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---- Microsoft Mail "VMS Mail" message ---- From: HeimannJ on Wed, Feb 17, 1993 2:05 PM Subject: RE: Trimix To: techdiver David Story, in response to my note stating that custom mix models are still being validated, writes: >Actually, Dr. Buehlmann has done significant testing of this kind of >mix diving, and the commercial diving community very rarely dives air. >I think there are more dives than you imagine. I misspoke. I meant to say that custom mix TABLES are usually being validated on the diver. The Navy has been diving mix since at least 1939 (salvage of the USS Squalus) and various tables based on Buehlmann's model have been in use since the sixties (this is my preferred table). When a custom mix table is cut for you, it is based on a particular model, e.g., Buehlmann, but with an often unique of assumptions and safety factors (what Dr. Bill Hamilton, probably the foremost sport and commercial table cutter, calls J-factors - J for Jesus). These include things like ascent rate, deco mixes used, probable tolerances in your mix, conservatism, etc. If the table cutter has made the right set of assumptions, and the table is used correctly, then you will probably be OK. If not, you might get into trouble. The only way you will know is to test it. As an example, consider the Navy model (I happen to have a version of this running on Microsoft Excel, by the way, if anyone would like to play with it). Arguments about its safety aside, it was the standard for years. Now if one cuts a table using the straight model, one finds that the stops for certain profiles are signifcantly shorter than what are in the tables. I have been told that this is because when the tables were initially tested, certain profiles were found to cause unacceptible bends incidence, and were tweaked until they were acceptible. Deco models (in fact, most physical models) are basically exercises in curve fitting. A modeller has a certain set of empirical data about what dive profiles cause bends, and a mathematical model which is thought to represent the important underlying phenomena. The parameters of the model are adjusted until it gives the closest possible fit to the data. The modeller knows that the model is only an approximation to what is really going on physically, however, and that certain phenomena are not necessarily addressed. Hence the predictions of the model - specific tables, that is - must be validated experimentally to assure that they are safe. For much more discussion of specific models, I refer you to Lippman's book "Deeper into Diving." Buehlmann's model in particular is discussed in his book "Decompression / Decompression Sickness," but is unfortunately out of print at the moment. >O2 toxicity (1.5ATA conservatively, 1.6ATA at the "limit") limits >working air depths to less than 200fsw. Nitrogen narcosis greatly >reduces the amount of work that can be expected of a diver below >(approximately) 150-180fsw. Trimix is important shallower than you think. I myself am interested in mix mostly for this depth range, since that's the range in which many of the interesting wrecks in my area lie. I have done a number of cold-water wreck dives in the 160-200 fsw range where I felt perfectly comfortable when swimming around the outside, but started to notice performance impairment when penetrating. Narcosis seems to limit one's ability to cope with complex tasks, such as managing reels and lights while trying to avoid silt and entanglement. My adventurous side says that I sure could get a lot more neat stuff on mix, since I would have a clearer head and could penetrate deeper and farther. My rational side says that I should practice penetration skills until they are virtually automatic, and then I will be more comfortable when heavily narced. At that point, I will be even more comfortable doing the same dive on mix, with its added logistical complexity.
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