safety Data Sheet Safety Data Ref: Date: Issue #:

03 01 2008 Revision D

The information in this safety data sheet is based on best knowledge available at the time and current national legislation. It provides guidance on health, safety and environmental aspects of the product and should not be construed as any guarantee of technical performance or suitability for particular application. As specific conditions of use are outside the control of the supplier, the user is responsible for ensuring that the product is used in a safe way and the requirements of relevant legislation are complied with.

Note: The information in the original manufacturer's MSDS has been reformatted to match a standard format in this Appendix. This information may be outdated.

1 2 3

4

Identification of Substance Composition Hazards Identification

Commercial Tribolube®-71 Name Non-irradiated PTFE Immediate No hazardous components concerns were knowingly incorporated into this lubricant. This product is not considered hazardous according to the OSHA Hazardous Communication Standard 29CFR 1910.1200. General Avoid heating above 250C. Health Mild irritant to the skin upon prolonged exposure for some individuals. No known medical condition that might be aggravated by exposure. HMIS Ratings Health: 1 Fire: 0 Reactivity: 0 Hazard Scale 0 = Minimal, 1 = Slight, 2 = Moderate, 3 = Serious, 4 = Severe,* = Chronic Hazard First Aid Inhalation Slightly toxic by inhalation (4 Measures hr. LC50 1,000-5,000 ppm; 8-40 mg/l). If discomfort occurs, move to fresh air, contact physician. Eyes Flush with fresh water. If irritated, contact physician. Skin Wipe off and wash with soap and water, If irritation developes, contact physician. Very low toxicity by contact (LD50> 10,000 mg/kg) Ingestion Very low toxicity by ingestion

5

Fire Fighting

6

Accidental Release

7

Handling & Storage

8

Exposure Controls Personal Protection

(oral LD50> 5,000 mg/kg). Give large amounts of water, contact physician. Melting Point Above250C Auto-ignition Nonflammable Temperature Fire Fighting Self contained breathing Proceedures appratus and protective clothing should be worn in fighting fires involving chemicals. Decomposition at temperatures above 290C may cause the evolution of toxic gaseous fluorine compounds. Unusual Fire Toxic fluorine gases are byand Explosion products of combustion. Hazards Spills Scrape up with proper tools. Wipe up with absorbant cloth or paper towel. Apply non-skid absorbant material to floor. Collect waste material for salvage or disposal. Precautions Use reasonable care. Do not store above 250° F or near flammables or explosive material. Toxic vapors may evolve above 250C. Provide adequate ventilation if used above this temperature.

Respiratory Required if product is being used as a mist Eyes Safety glasses recommended

9

Physical & Chemical Properties

10

Stability & Reactivity

11

Toxicological

Skin Plastic disposable gloves. Plastic apron or fabric laboratory coat recommended. Hygiene Do not contaminate smoking materials. Wash hands and/or contaminated area after exposure.inventory. Operating See Characteristics Test Data Temperature page Range Boiling Point Not applicable Specific 1.90 Gravity Vapor 10-6at20C(mmHg) Pressure Vapor Density Not applicable Solubility Not soluble in water Reactivity Non-reactive in water Appearance White Odor Odorless Chemical Stable under normal conditions Stability Conditions to Heating above 250C Avoid Incompatability Molten alkalai metals, interhalogen compounds, strong or nonaqueous alkalai and Lewis acids above 100C Hazardous Toxic fluorine gases Decomposition Hazardous Will not occur Polymerization Threshold limit LD50>40 g/kg (non-toxic)

Information

Ecological Information 13 Disposal Considerations

value OSHA LD50>40 g/kg threshold limit value ACGIH LD50>40 g/kg threshold limit value Carcinogen- Not applicable NPT Program CarcinogenNot applicable IARC Program

12

14 15 16

Transport Information Regulatory Information Other Information

Waste Dispose of in accordance with Disposal current Federal, State, and Local regulations Transport None Classification

Comments Tribolube-71® is intended for use as a lubricant in open and closed circuit oxygen, nitrox, and air scuba applications. Its use is not limited to scuba applications. Tribolube-71®is LOX/GOX compatible. Manufacturer Aerospace Lubricants Inc. 1600 Georgesville Road Columbus, OH 43228 Tele: 614-878-3600 Fax: 614-878-1600 Web: www.aerospacelubricants.com

Tribolube-71 Aerospace Lubricants, Inc.

Characteristics Test Data

TRIBOLUBE®-71 Fluorinated Polyether Greases CHARACTERISTICS TRIBOLUBE-71 primary characteristic is that it uses PTFE that has not been irradiated. Nonirradiated PTFE over long term exposure has demonstrated it does not corrode aluminum or anodized aluminum. Tribolube-71 has a very wide temperature range, nonreactive with strong acids, oxygen, fuels, and solvents. It is an excellent anti-wear, extreme pressure lubricant with long life. Tribolube-71 is qualified to MIL-PRF-27617 types 1, 2, & 3.

PERFORMANCE TEST TEST METHOD

APPLICATIONS TRIBOLUBE-71 is intended for use in scuba applications as well in small and large diameter ball, roller, needle, and plain bearings, threads, valves, gears, and screw actuators. It is compatible with most elastomers and plastic seals, gaskets and O-rings. Although this lubricant is very inert, newly exposed rubbing surfaces of aluminum and magnesium may react with the greases under certain conditions.

CONDITION

TYPICALVALUES

Temperature Range NLGI Number Unworked Penetration Worked Penetration Oil Separation

Evaporation

-100° to450° F 1 ASTM D1403 ASTM D1403 FED-STD791 Method 321 ASTM D2595

@77° F

294

60 Strokes

299

30 hrs @ 300° F

9.20%

30 hrs @ 350° F

10.53%

30 hrs @ 350° F

11.7%

22 hrs @ 400° F 0.7% 22 hrs @ 450° F 2.88% Low ASTM D- @ -40° F, starting 221 gm-cm Temperature 1478 60 min running 85 gm-cm Torque @-65° F, Starting 507 gm-cm 60 min running 130 gm-cm @-100° F, starting 1,157 gm-cm 60 min running 728 gm-cm Copper FED-STDCorrosion 791 24 hrs @ 212° F 1b Method 5309 Load Wear Index ASTM D144.34 2596 Last Non-seizure Load/Wear Scar None Last seizure Load/Wear Scar 620 kg/1. 87 mm Weld Point Load 800 kg Steel-on-Steel ASTM D- 1200rpm,40kg, 1 Wear 2266 hrs @ 167° F, 1.02 mm 52100 steel 1200rpm,40kg, 1 1.09 mm hrs, @ 400° F

High Temperature Performance Film Stability & Steel Corrosion Water Washout Resistance to Aqueous Solution LOX Impact Sensitivity Fuel Stability Fuel Resistance Oxidation Stability

52100 steel ASTM D10,000 3336 rpm@400° F 5 lb. load Mil-G168 hrs @ 212° F 27617D ASTM D1 hrs @ 105° F 1264 FED-STD791 168 hrs @ 77° F Method 5415 ASTM D- 20 impacts from 2512 1,100 mm FED-STD@77° F 791 Method 8 hrs @ 77° F 5414 ASTM D- 100 hrs @ 212° F 942 500 hrs @ 212° F

+2,800 hrs Pass 0.1% Pass No Reaction 0.20% Pass -1psi -1.5 psi

Tribolube EPO2

Aerospace Lubricants, Inc.

