Dive Computers – Insights for Divers & Professionals

Incl. 58 illustrations

© 2014 by Wolfgang Wild. All rights reserved.

Publisher: epubli GmbH, Berlin (Germany)

www.epubli.de

ISBN: 978-3-7375-0616-8

No part of this book may be reproduced in any written, electronic, recording, or photocopying without written permission of the publisher or author. The exception would be in the case of brief quotations embodied in the critical articles or reviews and pages where permission is specifically granted by the publisher or author.

Although every precaution has been taken to verify the accuracy of the information contained herein, the author and publisher assume no responsibility for any errors or omissions. No liability is assumed for damages that may result from the use of information contained within.

Cover Design: propublishing

Cover Pic: innovasub.com, with kind permission

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Introduction

What exactly does “conservative” mean in diving?

Dive Tables – the STUP effect

An example for reliably developed, validated and documented dive tables – the Recreational Dive Planner (RDP)

Reflections on a proper ascent rate

Excursus – How do divers control their ascent rate?

Saturation, halftime and M-value

Halftime

M-value

Doppler ultrasound and other ways to detect bubbles in our tissues

Understanding decompression sickness

”Tissue Model” vs. “Bubble Model”

Tissue Model / 1-phase model

Bubble Model / 2-phases model – Part One

The Buhlmann models ZH-L12 and ZH-L16

Gradient factors

Bubble Model / 2-phases model – Part Two

The attack on bubbles at depth

The VPM model

The RGBM model

Deep Stops – what is that?

Which stop-depth is the correct one?

What to do? STUP is for divers no real alternative!

Dive computers in practice

From table towards computer – multilevel diving

Preliminary evaluations of dive data banks – Wienke & DAN

Dive Computers 1994 – results of own practice tests

Dive Computers 2009 – practice test magazine Diver

Dive Computers 2009 – practice test magazine Scuba Diving

Practice tests in the hyperbaric chamber – Catalina Island (USA) 2011

The crux with technology

Closing chapter and perspectives

Requirements for the validation of dive computers

Dreams of the future – alternatives to the traditional dive computer

Bibliography and Internet Links

1 – Introduction

Dive Computers – Computers?

Here a selection of dive computer recalls published on the homepage of the Consumer Product Safety Commission (CPSC) which is in charge of consumer protection in the United States of America:

• AERIS Epic – decompression hazard

• Atomic Aquatics Cobalt – impact injuries and drowning hazard

• Dacor Darwin Air – serious injuries, including decompression sickness

• Hollis DG03 – drowning hazard

• Mares Nemo Air – drowning hazard

• Oceanic ATOM 2.0 – decompression hazard

• UWATEC Aladin Air X NitrOx – possible decompression sickness

Source: www.cpsc.gov

These few examples of dive computer recalls do not only concern various manufacturers, but also different mathematical calculation models (aka algorithms) and varied technical problems of our popular dive companions: A display which fails to display under water, a leaking O-ring leading to wrong remaining air times under water, a software defect which is known by the manufacturer who is forced seven years later to admit this before a United States District Court (see bibliography, San Francisco Chronicle) etc. etc. The reader will be well aware of such product recalls not being limited to diving equipment, and they are not only nerving, but actually inacceptable for us as consumers when our safety is concerned. Visiting above US government website cpsc.gov or using your favorite search engine and the proper term for “product recall” in your language reveals horrifying results. Aren’t we supposed to have fun when we go diving, and isn’t safety a prerequisite for that?

From the mechanic-pneumatic one-tissue (!) Decometer in 1959 (USA) via the legendary EDGE (USA, 1983) and Deco-Brain (Switzerland, 1983/1985) to present-day models, the dive computer has come a long way – more than half a century.

And the research which divers know as decompression theory is quite a bit older. If we take John Scott Haldane as “father” of all dive tables in 1908 this history extends well over one hundred years.

All diver training associations still regard it important that divers learn in any way why and how our body tissues are being enriched by gases which constitute the gas mixture divers breathe under water. And it is also still regarded essential for divers to get to know and manage the tools that are designed to avoid problems from this absorption of gases which always takes place in us when we use a “self-contained under water breathing apparatus” (aka “scuba”) to seek adventure and recreation in our fascinating under water realm.

