Two other important considerations…

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Posted by R Bear on February 13, 2001 at 12:14:47:

In Reply to: Re: I'd like to take a stab at this... posted by seahunt on February 13, 2001 at 09:34:40:

Two other important considerations…
The first one is altitude: Unlike some of these other topics though I think that most of us have a pretty good grip on this one. People that dive in the mountains take an altitude diving class and modify their tables accordingly. All of us were taught in BOW not to fly directly after diving. A good thing about altitude is that it shouldn't sneak up on you like some of the other things. If you get bent from swimming like mad to catch the boat or because you were dehydrated from sunburn but didn't realize it, those things may have sneaked up on you. Altitude shouldn't.

The other one is Temperature: A supersaturated liquid will exert a pressure greater than ambient. This pressure goes down with temperature. The problem here is that the pressure drop does NOT obey the gas laws. The pressure is proportional to the delta above the freezing temperature of the liquid. As a reminder, the gas law relationship between pressure and temperature is proportional to the delta above absolute zero. Why is that important? If your tanks cool from 98 to 65 they will drop from 3000 PSI down to about 2800 PSI. But if your hand has a nitrogen load of 10 PSI over ambient and cools from 98 to 65 it will be at 5 PSI over ambient. This severely retards off-gassing. I read about a drysuit diver who was warm except for his hands (which were numb). He got skin bends in his hands. The next example is scarier. A wetsuit diver decided to hit the hot tub after getting badly chilled on a dive. This was bout an hour after the dive. Within 10 minutes of getting in the (hot) water he developed DCS symptoms.
Cold flesh doesn't off-gas well. Mercifully it doesn't on-gas as well either. Part of this is because of physics, part is because of a reduction in circulation. The old rule of thumb many of us were taught is to use the tables like you were deeper than you really were, or throw yourself into the next higher repetitive dive group. A better rule is "Don't get cold. Abort the dive if you do." A final thought is that if you do get cold it may be better to warm up slowly. The guy in the hot tub had residual nitrogen just waiting to get out. If he had warmed up over the course of an hour, it would have taken that nitrogen an hour to come out of his blood as well and it would have been released slowly over the entire hour.


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