PERFLUOROPOLYETHER CORROSION INHIBITOR

TRIBOLUBE EPO2 FLUID is a low molecular weight Perfluoropolyether (PFPE) designed to be used as a passivator for

the surface of brass. Tribolube EPO2 is formulated to be compatible and completely miscible with all PFPE fluids at all temperatures. In addition to being used for extended protection, Tribolube EPO2 can also act as a protectant for other materials involved in but not limited to SCUBA gear. Tribolube EPO2 is LOX/GOX compatible. Typical properties are as follows: TEST ODP (Ozone Depletion Potential) Typical Boiling Point (C) Flash Point (C) Density @ 25C (g/cm3) Surface Tension @ 25C (dyne/cm) Viscosity @ 25C (cSt) Vapor Pressure @ 25C (torr) Heat of vaporization at boiling point (cal/g) Solubility in water [ppm(wt)]

Safety Data Ref: Date: Issue #:

Typical Test Results Zero 90 None 1.88 14 0.75 100 17 14

03 01 2006 Revision D

The information in this safety data sheet is based on best knowledge available at the time and current national legislation. It provides guidance on health, safety and environmental aspects of the product and should not be construed as any guarantee of technical performance or suitability for particular application. As specific conditions of use are outside the control of the supplier, the user is responsible for ensuring that the product is used in a safe way and the requirements of relevant legislation are complied with.

Note: The information in the original manufacturer's MSDS has been reformatted to match a standard format in this Appendix. This information may be outdated. 1 2

Identification of Substance Composition

3

Hazards Identification

4

First Aid Measures

Commercial Tribolube® EPO2 Name Composition Propene, 1,1,2,3,3,3- hexafluoro, oxidized, polymerized Immediate No hazardous components concerns were knowingly incorporated into this lubricant. This product is not considered hazardous according to the OSHA Hazardous Communication Standard 29CFR 1910, 1200. General Avoid heating above 250C. Health Mild irritant to the skin upon prolonged exposure for some individuals. Decomposition products formed at high temperatures (>250C) may cause "polymer fever." No known medical condition that might be aggravated by exposure. HMIS Ratings Health: 1 Fire: 0 Reactivity: 0 Hazard Scale 0 = Minimal, 1 = Slight, 2 = Moderate, 3 = Serious, 4 = Severe,* = Chronic Hazard Inhalation Slightly toxic by inhalation (4 hr. LC50 1,000-5,000 ppm; 8-40 mg/l). If discomfort occurs, move to fresh air, contact physician. Eyes Flush with fresh water. If

5

Fire Fighting

6

Accidental Release

7

Handling & Storage

irritated, contact physician. Inform physician product is inert/non-toxic. Skin Wipe off and wash with soap and water, If irritation developes, contact physician. Ingestion Very low toxicity by ingestion (oral LD50> 5,000 mg/kg). Give large amounts of water, contact physician. Inform physician product is inert/nontoxic. Flash Point N/A Auto-ignition Nonflammable Temperature Fire Fighting Self contained breathing Procedures appratus and protective clothing should be worn in fighting fires involving chemicals. Unusual Fire Decomposition at temperatures and Explosion above 290C may cause the Hazards evolution of toxic gaseous fluorine compounds. Toxic fluorine gases are by-products of combustion Spills Scrape up with proper tools. Wipe up with absorbant cloth or paper towel. Apply non-skid absorbant material to floor. Collect waste material for salvage or disposal. Precautions Use reasonable care. Do not store above 250° F or near flammables or explosive material. Toxic vapors may

evolve above 250C. Provide adequate ventilation if used above this temperature. Avoid spills, causes slippery surface. 8

Exposure Controls Personal Protection

9

Physical & Chemical Characteristics

10

Stability & Reactivity

Respiratory Required if product is being used as a mist Eyes Safety glasses recommended Skin Plastic disposable gloves. Plastic apron or fabric laboratory coat recommended. Hygiene Do not contaminate smoking materials. Wash hands and/or contaminated area after exposure. Boiling Point 90C Specific 1.78 Gravity Vapor 100 torr at 25C (mm Hg) Pressure Vapor Density Not applicable Solubility 14 ppm in water Reactivity Non-reactive in water Appearance Clear liquid Odor Odorless Chemical Stable under normal conditions Stability Conditions to Heating above 250C Avoid Incompatibility Molten alkalai metals interhalogen compounds,

11

12 13 14 15 16

strong or nonaqueous alkalai and Lewis acids above 100C Hazardous HF (Hydogen fluoride, Decomposition TVL/TWA = 2.5 mg/m3) and COF2 (Carbonyl fluoride, TVL/TWA = 5 mg/m3) are both toxic; form from thermal decomposition in air (at temperatures greater than 290C) Hazardous Will not occur Polymerization Toxicological Threshold limit LD50>40 g/kg (non-toxic) Information value OSHA LD50>40 g/kg threshold limit value ACGIH LD50>40 g/kg threshold limit value Carcinogen- Not applicable NPT Program Carcinogen- Not applicable IARC Program Ecological Information Disposal Waste Dispose of in accordance with Considerations Disposal current Federal, State, and Local regulations Transport Transport None Information Classification Regulatory Information Other Manufacturer Aerospace Lubricants Inc.

Information

1600 Georgesville Road Columbus, OH 43228 Tele: 614-878-3600 Fax: 614-878-1600 Web: www.aerospacelubricants.com

End Note

References

1

Joiner JT. 2001. NOAA Diving Manual. 2001. 4th Edition. United States National Oceanic and Atmospheric Administration, Office of Undersea Research, 742 p. Best Publishing Company, Flagstaff, AZ.

2

Hamilton RW, DJ Kenyon, RE Peterson, GJ Butler, and DM Beers. 1988. REPEX: Development of repetitive excursions, surfacing techniques, and oxygen procedures for habitat diving. National Undersea Research Program Technical Report 88-1A.

3

Hamilton RW, DJ Kenyon, and RE Peterson 1988. REPEX habitat diving procedures: Repetitive vertical excursions, oxygen limits, and surfacing techniques. National Undersea Research Program Technical Report 88-1B.

4

Anon. 1993. Sodasorb: Manual of CO2 absorption. W.R. Grace &Company, Lexington, MA, 52 p.

5

U. S. Navy. 1991. Technical Manual: Operation and Maintenance Instructions (Organizational Level) for Underwater Breathing Apparatus LAR V. SS600-AJ-MMO010/6Z326.142p.

