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Tables
Ever since the invention of a reliable means to supply compressed gas to divers, diving operations have been limited by the availability of appropriate decompression tables and methods. Now, with increased interest in alternative breathing mixes, advanced decompression methods, and exposures outside the range of standard air tables, special application tables are becoming an important tool in the high tech diver's arsenal and are growing in use.
Conceptually, special tables are no different than their public domain counterparts. Each represents a decompression procedure designed to bring a diver back to the surface without incident after carrying out a planned exposure. Where public domain tables span an entire range of dives, for example 0-190 fsw with bottom times ranging from 5 to several hundred minutes depending on the depth, and typically involve a single gas mix, usually air, special tables are typically calculated for a specific operation involving a narrow range of exposures, multiple gas mixes, decompression procedures and custom J-factors ("Jesus" factors added for conservatism).
Another important difference.Though most public tables have been tested on a significant number of dives, this is not always the case with special tables for which limited diving data may exist, a reason to proceed with caution. Anyone with a computer and computational algorithm can generate a table. The real issue is what the numbers mean. As Dr. Bill Hamilton points out in the accompanying article, special tables are only as good as the experience and judgement behind them. For that reason decompression engineering is still as much of an art as it is a science.
Currently special tables are being used by a small minority of technical divers and there are only few suppliers.For good reason. Planning and conducting dives involving special tables requires a degree of skill, sophistication and understanding that does not yet exist in the community as a whole.Training, experience, and a supporting infrastructure are needed, and the potential for injury is no small matter. For that reason, many professionals in the field remain leary of producing tables for just anyone, particularly in light of the current legal climate in the industry.
As special mix diving becomes more accessible, the need for special tables will continue to grow, and with it, the number of reliable suppliers. Until that time it's wise to proceed with caution, and work with qualified, experienced individuals who know what they're doing. As Dr. Hamilton explains, there is a lot more involved than simply cutting a table. Michael Menduno
Understanding Special Tables: Some things you should know
by Dr. R.W. Bill Hamilton
Many of you have asked about the preparation ofspecial application tables. This is a somewhat sensitive subject that raises many issues , and I think it worthwhile to discuss. One issue is whether professional diving physiologists and decompression consultants should support the extreme exposure diving that some people want to do. Another is whether a calculated table is reliable enough to keep the user out of trouble.
When I first began working with the technical diving community, I was approached by several divers who told me what they were planning to do, and it appeared they would make up the tables themselves if I could not provide them. Thus blackmailed, and having the humility at least to realize that some of the more experienced high tech divers could easily construct a table as well as I could, I went ahead with it. With some trepidation, I might add. Fortunately, it has worked out well, and because of the excellent feedback and sufficient repetition I now am relatively comfortable with the process. Note, I do not view this business as "selling tables", rather it is a professional service provided to clients that includes the generation of tables. As you will see this involves much more.
Generating a special or custom table typically begins by finding out something about the client. The individual must be totally comfortable with gas and pressure physics and physiology, oxygen safety with regards to both the gas and understanding its physiological effects, gas logistics and decompression methods, as well as the technical and operational aspects of diving they're doing.
The important thing to understand is that a reliable decompression table is not based on a formula alone. With today's state of knowledge, no matter what computational algorithm is used to produce a decompression table, it's basis is still empirical; tables are based on field experience (data! -ed.)&endash;"what works, works." Although we have sophisticated computational methods&endash;and they are getting better&endash;there is a fair degree of judgement involved in incorporating the experience of yesterday's dive into tomorrow's table. For that reason, table development is still something of an art that extends beyond the ability to solve exponential gas loading equations.
One reason for this is the statistical nature of the process; it takes many, many dives to know anything at all about incidence rates inherent in a set of tables. For example, if an individual does five dives with no problems, this only means that at a 95% confidence level, the tables have a bends incidence of less than 50%. It takes literally hundreds of clean dives to be sure the incidence is less than 1%. Even a couple dozen clean dives should not be greeted with too much confidence. This is especially true if many of these are not conducted for the full time and depth.
To help mitigate this situation, the relatively new technique of applying maximum likelihood analysis to decompression is making it possible to estimate the risk involved with a given table, based on a set of previous dives. In addition, guidelines have been developed for the development and validation of new and revised tables, and the ongoing management of existing ones, seeValidation of Decompression Tables, Report 74(VAL) 1-1-88, the Undersea and Hyperbaric Medical Society, Bethesda, MD. Under these guidelines,tables based on documented experience can be introduced at the " operational evaluation" stage, if done with sufficient care.
