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A Primer for Technical Diving Decompression Theory

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SCUBA AA PPRRIIMMEERR FFOORR TECH TTEECCHHNNIICCAALL DDIIVVIINNGG DDEECCOOMMPPRREESSSSIIOONN PHILIPPINES TTHHEEOORRYY 1 | P a g e ©Andy Davis 2015 www.scubatechphilippines.com Sidemount, Technical & Wreck Guide | Andy Davis First Published 2016 All documents compiled in this publication are open-source and freely available on the internet. Copyright Is applicable to the named authors stated within the document. Cover and logo images are copyright to ScubaTechPhilippines/Andy Davis. Not for resale. This publication is not intended to be used as a substitute for appropriate dive training. Diving is a dangerous sport and proper training should only be conducted under the safe supervision of an appropriate, active, diving instructor until you are fully qualified, and then, only in conditions and circumstances which are as good or better than the conditions in which you were trained. Technical scuba diving should be taught by a specialized instructor with training credentials and experience at that level of diving. Careful risk assessment, continuing education and skill practice may reduce your likelihood of an accident, but are in no means a guarantee of complete safety. This publication assumes a basic understanding of diving skills and knowledge. It should be used to complement the undertaking of prerequisite training on the route to enrolling upon technical diving training. 2 | P a g e ©Andy Davis 2015 www.scubatechphilippines.com This primer on decompression theory is designed as a supplement to your technical diving training. Becoming familiar with the concepts and terms outlined in this document will enable you to get the most out of your theory training with me; and subsequently enjoy safer, more refined dive planning and management in your technical diving activities. I have collated several highly-regarded articles which explain key concepts behind the design and use of the Bühlmann ZH- L16 B/C GF algorithm. This algorithm is generally the preferred decompression model in technical diving computers and planning software. If you need any help whilst self-studying this topic, please don’t hesitate to contact me: [email protected] 3 | P a g e ©Andy Davis 2015 www.scubatechphilippines.com Contents Gradient Factors - A Simplified Primer ................................................................................................................ 5 Flexible Control of Decompression ...................................................................................................................... 11 Gradient Factors .................................................................................................................................................... 15 History .............................................................................................................................................................. 15 Decompression Basics ..................................................................................................................................... 16 Tissue Saturation and Ascent Ceiling ............................................................................................................. 17 M-Values .......................................................................................................................................................... 19 Gradient Factors .............................................................................................................................................. 21 Practical Applications and Safe Diving Habits ............................................................................................... 24 Understanding M-Values ...................................................................................................................................... 28 What are M-Values? ....................................................................................................................................... 28 Historical Background ..................................................................................................................................... 29 The Workman M-Values ................................................................................................................................. 31 The Bühlmann M-Values................................................................................................................................. 32 DCAP And DSAT M-Values .............................................................................................................................. 34 Comparison of M-Values ................................................................................................................................ 34 Consistency of M-Values................................................................................................................................. 37 Format for M-Values ....................................................................................................................................... 37 M-Value Characteristics .................................................................................................................................. 37 The Ambient Pressure Line ............................................................................................................................. 38 The Decompression Zone ............................................................................................................................... 39 Multiple Inert Gases ........................................................................................................................................ 39 What Do M-Values Represent? ...................................................................................................................... 40 M-Values and Conservatism ........................................................................................................................... 42 M-Value Relationships .................................................................................................................................... 43 Analysis of Profiles .......................................................................................................................................... 44 Universal Reference Values ............................................................................................................................ 44 Margin of Safety .............................................................................................................................................. 46 4 | P a g e ©Andy Davis 2015 www.scubatechphilippines.com Gradient Factors - A Simplified Primer By Steve Lewis This short article is based on a series of presentations made in various locations during the late winter and spring of 2015, and is a shortened version of a more detailed treatment to be included in an upcoming book. Let’s start off with a disclaimer and an outline of some assumptions made while working on this blog post. First off, I did not start out intending to write a definitive piece on decompression theory or on the stellar work of Professor Albert Bühlmann. Also, this contains no detailed explanation of the internal workings of the maths behind a decompression algorithm and the challenges it meets while trying to model human physiology. Simply put, this was written to help the average punter better understand what gradient factors describe; and the potential impact of playing around with the GF settings on your personal dive computer (for example, a Shearwater Petrel). Secondly, I’ve made several assumptions… not the least of which is that readers have a basic understanding what happens to an individual after spending more than a couple of minutes sub-surface breathing compressed gas. (That’s just another way of saying that I’m writing for certified and reasonably experienced divers who understand that diving can result in decompression stress of one flavor or another.) OK, that settled, here we go. All dives are decompression dives, ergo all divers are decompression divers. Our acknowledgement of this and the depth of that acknowledgement can greatly influence our behavior when we dive: most specifically, our ascent behavior. How deep we dive, what gases we breathe, and how long we spend at depth vary considerably from dive to dive; but all dives share one common threat for individual divers: the risk of getting bent. 5 | P a g e ©Andy Davis 2015 www.scubatechphilippines.com Smart divers consider many factors when planning dives, many of which help to alleviate that risk. One is choosing to dive a “conservative profile.” I guess there are many interpretations of what that means… but for our purposes here, it means choosing a decompression algorithm that’s proven, and choosing a setting for that algorithm that will generate stress-tolerant dive tables. Many PDC (personal dive computers) use a Bühlmann ZHL-8 or ZHL-16 algorithm to model decompression stress in divers and their decompression obligation on ascent. In a way this is odd because both deco models are not only dated (Herr Doktor Professor Bühlmann died of heart failure in early 1994), but are based on the faulty premise that we can prevent bubbles from forming in our bodies during our ascent if we follow the schedules the algorithm kicks out. The data set on all recreational diving, but in particular staged decompression diving, has grown considerably since the Professor’s unfortunate and untimely death, and
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