6

U. S. Army. 1991. Military Diving, Volume 2. FM 20-11-2. Department of the Army.

7

U. S. Navy. 1984. Purging procedures for the Dräger LAR V underwater breathing apparatus. NEDU Report 5-84.

8

Williams S. 1975. Engineering principles of underwater

breathing apparatus. In Bennett PB and DH Elliott, (Eds). The physiology and Medicine of Diving and Compressed Air Work 2nd Edition, pp. 34-46, Bailliere Tindall, London. 9

10

Morrison JB and SD Reimers. 1982. Design principles of underwater breathing apparatus. In Bennett PB and DH Elliot (Eds), The Physiology and Medicine of Diving 3rd Edition, pp. 55-98, Bailliere Tindall, London. Carmichael RM, J Dituri, KR Hamlin, E Kalayci, J Kellon, A Krasberg, B Mee, and T Mount. 1998. Halcyon: The doing it right equipment company, (Halcyon user's manual), 42 p.

11

Glover TJ. 1997. Pocket Ref. "Elevation vs. Air and Water," p.13. Sequoia Publishing, Inc. Littleton, CO, 543p.

12

Dräger. 1996. Dolphin I Mixed Gas Rebreather: Instructions for Use. P/N 9021158. 81p.

13

Dräger. 1983. Operating Manual, Closed-cycle Oxygen Respirator, Model LAR V. P/N 2214.15e. 44p.

14

U.S. Navy. 1975. Evaluation of the Dräger LAR V pure oxygen scuba. NEDU Report 11-75.

15

Dräger Dive Safety Inc. 1999. OXYguage PPO2 Oxygen Monitor Operating Manual. P/N 4055795. 28p.

16

Knafelc M. 2001. Technical Memorandum: U.S. Special Operations Forces UBA: Steam Machines, Inc., PRISM. NEDU TM 01- 03. 12p.

17

Anon, 2006, "Pre-Dive Planning and Preparations: Sample Drop in Oxygen Fraction." Halcyon RB80 website. www.halcyon.net

18

Ericsson M, O Franberg and P Lindholm. 2008. "Undersokning av Halcyon RB80 halvsluten dykapparat i samband med dykeriolyeksfall." Swedish Armed Forces Diving and Naval Medicine Centre (DNC) and Swedish Armed Forces Diving

and Navel Medicine Centre (DNC) and Swedish Defence Research Agency (FOI). 25 July 2008. 29p.

GLOSSARY • - a symbol calling for multiplication

A absolute pressure - the total pressure on a person or object relative to a vacuum (underwater, it includes the total of air and water pressure) absorbent - chemical compound used to remove carbon dioxide from gas absorbent bed monitoring - a method of measuring scrubber use by tracking the heat front caused by the absorbent reaction in the scrubber media accumulator - a volume chamber used in some mixed gas CCRs to add a preset volume of oxygen to the breathing loop ACG syndrome - a condition in which a caustic solution (formed by absorbent being dissolved in water in the breathing loop) is aspirated by the diver via the mouthpiece acidic gas - any gas that is acidic in nature, such as carbon dioxide acoustic jet - see sonic valve active system - a type of rebreather in which gas addition is automatic, and independent of the diver acute oxygen toxicity - see CNS toxicity ADBOV - automatic diluent bailout valve adult respiratory distress syndrome - a progressively deteriorating lung condition stemming from pneumonia, chemical pneumanitis, or

other factors, which often leas to death ADV - automatic diluent valve AGE - arterial gas embolism allergic reaction - an immune response of the body to a contaminant or contact agent ambient pressure - the pressure around an object or person ANDI - American Nitrox Divers Inc. annular axial flow - a canister design in which the gas passes in a straight path through a donut shaped scrubber in a uniform direction, traveling in a direction parallel to the "hole" of the scrubber annular corrugation - a breathing hose design providing a series of regular, distinct corrugations ARDS - adult respiratory distress syndrome arterial gas embolism - a malady resulting from pressure imbalances causing a rupture of the lungs, allowing air bubbles to enter the arteries ata - atmospheres absolute atm - atmospheres atmosphere - a unit of pressure equal to that of the weight of air at sea level, or 14.7 psi (1.01 bar) Au - gold automatic diluent bailout valve (ADBOV) - a rebreather mouthpiece that incorporates an automatic diluent valve and an open circuit second stage

automatic diluent valve - a valve that automatically adds diluent into a mixed gas CCR when breathing loop volume is reduced averaging protocol - a system in which the multiple oxygen sensor values in a mixed gas CCR are averaged to arrive at the PO2 value used by the electronics to control oxygen addition axial flow - a canister design in which the gas passes in a straight path through the scrubber in a uniform direction

B back-mounted counterlung - a counterlung design in which the counterlung is carried on the diver's back bailout - a procedure used in the event of a failure of the primary dive equipment in which a redundant gas supply system is used to ascend to the surface bailout bottle - a compressed gas cylinder and attendant OC regulator carried to provide an alternate gas supply system in the event of a failure of the primary dive equipment bailout valve - a rebreather mouthpiece that incorporates an open circuit second stage, so it can be used in both closed and open circuit modes Ba(OH)2 - barium hydroxide bar - a metric measure of pressure approximately equal to one atmosphere BC - buoyancy compensator Betadine® - trade name for the disinfectant povidone-iodine Boyle's Law - at constant temperature the volume of a quantity of gas in a flexible container varies inversely with its pressure

BOV - bailout valve breakthrough - the point at which a scrubber fails to absorb all of the carbon dioxide from an exhaled gas, typically defined as when the canister passes 0.5% CO2 SEV breathing bag - see counterlung breathing loop - those parts of a rebreather through which breathing gas circulates, typically including the mouthpiece, hoses, water traps, scrubber, counterlung, and lungs breathing resistance - see work of breathing BSAC - British Sub-Aqua Club bubble check - the practice of visually examining a buddy's rebreather upon descent, looking for bubbles that might indicate a leak or loss of system integrity buddy mount - a display showing PO2 designed to be monitored by the diver's dive partner bulk loading - a problem seen during deep dives on rebreathers, where the density of the breathing gas inhibits carbon dioxide absorption bypass valve - a valve that allows a diver to manually add gas from any supply gas cylinder into the breathing loop