The basic table computational methods used by most decompression consultants are still primarily Haldanian which uses compartments (commonly but inappropriately called " tissues"), exponential gas loading equations, and ascent constraints called M-values. Promising variations include bubble growth or bubble generation factors as well, but these too are usually fed by gas loadings. The Haldane method has it's defects, but is workable as long as one takes its weaknesses into account and stays sufficiently close to past experience i.e.what worked in the past. Personally, I prefer the "Haldane-Workman-Schreiner" method, which is a considerable advancement over Haldane.
Once a formula is decided upon, it is necessary to consider other factors involved in decompression management such as the range and variation of gas mixtures and particularly oxygen management; oxygen is the key to reliable decompression. The table should take into account the oxygen tolerance limits of the diver, both central nervous system (CNS) and "whole body". The rates of travel, gas mix changes, etc. should be included in the table as instructions where they are needed, as well as the accumulated decompression time, or " decom time ", as well as the "stop time" at each stop (For a discussion of decompression time management, see Decompression Strategies, M. Menduno, technical Diver 2.2 , Summer 1990).
For technical dives, gas switches are usually determined by the specific dive plan. These are of course worked out inconjunction with the dive team.
Once a basic operational approach is established for a dive, it is usually a relatively simple matter to make adjustments for the many factors involved such as extensions of depth and time, J-factors etc. However to consider all of these, check the process thoroughly, and be sure the user has the right instructions is not a trivial matter. For that reason, I feel it's important to label tables and the corresponding computations with a unique traceable basecase name, and I encourage others to do the same rather than just calling a table, the "DCAP 250/30" or the"Wakulla tables . " That way the origin of any table or computation can be traced and reviewed, an otherwise time consuming task that can result in confusion.
Another important thing that I feel is necessary is to provide the user with instructions on how to use the tables. Although you obviously cannot tell the user everything he or she needs to know in order to perform a sophisticated dive operation requiring special tables, it is important that they include at least a description of each item on the table and how to use them. Finally, although it's not something that can always be enforced, I feel it is important that clients agree to provide feedback as to the results of using a set of tables. The data we get from such feedback is valuable and allows us to continue to improve the methodology.
Generating special tables for a range where the parameters are familiar and there is sufficient available experience typically represents about a days work and can run anywhere from $200-500 and up depending on the supplier. More exotic tables, requiring the acquisition and analysis of specific data, takes a good deal more time and thus can cost a great deal more (often a couple of orders of magnitude more for the development of a full set of commercial or professional tables). Note the time it takes may bear very little relation to the value of the table, but it does have an impact on the cost.
No table or decompression method is completely "safe." In fact I do not even use that word inconjunction with decompression. It's wrong on several counts. First, no significant dive is free of the risk of decompression sickness. Second,DCS is not an accident ; it happens and will continue to happen as a predictable part of diving. The dive team should plan for it, and in doing so can reduce the consequences to as near zero as possible.When DCS does occur, it can and should be treated promptly and adequately, and if this is done the chance of residual injury is quite small.
The real accidents that take divers lives are due to operational problems like running out of gas, getting lost or entanglement, not pushing a dive computer or running over a table. In my opinion this is where some current technical diving practices seem to be operationally inadequate.
Technical dives are operations. As such they should have a leader and or safety manager, planning, cooperation, and the predetermined ability to cope when any step falls short. This means building in redundancy throughout; in equipment, extra gas for contigencies, extra oxygen on board, and ensuring that other team members, especially the boat captain, know what's going on and what to do if things don't go as planned. Having understanding and agreements before the fact, can go a long way to allay the consequences of a glitch in the operation.
Technical diving can also be risky. The divers assume this risk, fully and personally, but this does not justify taking unneccesary risks. Tricks like diving on air to 290 feet are selfish and irresponsible. The diver who does not get away with this may be the only one to die, but will not be the only one to suffer. Accidents reflect on all divers, and can set progress back in a variety of ways. If you want to keep pushing the envelope, fine, but do it intelligently with care and responsibility.
The decision to use special tables depends on the specific operational plan for the dive, whether special breathing mixtures or accelerated decompression methods are to be used, and whether or not a general purpose table is applicable. Whatever your decision, make sure that the tables you're planning to use were generated by a reputable supplier and are in fact, appropriate for the dives you're planning to do. It goes without saying that you should have the necessary experience, training, and equipment required for the operation in question. No matter how reliable, no table can make up for lack of judgement or the inability of the diver to carry it out.