Some diver training associations continue to proceed via the dive table, others have abandoned the tables completely, relying now fully on the dive computer and respective simulation programs for teaching divers the dive planning basics.

In this eBook the reader will not be pestered with lengthy mathematical equations of numerous dive computer algorithms – these can be studied in a wealth of internet articles and specialized literature that partially meets scientific requirements.

The intention of this eBook is rather to offer interested divers and diving professionals with pictures, illustrations, tables and charts deeper insights into the background and the potential of dive computers.

This will include the understanding of dive tables and some underlying specifics of the decompression models of modern dive computers.

The reader of this eBook can expect that the notorious STUP effect will be addressed – and that topics like ascent rate, ascent techniques and stop-depth are discussed is indispensable.

How does the future of dive computers look like? More gadgetry at the wrist of the diver with miniature display sizes overloaded by supposedly important information? A different path is taken by the SDC (smartphone dive computer), which leverages the enormous processing power of modern smartphones and their reasonable display sizes. This eBook also takes a look at such developments.

Finally a brief remark on the numerous sources referenced in this eBook: These are not only listed in the detailed bibliography but usually also directly in the text so that the reader is spared from frequently having to jump forward into the appendix and back into the text. And as many sources are publicly available in the internet the bibliography is providing corresponding links.

The valued reader will easily realize that the topics discussed in this eBook are addressed to all divers and diving professionals of all diver training associations.

Enjoy browsing through this eBook on your iphone, kindle, kobo, sony reader, tolino, tablet …

2 – What exactly does “conservative” mean in diving?

From the years in which divers had no alternative than using dive tables for their dive planning (because dive computers simply did not yet exist), older divers will remember the grandiose statement:

Our xyz dive table is more conservative than others.

Also dive computers and simulation programs for PC and/or tablet put forward that claim. What is suggested with such a claim or promised to divers?

Dive Tables – the STUP effect

A former both popular and trivial comparison of dive tables looked like this:

Take several dive tables, note their no-decompression limits in a graph – and without further ado the table with the shortest no-decompression times for certain depths was entitled to be the “safest” table, because this one obviously was the “most conservative”.

The following tables for air just serve as examples; the first three of them are dive tables which are or used to be popular in German speaking Europe.

Notes: “VDST” means “Verband Deutscher Sporttaucher”, which is the German CMAS federation branch; University Zurich [correctly spelled Zuerich with u-umlaut] refers to the hyperbaric chamber in the capital of Switzerland that Albert Buhlmann [correctly spelled Buehlmann with u-umlaut] used for developing his famous ZH-L algorithm which is discussed later in this eBook.

[Note for eBook readers with a device which cannot display colors: The upper broken line depicts the Deko '92 table, the lower dotted line shows the US Navy tables.]

The graph shows us the dive table with the most conservative values – correct?

Not really. There is another dive table with significantly shorter no-decompression times and no-decompression limits:

[Note for eBook readers with a device which cannot display colors: The upper unbroken line (green) shows the STUP effect.]

The reader will ask himself with good reason: From which dive table does this green line originate? Well, honestly spoken, from no dive table available on the market. But this table can be “produced” easily and quickly: Just take a pen, place above the former most “conservative” line some points and connect these. And – lo and behold – you have generated a safer because more conservative “dive table”. (Using EXCEL this is a matter of seconds ...)

Question: Is there perhaps even an enhancement of this awfully economic developing procedure?

Answer: Yes. The STUP principle – Stay Up. Because for divers it is of course safest not to go diving, staying at the surface or even better at home instead; this has the advantage of sparing all the grappling with dive equipment and avoiding the uncertainty of potentially being hit by decompression sickness (DCS).

Note: Actually even at home you can not be sure not to get hit by DCS. Raymond Rogers has pointed to that chance when criticizing a popular mathematical equation used by many decompression scientists that time; Rogers concluded that if this equation was valid an unlimited no-decompression limit could only be found to be “a fraction of a foot deep”:

“This would imply that if you lay in your half-filled bathtub long enough, you’d get decompression sickness when you stood up!”