C CaCO3 - calcium carbonate calcium carbonate - a naturally occurring benign compound (sea shells, coral, and limestone), which comprises the bulk of the end product of most used carbon dioxide absorbents

calcium hydroxide - an alkalai compound commonly used as a carbon dioxide absorbent calibration bag - a device used in conjunction with a stopwatch to measure gas flow, used to confirm mass flow controlled SCRs are performing as designed canister - a device built to hold absorbent used to remove CO2 in exhaled breathing gas canister duration - the amount of time a freshly filled absorbent scrubber canister will last before breakthrough occurs Ca(OH)2 - calcium hydroxide carbon monoxide - an oxide of carbon that binds to hemoglobin much more readily than oxygen, potentially causing carbon monoxide toxicity carbon monoxide toxicity - a form of cellular suffocation caused by carbon monoxide binding to the hemoglobin, wholly or partially preventing oxygen transport to tissues by blood carbonic acid - a naturally occurring acid formed when carbon dioxide dissolves in water caustic cocktail - a condition in which a caustic solution (formed by absorbent being dissolved in water in the breathing loop) is delivered to the diver via the mouthpiece CCR - closed cicuit rebreather, usually referring to closed circuit mixed gas rebreather CE Mark - a mark indicating conformance with minimum design/ manufacturing standards in Europe cf - cubic feet

cfm - cubic feet per minute CFR - U.S. Code of Federal Regulations channeling - passage of gas through the scrubber bed in a nonuniform manner, resulting in much of the gas volume having a very short dwell time check valve - see mushroom valve checklist - a tool used to ensure pre- and post-dive maintenance actions are performed chemical pneumanitis - fluid in the lungs caused by chemical burns in the interior of the lungs Christo•Lube® - an oxygen compatible lubricant manufactured by Lubrication Technology, Inc. closed circuit mixed-gas rebreather - a type of scuba equipment that recirculates all of the exhaled breathing gas, and blends oxygen and diluent to comprise the breathing gas closed circuit rebreather - a type of scuba equipment that recirculates all of the exhaled breathing gas CNS - central nervous system CNS toxicity - a severe form of oxygen poisoning resulting from relatively short exposures to high partial pressures of oxygen (PO2 ≥ 1.3 atm), also known as acute oxygen toxicity CO2 - carbon dioxide co-axial flow - a canister design in which the gas passes first through the outer absorbent filled portion of a cylindrical scrubber, then reverses direction passing through the hollow center of the canister

color reversion - the effect in which a color indicating absorbent changes from its "used" color state back to its "unused" color state at some time after the absorbent has been removed from service (note that this does NOT mean that the absorbent is once again usable) computer/algorithm testing - testing of the programming and models used in microprocessors controlling oxygen addition and/or inert gas status computer control - a microprocessor used to electronically control the injection of oxygen into the breathing loop in a rebreather constant fraction - a gas mixture that maintains a stable percentage of constituent gases as pressure changes constant mass flow rebreather - type of SCR which continuously injects a fixed quantity of gas into the breathing loop, containing sufficient oxygen to replace that metabolically used by the diver (also known as an "active" system, since it continually adds gas) constant mass ratio rebreather - a type of RMV-keyed SCR in which a fixed mass of gas is injected at a rate proportional to the ventilatory rate of the diver constant partial pressure rebreather - a type of rebreather that uses two gases to maintain a constant partial pressure of oxygen in the breathing gas constant PO2 dive tables - dive tables developed for use with constant partial pressure CCRs, based on the EAD concept for different PO2 constant ratio rebreather - a type of SCR that uses two gases to maintain a constant ratio or fraction of oxygen in the breathing gas constant volume ratio rebreather - a type of RMV-keyed SCR in which the volume ratio is fixed, i.e. the size of the exhaust counterlung is unchanging

contraction phase - the first phase of an oxygen seizure, characterized by the victim's muscles becoming completely rigid contraction/tonic phase - affected diver's muscles contract and he becomes completely rigid. The airway is closed, and the diver does not breathe. Symptom of CNS oxygen toxicity. ConVENTID - an acronym used to remember the symptoms of CNS oxygen toxicity counterlung - the rebreather component that provides a volume reservoir for exhaled gas as a diver breathes CPDT - constant PO2 dive tables cryogenic rebreather - a rebreather system that uses liquid gas supplies to provide gas to the user cryogenic scrubber - a scrubber that freezes CO2 by changing from a gas to a solid, leaving oxygen and diluent in the gaseous phase to continue cycling in the loop.

D D0 - the depth at which the partial pressure of nitrogen in a breathing mix equals 0.79, the same as breathing air on the surface Data - depth measured in atmospheres absolute, also called pressure at depth Dalton's Law - the pressure exerted by any gas component in a mixture of gases is equal to the fraction of that gas in the mix times the absolute pressure of the total gas DCI - decompression illness DCS - decompression sickness

decompression illness - a generic term that includes both decompression sickness and arterial gas embolism, used because differentiation between the two maladies is often difficult or impossible to determine in the field decompression sickness - a malady caused by inert gas coming out of solution from body tissues too rapidly following a hyperbaric exposure digital display - an instrument that uses a liquid crystal display to indicate PO2 in the breathing loop diluent - a gas (typically air) used in a rebreather to reduce the fraction of oxygen in the breathing gas, allowing the system to be used at depths greater than 20 fsw (6.1 msw) direct gas analysis - a method in which CO2 in the breathing loop is determined by directly measuring or analyzing the gas dive/surface valve - rebreather mouthpiece DiveSorb® - a soda lime absorbent manufactured by Dräger diving bell - an early type of dive equipment consisting of a chamber at ambient pressure, in which the bottom was open to allow pressure to equalize DrägerSorb® - a soda lime absorbent manufactured by Dräger for use in medical applications DSV - dive/surface valve or the rebreather mouthpiece dust content - the amount of dust or fine particulate matter contained in an absorbent, impacting the rate that absorbent will dissolve should it come in contact with water dwell time - the amount of time exhaled gas remains within the scrubber

E EAD - equivalent air depth EANx - enriched air nitrox of undefined mix (i.e., EAN32 would be nitrox with 32% oxygen) eCCR - electronically controlled CCR electronic control - an analog circuit used to electronically control the injection of oxygen into the breathing loop in a rebreather electronically controlled CCR - a mixed gas closed circuit rebreather in which oxygen addition is governed by electronics, done independently of direct diver control elimination protocol - a system in which the values from the lowest and highest reading oxygen sensors are eliminated when controlling oxygen addition in mixed gas closed-circuit rebreather electronics Epinephrine - a drug used to alleviate symptoms of severe allergic reactions epi pen - a syringe-like device containing and used to administer epinephrine ESA - emergency swimming ascent equivalent air depth - the depth at which a diver who is actually breathing nitrox would be on-gassing nitrogen at the same rate had he been breathing air instead esophageal swelling - swelling of the esophagus (windpipe) caused by exposure to chemicals (such as absorbents) or allergy-inducing agents exhalation bag - see exhalation counterlung

exhalation counterlung - in a splint counterlung design, the counterlung that provides reservoir volume just downstream of the exhalation hose, and usually is designed to also function as a water trap exhalation hose - the breathing hose returning exhaled gas to the scrubber and counterlung from the mouthpiece and diver exothermic - a chemical reaction characterized by the generation and release of heat ExtendAir® - a solid state scrubber media manufactured by Micropore