Dr. R.W. Bill Hamilton is a diving physiologist and principal of Hamilton Research Ltd. with over 20 years of decompression management experience in the hyperbaric and aerospace industries. He can be contacted at 80 Grove St., Tarrytown, NY 10591
Callouts: With today's state of knowledge, no matter what computational algorithm is used to produce a decompression table, its basis is still empirical. "What works, works. "
DCS is not an "accident"; it happens and will continue to happen as a predictable part of diving. The dive team should plan for it, and in doing so can reduce the consequences to as near zero as possible.
Tricks like diving with air to 290 feet are selfish and irresponsible. The diver who does not get away with this may be the only one to die, but will not be the only one to suffer. Accidents reflect on all divers, and can set progress back in a variety of ways.
Reccomended Reading:
1. Hamilton R.W. 1989 Dec., Tolerating Exposure To High Oxygen Levels: Repex and Other Methods. Marine Tech Soc J 23(4): 19-25.
2. Hamilton R.W. 1990. Processing Decompression Tables. In: Knudsen G, Lindrup AG. Vitenskapelig FUDT seminar innen dykkemdisin/ fysiologi. Bergen: Norwegian Underwater Technology Center.
3. Hamilton RW, Kenyon DJ 1990. DCAP Plus: New Concepts In Decompression Table Research. In: MTS: Science and Technology For A New Ocean's Decade. Volume 3. Washington: Marine Technology Society.
4. Schreiner HR, Hamilton RW, Validation of Decompression Tables, Report 74(VAL) 1-1-88, the Undersea and Hyperbaric Medical Society, Bethesda, MD.
Box:
The Role Of DCs
Today's dive computer (DC) technology is not well suited to deeper mixed gas diving applications. Though air-based DCs have replaced tables for most shallow water operations, and will eventually be adapted for EAN use, special mix diving generally involves too many variables to be incorporated into an non-interactive program. What's more the square wave profiles inherent in tables more closely approximates the typical deep diving profile and adds a degree of conservatism, which is needed. As a result, dive computers are not likely to replace the need for special tables anytime soon.
BOX:
"Doubtless no law of chemistry is broken by the action of nervous cells, and no law of physics by the pulse of nervous fibers, but something requires to be added to our sciences in order that we may explain these subtle phenomena."
William Jevons (1873)
Sample Trimix Table
KWD USS Wilkes Barre Table Depth: 250 FSW
RWH/ BD 91May31 Bottom Time: 45 MIN
D159TO.HO3 Bottom Mix: TX 17/50
Bottom PO2: 1.46 ATM
DEPTH STOP DECOM
FSW TIME TIME MIXTURE COMMENTS
----------------------------------------------------------------------------------------------------------- 00 00 00 AIR Descend To Bottom Comfortable 00 00 TX 17/50 Rate Breath Trimix 17% O2, 50%He,
250 45 00 TX 17/50 Balance N2, From Surface.Ascend
140 02 04 TX 17/50 To First StopAt 60 FSW/ MIN,
130 01 05 Tx 17/50 Rate 30 FSW/ MIN After First Stop.
120 02 08 Tx 17/50
110 03 11 EANx36 Breathe EAN 36%At 110 FSW
100 03 14 EANx36
90 04 19 EANx36
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx STOPS INTENTIONALLY OMITTEDxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
30 33 114 EANx36
20 27 141 Oxygen Breathe100% O2, 20 FSW
10 49 190 Oxygen ToSurface
00 00 191 Reach Surface
TOTAL Time = 236 MIN
DECOM Time = 191 MIN
OTU's = 299
This is an example of a special table calculated for a 45 minute exposure at a maximum working depth of 250 fsw, using a trimix 17/50 (17% O2, 50% He, balance N2) bottom mix and an intermediate mix of enriched air (EANx36: 36% O2, Balance N2) and oxygen for decompression. The table was generated using the Hamilton Research Ltd. DCAP computational program with the Tonawanda II algorithm and the ascent-limiting matrix designated 11F6, without added "conservatism factors." Rates and gas switchs are given in theComments section, but a more detailed set of instructions should accompany a table of this complexity. Note the individual stop times and cummulative decompression times are included. The diver starts the clock upon leaving the bottom and leaves each stop at the time in the "DECOM TIME" column, and thus doesn't have to worry about individual stop times. This is an easier way to manage a decompression than timimg stops, it involves fewer chances for error, and this method enables a dive to be "reconstructed" at any time.For a dive with a planned bottom time, RUNNING TIME, which starts on leaving the surface is preferred by some technical divers.See Decompression Strategies, technical Diver 2.2, for a discussion. Note, the cummulative oxygen doseage, measured in oxygen tolerance units (OTUs) is also displayed. For a discussion see Reference #2 above.