Source: Rogers, Raymond, Renovating Haldane, The Undersea Journal, Third Quarter 1988, p. 17

The reader and enthusiastic diver will understand how this hint by Rogers is meant (animated, winking smileys unfortunately don’t show well in eBooks …). And smiling while reading this eBook is explicitly allowed.

So, this approach is of course not the way to seriously discuss the topic “conservative” no-decompression times.

If we really want to be able to test the “performance” of a dive table (or of a dive computer) and compare it with other dive tables (or dive computers) then we’d need to know from the developer (or more modern: from the designer) of the dive table resp. dive computer in which way the concept or design had been created. Which parameters (besides depth resp. pressure, and time) found their way into the design – and, above all: How has been validated whether a diver using it will come back home from his dives safely and healthily – i.e. without being hit by DCS (very likely not being hit, at least).

Unfortunately there is an inglorious “tradition” among the designers which perpetuated from dive tables to dive computers: Only with few exceptions the diver receives answers on such questioning – usually he doesn’t hear anything (or nothing specific).

What is the common language regime in advertising, promotion, and instruction manuals? ”Modified Haldanean model”, “US Navy values, adapted for recreational divers”, “Buhlmann ZH-L16 with gradient factors”, “based on RGBM”, etc.

It can be that a diver who respects the no-decompression limits of a certain dive table or dive computer will surface without any problems, and will continue diving after some surface interval, safely and healthy. But it can also be that this diver could have been even longer under water for some time.

Briefly speaking: Perhaps the diver has shed wonderful time under water with this dive table or dive computer, because he surfaced too early and therefore did not see the Whale Shark, Nautilus or any other creatures of our dreams. And in this case he’d also have shed valuable breathing gas because usually tank fills cost money.

Note:

Let us finally check how the dive tables which we have used above differ when strained for a repetitive dive:

To properly do that (divers who were trained to handle dive tables know this) we need the so-called pressure group (PG) which represents the residual nitrogen from the previous dive, and we need of course the duration of the pause at the surface, the so-called surface interval (SI). For our example let’s assume that we want to return after a surface interval of one hour again to the same depth of the first dive, which was 18 meters/60 feet (in the hope to revisit the curious-playful Octopus there). How long could we stay there for another no-stop dive? For dive number one let’s assume a dive time of 50 minutes:

• Dive #1 – depth: 18 m/60 ft, time: 50 minutes

• SI – 60 minutes

• Dive #2 – depth: 18 m/60 ft, time: maximum no-stop time

Above dive tables would allow the following no-stop limits for the second dive to 18 m/60 ft:

• 6 minutes – Deko 2000 and US Navy (2007)

• 10 minutes – Deko '92

• 24 minutes – NAUI (1995)

• 34 minutes – Hyperbaric Chamber University Zurich (1986) and RDP/DSAT (1988)

What does that mean? Do these results mean that the tables allowing only 6 minutes are several 100% “safer” than the others, and the extensively validated DSAT table with its 34 minutes should be regarded as “unsafe”, then? Think of the following: Grammatically a comparison of the adjective “safe” is possible (safe – safer – safest), but content-wise this does not make any sense. Either safe – or not. And as we have learned before: only STUP is safe, somewhat at least. So the answer to above question is as before: Can be, but not necessarily. And, being honest: Who of us would really assemble all our equipment for just another 6 minutes under water? Alternatively, the flexible diver could have switched to a different table from the very beginning …

Bottom line: A pure comparison of dive table no-stop times is of no substantial value for the diver – and this is equally true for dive computers. Divers need to know much more about no-stop times, in particularly where they originate from and how they were tested (i.e. validated).

An example for reliably developed, validated and documented dive tables – the Recreational Dive Planner (RDP)

From a diver’s perspective there is little to complain about the development and validation of the Recreational Dive Planners (RDP) by DSAT (Diving Science and Technology): The mathematical equation which is used to calculate absorption and desaturation has been worked out in a time consuming procedure (for details please see the documentation referenced below); then extensive “dry dive” testing followed in the hyperbaric chamber under medical supervision and guidance; and if such dry dives produced bubbles that could be heard and documented by the use of Doppler ultrasound technology, exceeding a beforehand defined grade, the coefficients of the mathematical equation were adapted.