F FAA - U.S. Federal Aviation Administration FFM - full face mask ffw - feet of fresh water FiO2 - fraction of oxygen in the gas inspired by a diver flask - a spherical vessel used to contain gases at high pressures, differing from cylinders in shape flow meter - a device used to measure gas flow, used to confirm mass flow controlled SCRs are performing as designed flush (breathing loop) - see loop flush FO2 - fraction of oxygen, or the percentage of oxygen in a breathing gas expressed as a decimal fraction fraction of oxygen - the percentage of oxygen in a breathing gas expressed as a decimal fraction

front-mounted counterlung - a counterlung design in which the counterlung is carried on the diver's chest/abdomen FSO2 - fraction of oxygen in the supply gas fsw - feet of sea water fuel cell - see oxygen sensor full face mask - a mask covering the entire face, including eyes, nose, and mouth

G gas efficiency - the comparative efficiency or gas economy of a type of diving equipment gauge pressure - absolute pressure less one atmosphere (i.e., at sea level on the surface would be zero) grain mesh size - a measure of the number of grains of absorbent that will fit into a given area granny line - a line attached to the anchor line at the surface and trailing to the stem of the vessel, which enables divers to pull themselves handover-hand to descend down the anchor line, rather than having to swim against a current

H H2CO3 - carbonic acid Halocarbon® - an oxygen compatible lubricant hardness - a measure of the percentage of absorbent that remains after a proscribed mechanical agitation test, reflecting its resistance to powdering or forming dust

hazardous material - a regulatory classification of a material indication that transport, handling, and disposal are regulated HDD - head down display HDS, USA - Historical Diving Society, USA head down display - a display within the dive mask that can be monitored by the diver at all times head up display - a display that can be seen by the diver looking through the diving mask helical corrugation - a breathing hose design in which corrugations are continuous, wrapping down the length of the hose in a manner similar to a spring high performance grade - an absorbent formulation that contains a relatively high (usually 15-18%) moisture content HP grade - see high performance grade HP Sodabsorb® - a soda lime based absorbent manufactured by W.R. Grace and Company containing a higher moisture content than their standard Sodabsorb® HUD - head up display hydrostatic head - a pressure differential caused by the weight of a volume of water, or the difference in water pressure of a system open at two different depths hypercapnia - carbon dioxide poisoning, caused by breathing elevated levels of CO2 hyperoxic - a gas in which the PO2 is greater than 0.21 atm hyperoxic myopia - a form of oxygen toxicity that causes myopia (near sightedness)

hypoxia - a medical condition in which one is impaired by lack of oxygen hypoxic - a gas in which the PO2 is less than 0.21 atm

I IANTD - International Association of Nitrox and Technical Divers independent testing - testing conducted by a group or agency unassociated with the manufacturer inert gas narcosis - a feeling of intoxication or impairment resulting from the anesthetic effects of breathing nitrogen or other inert gases at high partial pressures inhalation bag - see inhalation counterlung inhalation counterlung - in a splint counterlung design, the counterlung in which gas is stored immediately prior to traveling into the inhalation hose inhalation hose - the breathing hose bringing gas to the mouthpiece and diver IR - infrared

K K - ventilatory equivalent for oxygen K2CO3 - potassium carbonate kg - kilogram (equivalent to 2.2 pounds) KOH - potassium hydroxide

L

L- liters LAR V - an oxygen CCR manufactured by Dräger LED - light emitting diode lenticular oxygen toxicity - a form of oxygen toxicity that affects the lens of the eye lifesaver change - a loop flush performed prior to ascent, ensuring the gas in the breathing loop is not hypoxic light emitting diode - a small light, commonly used to indicate breathing loop PO2 in rebreathers LiOH - lithium hydroxide lithium hydroxide - an extremely reactive alkalai compound rarely used as a carbon dioxide absorbent in underwater breathing systems loop flush - a complete replacement of the gas in the breathing loop with fresh gas from the gas supply or diluent cylinder (accomplished by three cycles of exhausting the gas in the breathing loop by inhaling through the mouth and exhaling through the nose until no further gas may be inhaled, followed by completely filling the breathing loop with fresh gas) lpm - liters per minute

M manual override - a mechanism allowing the diver to inject gas from any supply cylinder manually

manually controlled CCR - a mixed gas closed circuit rebreather in which some or all of the gas addition is done by the diver mass flow controller - a component of some SCRs that injects a fixed number of gas molecules into the breathing loop mass flow steady state equation - an equation derived by looking at steady state conditions in a mass flow controlled SCR, which predicts the fraction of oxygen inspired (FiO2) by the diver Material Safety Data Sheet - a document detailing the composition, health and environmental hazards, safety precautions, and first aid for a chemical or compound manufactured and offered for sale maxPO2 - maximum allowable oxygen partial pressure maximum operating depth - the maximum depth at which it is safe to use a breathing gas or rebreather mCCR - manually controlled closed circuit rebreather mechanical display - an instrument that uses a jeweled needle mechanism to display PO2 to in the breathing loop metabolic rate - the amount of oxygen an individual consumes in liters per minute (lpm) mfw - meters of fresh water middle ear oxygen absorption syndrome - barotrauma injury of the tympanic membrane and middle ear caused by absorption of oxygen from the middle ear space following surfacing from a dive in which the breathing gas used for ear equalization had a high FO2 minimum loop volume - the optimal volume of gas in the breathing loop for CCRs, in which the counterlung is just at the completely deflated state at the end of a normal inhalation

minSP - minimum set point minimum set point - the lowest set point which can be used at a particular depth in a mixed gas CCR to avoid having a hypoxic mix in the breathing loop mixed gas CCR - a closed circuit rebreather utilizing both oxygen and diluent gases, also known as a constant PO2 rebreather MOD - maximum operating depth of an SCR or open circuit nitrox mix based on oxygen partial pressure, measured in fsw or msw MODIP - maximum operating depth based on input pressure MODN - maximum operating depth based on narcosis MODO - maximum operating depth based on oxygen levels, measured in fsw or msw mouthpiece - the rebreather component from which the diver breathes, incorporating the mushroom valves that provide directional control for gas flow, and a means of providing a water-tight seal to protect the breathing loop when not in the diver's mouth mouthpiece protocol - the rules and procedures governing proper use of rebreather mouthpieces MSDS - Material Safety Data Sheet msw - meters of seawater muscle memory - the subconscious retention of a motor skill that has been repetitively performed mushroom valve - a non-return valve located in rebreather mouthpieces that controls gas flow direction within the breathing loop myopia - near sightedness

N N2 - nitrogen Na2CO3 - sodium carbonate NaOH - sodium hydroxide NAUI - National Association of Underwater Instructors NEDU - U.S. Navy Experimental Diving Unit negative pressure check - a test performed to see if a breathing loop or other portions of a rebreather leak gas when the ambient pressure is greater than that inside the system, conducted prior to diving nitrogen narcosis - drowsy state induced by breathing air under higher than atmospheric pressure NOAA - U.S. National Oceanographic and Atmospheric Asssociation normoxic - a gas in which the PO2 is 0.21 atm, as in air at sea level nose breathing - the practice of breathing through the nose, or if underwater of regularly exhaling through the nose

