Posted by R Bear on February 13, 2001 at 07:57:08:
In Reply to: Deco issue to JW posted by seahunt on February 12, 2001 at 14:31:22:
So here are the five things you mention.
j-factors (fat content)
gas selection
workload
physical condition
hydration
J Factors: This is the one I know the least about in the context of diving. I suspect it is the least important of the five things mentioned, but confess that I am not sure about that. I do know that one can achieve good cardiovascular fitness in spite of being overweight (undertall? (-[:] ). I suspect that the fitness level is more important than the fat level.
Gas selection: This is probably the easiest to quantify. I won't try to teach a nitrox class on-line, but we all know that raising the PPO2 and lowering the PP of the inert gas means less on-gassing of inert gas. It is not quite as simple to quantify the more rapid on-gassing and off-gassing of He but a mathematical model will still apply. The same statement applies to maximizing a high O2 window while decompressing. I am not providing any numbers, but that is not a dodge. The numbers are simply beyond the scope of the post. Still the effects of EANX 32 (as an example) instead of air are (easily) quantifiable. Statements like, "I am fit. You are fat" are extremely difficult to quantify.
Workload: It is a fact that a gas will saturate an agitated liquid faster than a calm liquid. Conversely, gas will exit an over-saturated liquid more rapidly if that liquid is agitated. Unfortunately the human body is so much more complex than an (agitated) liquid that our best data on this comes from experience, not from physics. Experience shows that the (theoretical) faster on-gassing due to workload at depth is not very important. Workload at depth has so many very real consequences that this (possible) consequence is almost moot. These include things like increased blood CO2 and rapid use of gas. Where workload really becomes a hue factor in DCS is upon ascent. It is possible to take two adequately decompressed divers and force DCS on one of them by having him run sprints. Doppler tests show that a deco diver can release a stream of bubbles into his blood by doing as little as flexing, or hugging himself. The practical application of this is that when you suspect you are pushing your limits, you should treat yourself as if you are fragile. Float in the water and talk about the dive. That boat will still be there in ten minutes. You can get on it then after you have off-gassed a little more. Also feel free to pass that heavy tank (or set of tanks) to someone on the boat instead of demonstrating what a macho stud you are. This is the point in the dive where a heavy workload can bring problems to you.
Physical condition: This is the card that makes conservative and safe NOT synonyms. I think that the difference between the most fit individuals and the least fit individuals is a factor of approximately 1.4. This number is a bit of a guess (obviously) but I am working in part from dive tables. Compare the Navy tables (which were developed for VERY fit individuals) to NASDS tables (for the average diver) and you will get a sense of what I mean. I think there are dives where I could deco for an hour and get out but there is a less fit person out there who could deco for 80 minutes and still get bent. The normal thinking is that Mr. 80 minutes was MORE CONSERVATIVE on deco than me. But I was safe and he wasn't. The 1.4 applies backwards to NDL vs deco. Mr. Fit may have an NDL of 28 while Mr. Unfit has an NDL of 20. On the flip side Mr. Unfit may have to do 42 minutes of deco to equal Mr. Fit's 30 minutes. Unfit people tend to want to discount fitness as a safety factor. Obviously I think it is HUGE.
Hydration: I don't think there is any benefit to OVER-hydrating other than to be good and sure you are not DE-hydrated. I think that the number on severe dehydration is even higher than the number on major lack of fitness. Something like 1.5. The difference is that Mr. Unfit should recognize he is unfit and apply a multiplier to his NDL (or deco) times for safety. In theory Mr. Dehydrated could limit NDL to 2/3 (to account for 1.5) and be safe, but the real answer of course is to not be dehydrated.
By the way, I just found this board. I'm a cave diver from Florida. Some of you that don't know me may wonder why you should take my advice on this stuff when you don't know me. The answer is that you shouldn't. You should do your own research and verify that you have the right info to whatever degree of rightness that you desire and are able to obtain. On the other hand this is a pretty solid answer.
DSAO
Ron