Only then came the next, ultimate step in the validation process of the model: 228 real “wet dives” in the Puget Sound close to the Canadian border in the west of the USA, next to Seattle (yes, it’s a bit fresh there under water, around 12°C/54°F in summer, i.e. the test dives are to be regarded rather strenuous than just relaxing).

The test subjects were young and old divers, male/female, with much/little dive experience, with quite a bit bioprene / athletic / slim – a representative average, so to say. The dives were conducted over several days, incl. multilevel dives and repetitive multilevel dives.

After each dive the test subjects were checked by the means of Doppler ultrasound if any bubbles could be heard, and only after no problems could be stated any more the mathematical model (i.e. the algorithm) was released by DSAT.

Result of the lengthy validation process:

“No cases of decompression sickness were encountered in any test, either in the hyperbaric chamber or in open water. … Doppler detectable bubbles were found in 4% of the profiles. In subjects who did deep knee bends during compression, bubbles were found in 12% of the profiles. The Doppler Grade in the subjects while resting was almost exclusively Grade 1. The after-exercise grades were mostly 1 and 2, with one subject having a Grade 3. Some of these grades could have been caused by objects in the bloodstream other than bubbles, but as mentioned previously, when in question, it was resolved in favor of gas bubbles and a higher grade.”

Source: Richardson, Drew, The Recreational Dive Planner: History and Development, The Undersea Journal, First Quarter 1988, p. 8

Here the “Doppler Bubble Grade” scale from 0 to 4 according to Dr. Merrill Spencer, 1974 (Source: ibid.)

The total number of test dives (both hyperbaric chamber and open water) which were conducted and evaluated was 911. The validation was done externally, that means: not by DSAT itself, but rather under the direction of Dr. Michael Powell at the Institute of Applied Physiology and Medicine (IAPM) in Seattle, USA.

Today the complete final documentation is available for free download (link see bibliography):

Diving Science and Technology (DSAT), Development of no-stop decompression procedures for recreational diving: The DSAT Recreational Dive Planner (1994)

A preliminary version which had been mailed for information and review to over 800 experts in hyperbaric medicine, PADI instructor development professionals and various institutions appeared already end of 1997 under the title:

Diving Science and Technology (DSAT), Recreational Dive Planning … The Next Generation – New Frontiers in Hyperbaric Research; Santa Ana, California (USA) 1987

The following snapshot from the 1994 DSAT documentation (p. 27) shows a diver in the hyperbaric chamber trying by means of a rowing machine to drive as many potential bubbles out of any tissue niche where the might have been hiding. Only after these efforts and additional knee bends Doppler was used.

Real, wet test dives were only conducted after successful dry dives in the hyperbaric chamber.

Without going too deep into that (thanks to the free download possibility everything can be studied in detail today) only a few more aspects in bullet point form which might be of most interest for the reader of this eBook:

• As the development of the mathematical equation had been financially sponsored by PADI (Professional Association of Diving Instructors), they had secured the exclusive rights for PADI to develop and distribute a new dive planner for recreational divers based on the DSAT findings; so, in 1988 the Recreational Dive Planner (RDP) was launched. With the RDP divers had for the first time a dive table which does not penalize the recreational diver with too short no-decompression times and limits. Just to remember: the until then traditionally used US Navy tables had never been thought for recreational diving (and for female divers not at all). Some other dive tables reacted on the RDP by shortening their (somehow created) no-decompression limits to increase their safety margin.