O O2 - oxygen O2 ear - see middle ear oxygen absorption syndrome OC - open circuit, referring to open circuit scuba OCB - open circuit bailout mouthpiece

off-O2 effect - the onset or a recurrence of acute oxygen toxicity symptoms occurring several minutes after the PO2 is reduced OL - amount of oxygen lost from the breathing loop (lpm) open circuit - a type of scuba equipment in which breathing gas is used once, then vented to the ambient environment open circuit bailout mouthpiece - see bailout valve orifice - the hole in the sonic valve machined to a specific size to determine and limit gas flow OS - amount of oxygen added to the breathing loop (lpm) OTU - oxygen tolerance limit over-learning - repetition of a motor skill beyond that necessary for demonstration of the skill while concentrating on the activity, done to develop muscle memory of the skill overpressure valve - a valve that relieves pressure from a counterlung by venting excess gas volume over-the-shoulders counterlung - a counterlung design in which the counterlung wraps over the diver's shoulders, similar to a jacket style buoyancy compensator OXYgauge - a gauge that determines the PO2 in the breathing loop, manufactured by Dräger for use in the Dolphin I or modified Atlantis I SCRs and DrägerRay SCR oxygen cleaning - a process used to prepare components used in systems exposed to gas containing high fractions of oxygen at high pressures oxygen clock - a term referring to a diver's cumulative exposure to oxygen

oxygen rebreather - a type of rebreather designed to use 100% oxygen, limited to shallow depths oxygen sensor - a sensor used to measure the partial pressure of oxygen in a gas oxygen tolerance unit - a measure of oxygen exposure used in preventing whole body oxygen toxicity using the REPEX method

P PADI - Professional Association of Diving Instructors partial pressure - the pressure exerted by one component gas in a mixture of gases passive addition semi-closed rebreather - see RMV-keyed SCR passive system - a type of rebreather in which gas addition is powered by the breathing of the diver Pb - lead PCO2 - partial pressure of carbon dioxide pendulum axial flow - a canister design in which the gas passes bidirectionally (i.e., in and out) in a straight path through the scrubber percent-CNS method - a method of tracking oxygen exposure to prevent CNS toxicity problems performance standard - a measure against which testing results may be evaluated piezo-electric valve - an electromagnetic valve that remains open while a current is applied, typically used to inject oxygen in mixed gas CCRs

PN2 - partial pressure of nitrogen, in atm PO2 - oxygen partial pressure, in atm PO2 limit - the partial pressure of oxygen above which oxygen exposure is considered unsafe, i.e. most agencies recommend a limit of 1.4 atm PO2 for recreational diving poloxamer-iodine - a disinfectant marketed under the trade name Wescodyne® positive pressure check - a test performed to see if a breathing loop or other portions of a rebreather leak gas when pressurized and filled with gas, done prior to diving post-dive check - the procedure followed after a dive to ensure the rebreather is properly cleaned and maintained potassium carbonate - a benign product formed during the carbon dioxide scrubbing process potassium hydroxide - an alkalai compound commonly used in carbon dioxide absorbents povidone-iodine - a disinfectant marketed under the trade name Betadine® pre-dive check - the procedure followed prior to a dive to ensure the rebreather is operating in a nominal manner pulmonary oxygen toxicity - see whole body oxygen toxicity purge (breathing loop) - see loop flush

Q QS - flow rate of the supply gas into a breathing loop (lpm)

QV - vent rate of gas from a breathing loop (lpm)

R R1 - volume ratio, = VD/VE radial flow - a canister design in which the gas enters a hollow inner portion of a cylindrical canister and flows radially outward through the scrubber bed RAID - Rebreather Association of International Divers reactive plastic cartridge - a solid state scrubber media rebreather - a type of scuba equipment that circulates some or all of the exhaled gas, increasing gas efficiency Rebreather Advisory Council - a committee established by NAUI to review rebreather training standards and establish minimum equipment standards for the recreational diving community relaxation phase - the second phase of an oxygen seizure, characterized by the victim's muscles becoming completely slack reliability test - a test to determine the theoretical mean time between failures of a rebreather Repetitive Exposure Method - a method of tracking oxygen exposures to prevent whole body oxygen toxicity, developed by Bill Hamilton, et al. REPEX Method - see Repetitive Exposure Method residence time - see dwell time respiratory linked injection - a type of SCR in which gas replacement is tied to the diver's work effort by using the respiration rate

respiratory minute volume keyed rebreather - a type of SCR in which gas replacement is tied to the diver's work effort by using the respiration rate, also known as a "passive" system reverse block - a squeeze caused by ascent, in which the pressure of a void or air space inside the body exceeds that of the ambient environment RMV - residual minute volume RMV-keyed SCR - a type of SCR in which gas replacement is tied to the diver's work effort by using the respiration rate, also known as a "passive" system RPC - reactive plastic cartridge

S SAC rate - surface air consumption rate sanity breaths - a few breaths taken using an open circuit gas supply at the beginning of an emergency situation scfm - standard cubic feet per minute (cfm at 1 ata and 70°F (21C)) SCR - semi-closed circuit rebreather SCR mode - semi-closed rebreather mode scrubber - a device built to hold absorbent used to remove CO2 in exhaled breathing gas scrubber bed - the carbon dioxide absorbent when being used in a rebreather semi-closed circuit rebreather - a type of scuba equipment that recirculates some of the exhaled gas, and releases part to the ambient environment

semi-closed rebreather mode - a method for extending diluent supply in a mixed gas CCR in emergency situations sensor - see oxygen sensor sequential controller - see step controller valve set point - in mixed gas CCRs, the PO2 trigger value at which the unit automatically adds oxygen to the breathing loop soda lime - in mixed gas CCRs, the PO2 trigger value at which the unit automatically adds oxygen to the breathing loop Sodabsorb® - a soda lime based absorbent manufactured by W.R. Grace and Company sodium carbonate - a benign product formed during the carbon dioxide scrubbing process sodium hydorxide - an alkalai compound commonly used as a carbon dioxide absorbent Sofnolime® - a soda lime absorbent manufactured by Molecular Products, Ltd. solenoid - an electromagnetic valve that opens when a current is applied, typically used to inject oxygen in mixed gas CCRs solid state scrubber - an absorbent manufactured as a rigid solid component, rather than the more common granular absorbent material sonic valve - the part of the mass flow controller that governs the flow of gas, using the principle that the number of gas molecules able to pass through an orifice or hole is limited by flow at the speed of sound SPG - submersible pressure gauge

split counterlung - a counterlung design in which the counterlung is divided into two roughly equal volumes squeeze - damage and pain resulting from pressure imbalances between air spaces or cavities in the body and the ambient environment, caused by a failure to equalize steady state condition - a condition in which gas flow in and out of a breathing loop is in equilibrium, used to determine theoretical performance standards of differing types of rebreathers step controller valve - a needle valve controlled by a stepper motor, typically used to inject oxygen in mixed gas CCRs