• Later, DAN and PADI subjected the RDP to further testing: 4 (four) dives daily during 6 (six) consecutive days; these dives were conducted in the controlled environment of a hyperbaric chamber, i.e. dry dives; after each dive the 20 divers were evaluated for bubbles by the use of Doppler ultrasound. Result after 475 dives: „No decompression sickness and minimal detectable bubbles.“ (Richardson, Drew, How Much Diving Is Too Much?, The Undersea Journal, Second Quarter 1990, p. 14)

• Because DSAT had released the algorithm for free use (except for the production of dive tables), several smart diving equipment manufacturers seized the chance, ordered together the production of a chip resp. microprocessor and had it installed in their newly developed dive computers; this was truly smart because they saved a lot of development costs. As far as the author of this eBook is informed, in the USA these manufacturers were Dacor, Oceanic, Sherwood, and US Divers (even though not all dive computer models of these companies were equipped with the DSAT algorithm). In this way they had a dive computer which was calculating both saturation and desaturation with an extensively validated mathematical equation.

• In the RDP 14 theoretical tissues or “compartments” are used: 5 – 10 – 20 – 30 – 40 – 60 – 80 – 100 – 120 – 160 – 200 – 240 – 360 and 480 minutes (the term “compartment” will be discussed later). As “controlling tissue” for the RDP finally the 60-minutes compartment was selected. A few points to the concept of a controlling tissue which are probably known by diving professionals and may therefore be skipped by them. All dive tables and dive computers which are based on a so-called “tissue-model” (see further below) use a specific controlling tissue which is defined as follows: this is the theoretical tissue, which came closest to its maximum but still safe gas-loading during a dive. The criterion for this closest margin meant for the development of the Recreational Dive Planner (RDP), as already shortly mentioned <no Doppler Grade as defined by Dr. Merrill Spencer above Grade 3> or, in other words, no Doppler audible bubbles which could have lead to DCS symptoms. Result for utilizing this concept in the development and validation of the RDP: “No cases of decompression sickness occurred in any test.” (DSAT, Recreational Dive Planning … The Next Generation – New Frontiers in Hyperbaric Research, 1987, Executive Summary, p. 4)

• As said, the controlling tissue of the RDP is the 60-minute compartment, while for the original US Navy tables it’s the 120-minute compartment. For military divers with their given dive objectives and profiles a 120-minute compartment may be quite adequate, for recreational diving it is unnecessary conservative. The calculated (still safe) saturation level of this controlling tissue is further used for planning the maximum safe dive time for a repetitive dive which divers know to have shorter no-decompression times than the previous dive. Just so much at this point on that topic, later in this eBook a bit more. One additional hint regarding the number of compartments which are used in the algorithm of the RDP. For planning dives on sea level and to a maximum altitude of 300 meters/1.000 feet the RDP is based on above presented 14 compartments; for altitudes above 300 meters/1.000 feet as many as 20 compartments were used to calculate the RDP altitude conversion. (See Richardson, Drew, Deep, Repetitive Diving – A New Rule Applies, The Undersea Journal, Third Quarter 1989, p. 26)

• Following the RDP “Wheel” and RDP table launch in 1988, further dive tables were introduced for using the RDP not only with air but also with Enriched Air / Nitrox: the EANx32 and EANx36 RDPs, released in 1996; and years later also “electronic” dive tables followed: the eRDP (2005) und the eRDPML (2008). The eRDPML uses the validated data of the RDP in its original “Wheel“ version, so that even multilevel dives for up to three levels can be calculated as no-stop dives. [Note: This dive planner is no dive computer and can not be taken under water.]

Reflections on a proper ascent rate

The Recreational Dive Planner (RDP) has been tested and validated for a maximum ascent rate of 18 meters/60 feet per minute; for diving at higher altitudes, which means for the RDP beyond 300 meters/1.000 feet above sea level, the ascent rate is limited to 9 meters/30 feet per minute. Other dive tables and present-day dive computers usually stipulate 10 meters/33 feet per minute or even slower; other dive computers also use varying ascent rates for different depth ranges.

Note: The US Navy dive tables also prescribed a maximum ascent rate of 60 ft/min until 1993; since then 30 ft/min is the official US Navy limit – „with no change made in any of the table entries“.

Source: NEDU - US Navy Experimental Diving Unit, Graphical Analysis: Decompression Tables and Dive-Outcome Data; Panama City, Florida (USA) 2004, p. 2; download link see bibliography

What is known about the background of the ascent rate?