T task loading - stress induced by having a large number of tasks or actions to perform or concentrate on simultaneously TDI - Technical Diving International technical diving - any type of recreational diving conducted beyond generally accepted limits, using uncommon equipment, or in overhead environments. Examples include staged decompression diving, diving beyond 130 fsw (40 msw), cave or wreck diving, or diving using a helium based oxygen mixture technical diver - one who engages in technical diving temperature stick - a series of thermistors running the length of an absorbent bed used to measure the heat front during use transducer - a device which measures pressure electronically transducer testing - ensures that the transducer operates as designed, within accuracy parameters

Transportation Security Administration - the government agency in the U.S. that regulates and controls items and substances that may be shipped on commercial carriers Tribolube® - an oxygen compatible lubricant manufactured by Aerospace Lubricants, Inc. trim - the position in which a diver's body while at rest would settle trimix - a breathing has containing helium, nitrogen, and oxygen used to reduce the level of nitrogen narcosis experienced when breathing high PN2 during deep dives TSA - Transportation Security Administration type rating - a qualification on a rebreather certification card that defines the specific rebreather the cardholder is trained to use

U ultrasonic cleaner - a device which uses high frequency vibrations to clean parts in an aqueous environment, commonly used as part of the oxygen cleaning process

V variable volume ratio rebreather - a type of RMV-keyed SCR in which the volume ratio changes, i.e. the size of the exhaust counterlung is reduced as depth increases VD - in RMV-keyed rebreathers, the amount of gas vented from the discharge counterlung VE - in RMV-keyed rebreathers, the amount of gas exhaled ventilatory equipment for oxygen - a constant that represents the fact that the harder one works, the faster one breathes, a value which varies from person to person

vital capacity - the maximal amount of gas a person can exhale after a full inhalation VO2 - metabolic consumption of oxygen (lpm) volume ratio - in a RMV-keyed SCR, the ratio of the exhaust counterlung volume to the total counterlung volume voting protocol - in a mixed gas CCR, a system that first compares multiple oxygen sensor values, ignoring any that are outside of tolerance, before arriving at the PO2 value used by the electronics to control oxygen addition

W water trap - a rebreather component designed to collect and hold water in a place apart from the absorbent, minimizing the chance of caustic cocktail Wescodyne® - trade name for the disinfectant poloxamer-iodine whole body oxygen toxicity - a form of oxygen poisoning resulting from relatively long exposures to low to intermediate partial pressures of oxygen (PO2 > 0.5 atm), also known as pulmonary oxygen toxicity WOB - work of breathing work of breathing - a measure of breathing resistance, the amount of effort required to breathe using a type of diving equipment

ABBREVIATIONS • %CNS ADBOV ADV AGE ANDI ANSI ARDS ata atm Au Ba(OH)2 BC BOV BSAC CaCO3 Ca(OH)2 CC CCR CE cf cfm CFR cm CMAS CNS CO2

a symbol calling for multiplication percent CNS automatic diluent bailout valve Automatic Diluent Valve arterial gas embolism American Nitrox Divers Inc. American National Standards Institute adult respiratory distress syndrome atmospheres absolute atmospheres gold barium hydroxide buoyancy compensator bailout valve British Sub-Aqua Club calcium carbonate calcium hydroxide closed circuit closed circuit rebreather Certification European cubic feet cubic feet per minute U.S. Code of Federal Reglations centimeter Confederation Mondiale dos Activites Subaquatiques central nervous system carbon dioxide convulsions, vision, ears, nausea, twitching, irritable, ConVENTID dizziness

CPDT CPR D DAN Data DCI DCIEM DCS Dmax D0 DSO DSV EAD EANx EC eCCR EN ESA EU EUBS FAA FFM ffw Fg FiO2

FMECA FO2 FsO2 fsw GUE

constant partial pressure dive tables cardio-pulmonary resuscitation depth Diver Alert Network depth in atmospheres absolute decompression illness Defence and Civilian Institute for Environmental Medicine decompression sickness maximum depth of dive depth at which PN2 in a breathing mix is 0.79 atm diving safety officer dive/surface valve equivalent air depth enriched air nitrox European Community electronically controlled closed circuit rebreather European Norm emergency swimming ascent European Union European Underwater Barometric Society Federal Aviation Administration full face mask feet of fresh water fraction of a gas fraction of inspired oxygen Failure Mode Effect and Criticality Analysis fraction of oxygen fraction of oxygen in supply gas feet of sea water Global Underwater Explorers

H2CO3 HDD HDS HP HP H2O HSE HSE HUD

IANTD IP IR ISO K K2CO3 kg KOH kPa L LED lb LiOH LPM maxPO2

mCCR minSP mfw MOD MODO MODN

carbonic acid head down display Historical Diving Society high performance high pressure water Health and Safety Executive Hydrospace Engineering head up display International Association of Nitrox and Technical Divers intermediate pressure or input pressure infrared International Standards Organization ventilatory equivalent for oxygen potassium carbonate kilogram potassium hydroxide kilo pascals liter light emitting diode pound lithium hydroxide liters per minute maximum allowable partial pressure of oxygen manually controlled closed circuit rebreather minimum set point meters of fresh water maximum operating depth maximum operating depth based on oxygen maximum operating depth based on narcosis

MODIP MSDS msw mV N2

Na2CO3 NaOH NASA NAUI NEDU NOAA NSS-CDS O2 OC OCB OL OS OTU P PADI Pb Pcyl Pg

Pi

Pf PCO2 Pcon

PN2 PO

maximum operating depth based on interstage (input) pressure Material Safety Data Sheet meters of sea water millivolt nitrogen sodium carbonate sodium hydroxide National Aeronautical and Space Administration National Association of Underwater Instructors Navy Experimental Diving Unit National Oceanic and Atmospheric Association National Speleological Society- Cave Diving Section oxygen open circuit open circuit bailout mouthpiece oxygen lost from breathing loop oxygen added to breathing loop oxygen tolerance unit atmospheric pressure at altitude (atm or bar) Professional Association of Diving Instructors lead pressure of a cylinder at rated capacity partial presence of a gas starting pressure or input pressure ending pressure partial pressure of carbon dioxide pressure contained in a cylinder partial pressure of nitrogen output pressure

PO2

partial pressure of oxygen

R1 RAB RAID RMV RPC SAC SCR SCUBA SEV SFAIR SP SPG STP T TD TDI TSA TT UBA UK US USN Vcon Vcyl

vent rate of gas volume ratio Rebreather Advisory Board Rebreather Association of International Divers residual minute volume reactive plastic cartridge surface air consumption semi-closed rebreather self-contained underwater breathing apparatus surface equivalent value so far as is reasonably practical set point submersible pressure gauge standard temperature and pressure time of dive dive time Technical Diving International Transportation Security Administration allowable time at depth underwater breathing apparatus United Kingdom United States United States Navy volume contained in a cylinder volume of a cylinder at rated capacity

psi psia psig PV Qs Qv

pounds per square inch pounds per square inch, absolute pounds per square inch, guage pressure volume supply gas flow rate

VD VE

VO2 Vgas WCcyl WOB

gas vented from discharge counterlung amount of gas exhaled metabolic consumption of oxygen volume of gas needed for emergency reserve water capacity of a cylinder work of breathing

A absorbent absorbent bed monitoring absorbent grade accumulator ACG syndrome acoustic jet active SCRs ADBOV adult respiratory distress syndrome ADV AGE air embolism alarms Alexander the Great allergic reactions altitude altitude diving Ambient Pressure Diving American Heart Association American National Standards Institute American Red Cross American Safety & Health Institute Amsorb® ANDI anesthesia machine annual maintenance annular axial flow

annular corrugations AP Diving Arawak ARDS Aristotle Armstrong Medical arterial gas embolism Atlantis I automatic diluents bailout valve automatic diluents valve averaging protocol axial flow

B back-mounted counterlungs bailout bailout valve Balcombe, Graham barium hydroxide battery battery life evaluation BC Beckman Instruments Co. bellows counterlung Betadyne® Bio-Marine Industries Bond, James Borelli, Giovanni Alfonso Boyle's Law

breakthrough breathing loop BSAC bubble noise buddy checks buddy displays bulk loading buoyancy compensator

C calcium carbonate calcium hydroxide calibration calibration bag canister duration canister filling capacity carbon dioxide carbon monoxide carbonic acid cardiopulmonary resuscitation Carleton Technologies caves CCR 1000 CDBA CE central nervous system oxygen toxicity check valves chemical pneumonitis

Christolube® CisLunar Classic KISS closed-circuit rebreathers Clough, Stuart CMAS CNS oxygen toxicity CO2

co-axial flow Cochran color reversion Comex Comité Européen de Normalisation communications equipment computer/algorithm testing Conformite Europeenne constant fraction constant mass flow constant mass ratio constant PO2 Constant PO2 Dive Tables constant ratio constant volume ratio steady state equation Conventid counterlungs Cousteau, Jacques-Yves CPDT cylinder cylinder gas volumes

cylinder types cylinder volumes

D Dalton's Law Davis Escape Apparatus DC55 DCI DCS DD500 decompression decompression illness decompression sickness Decosoft® Defense and Civil Institute of Environmental Medicine (DCIEM) Delta-P Sentinel® Denayrouze, Auguste DESCO digital displays diluent direct gas analysis discharge counterlung disease transmission disinfection dive computers dive log dive planning Dive Rite dive tables

dive/surface valve DiveSorb® Divex Dolphin I Dolphin SCR Dräger DrägerRay DrägerSorb® Drägerwerk drain plugs DSAT durability testing dust content

E EAD eCCRs efficiency electrolung electronics elimination method emergency procedures emergency swimming ascent EN14143 equivalent air depth ergonomics evaluation ESA European Committee for Standardization Evolution

Experimental Diving Unit (NEDU) ExtendAir®

F Failure Mode Effect and Criticality Analysis Fernez, Maurice FFM Fischel, Halbert flasks flooding flow meter Fluess, Henry A. flying after diving fractions Freminet, Sieur frenzy front-mounted counterlungs full-face masks

G Gagnan, Émile gas control gas efficiency gas injection rate gas supply general travel hints

H Halcyon II®

Halcyon RB80® Hales, Stephen Halcyon hardness Hasenmayer, Jochen Hass, Hans HDDs head down displays head up displays helical corrugations high pressure nervous syndrome Hollis Gear hoses HUDs Hydrospace Engineering hydrostatic head hydrostatic tests hypercapnia hyperoxic hyperoxic myopia hypoxia hypoxic

I IANTD ice diving immediate pre-dive checks independent testing Innerspace Services Corp.

Inspiration/Evolution international travel Intersorb® Interspiro interstage pressure intersurgical Isler, Oliver

J J.H. Emerson & Co. Jetsam Technology

K K Kanswisher, John Khotinsky, Achilles Krasberg, Alan

L Lake, Simon Lambersen, Christian J. Lambert, Alexander LAR V LAR VI LAR VII LCDs Le Prieur, Yves LEBA LEDs

lenticular oxygen toxicity light emitting diodes linearity LiquiVision X1® lithium hydroxide longer-term maintenance lubricants

M manual override mass flow controlled SCRs mass flow controller mass flow controller jet Material Safety Data Sheet maximum operating depth maximum PO2 limit of 1.4 atm mCCR MDEA mean time between failures (MTBF) mechanical displays Micropore middle ear oxygen absorption syndrome mine rescue minimal performance standard minimum loop volume Minimum Surface Interval Table mixed gas CCRs mixed-gas closed-circuit rebreathers Mk 15

Mk 16 MOD MODIP MODN MODO Molecular Products, Ltd. Momsen, Charles monthly maintenance mouthpiece MSDS muscle memory mushroom valves

N narcosis 160 NAUI NEDU negative pressure check Neptune LP Neptune NX nitrogen nitrogen narcosis NOAA NOAA Oxygen Partial Pressure & Exposure Time Limits Nohl, Max Gene Normalair-Garrett normoxic condition Northrop

NSS-CDS

O O'Neill, Jerry O2 ear OC buddy Occupational Safety and Health Administration ocular toxicity Off-O2 effect open circuit open circuit bailout mouthpiece orifice OTS OUT over the shoulders counterlung overhead environment overpressure overpressure valve oxygen oxygen analyzers oxygen booster oxygen CCRs oxygen cleaning oxygen closed-circuit rebreathers oxygen compatible grease oxygen exposure oxygen metabolism oxygen Sensor oxygen tolerance units

oxygen toxicity oxygen variation

P PADI Palmer, Rob partial pressure passive SCR PCO2 Pelican® pendulum axial flow percent-CNS method perceptual narrowing piezo-electri valve pilot error Pirelli poloxamer-iodine Porpoise Pack Poseidon positive pressure check post-dive checklist potassium hydroxide povidone-iodine pre-breathing pre-dive checklist pre-dive planning pressure and depth Priestley, Joseph Prism 2

proportional valve pulmonary oxygen toxicity purging

R RAB radial flow RAID Reactive Plastic Cartridges rebreather benefits rebreather standards Rebreathers Australia Reimers Engineering RelyOn® REPetitive Excursion REPEX Method respiratory minute volume respiratory minute volume keyed reverse block ring weights RMV-keyed SCRs RMV-keyed semi-closed rebreathers Ronjat, Alain Rouquayrol, Benoit

S SAC rate safety stop sanity breaths

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