During a workshop of the well-reputed American Academy of Underwater Sciences (AAUS) in 1989 this question was investigated. The surprising discovery was that the traditional 60 ft/minute limit obviously does not originate from diving, but rather from a regulation for exiting submarines under water, and this regulation was not one issued by the US Navy, but rather from the British Royal Navy. In a presentation during this AAUS workshop Dr. Edward Lanphier, a member of the US Navy Experimental Diving Unit (NEDU) since 1951, commented this surprising finding as follows: “The concern seemed to be less with the rate of ascent itself than with the chance that the diver would miss his first decompression stop if he were coming up too fast.” So, of primary concern was not the ascent rate but to ensure that the diver leaving a submarine under water would be able to stop his ascent at the prescribed depth.

If divers reading this should be asking: So, the traditional maximum ascent rate has no documented physiological foundation? – here another quotation from the same workshop: “Bill Hamilton noted that from the way Ed Lanphier described it, the 60 fpm ascent rate was for operational reasons, rather than for optimal decompression. Ed Lanphier: Yes, surely.”

Source: Lanphier, Edward, A Historical Look at Ascent; in: Lang, MA & Egstrom, GH, Biomechanics of Safe Ascents Workshop, AAUS 1989, pp. 6 and 9; download link see bibliography

Let us reflect a moment on this interesting hint.

For practical reasons a stop before the final ascent to the surface serves two main purposes:

• To check for neutral buoyancy and adjust as necessary.

• To give the body time to get rid of excessive gas absorbed in the tissues during the dive – i.e. off-gassing or gas-washout, as it is called.

Diving instructors among the readers who have ever asked their students in the classroom to walk the distance of 60 feet/18 meters in one minute know how slow that is; so strolling instead of walking might be the more appropriate expression. Also well known is that under water air in the BCD (and the dry suit) expands during ascent. This means that on the way up to the surface, especially during the last 33 feet/10 meters where the water pressure decreases by 100% and the air volume in the BCD would increase proportionally (if the diver would not deflate appropriately) – during these final feet it becomes more and more difficult for the diver to control his ascent rate. This is especially difficult for the unexperienced diver because he not only has to release air from his BCD and/or dry suit, but he also should watch up to the surface to make sure nothing blocks his way home and to ensure free airways, and at the same time he should keep an eye on his depth gauge and watch or his dive computer to control his ascent rate. This is far from easy for novice divers. And we were just talking about 60 feet/18 meters per minute, and not about 30 ft/9 m per minute, or even slower …

In so far nothing has to be changed in the statement, which we could read in May 1989 in the US magazine Skin Diver as the headline of an editorial by publisher Bill Gleason:

60 FEET A MINUTE IS A LONG, LONG TIME

Following a suggestion in this Skin Diver editorial the author of this eBook conducted a small experiment with staff and candidates during an instructor training course: Along the ascent line which led from an anchored platform in 16 meters/52 feet to the surface, clearly visible depth markers were attached; clearly visible, however, only for the staff – the participating divers were asked to look into the beautiful clear waters of this Austrian lake and watch endemic fish swim by. Their only assignment was to ascend continuously and as slow as possible.

We can make it short: most of the divers were pretty much convinced that their ascent rate never exceeded the recommended maximum of 18 meters/60 feet per minute – which had meant a total of 52 seconds from a depth of 16m/52 ft [here the quick and easy calculation in feet: 1 minute = 60 seconds for 60 ft; this equals one second for one foot, or 52 seconds from the depth of 52 feet].

Well, for good reasons several staff was positioned at various depths next to the depth markers on the ascent line, and their assignment was to note the time which the divers needed from 52 to 40 ft, from 40 to 30 ft, from 30 to 16 ft, and from 16 ft to the surface.

The reader will already smell the outcome: A maximum of 60 feet per minute? Not really. For the last 16 feet to the surface several voluntary participants only needed 6 seconds, which equals to possibly record breaking, hardly to believe 160 feet per minute. Not only the time taking staff was frowning, imagine the faces of the participating divers. After all, we hadn’t invited dive beginners to this self-experiment, but ambitioned, future instructors.