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> >Some people who decompress on O2 combine their 10 and 20 foot stops @ 20 feet. > > This is referred to as "pulling a stop" and is actually more efficient. > > I've seen Gentile say this in his "Ultimate" wreck diving book. The > quote, from memory, is "The increased pressure gradient [of hanging at > 20 instead of 10fsw] actually causes the nitrogen to come out of > solution faster." > > Does this make sense? Assuming pure O2 decompression, why would > decompressing at 20 be faster than at 10fsw? The nitrogen is > responding to the ambient pressure (assuming inspired N2 pressure > constant) directly: why would it come out faster at a higher ambient > pressure? The part which doesn't quite make sense to me is saying that the pressure gradient is increased by hanging at 20 feet instead of 10 feet. With 0% inert gas in the breathing mix the pressure gradient is just the partial pressure of inert gas in tissues, which is independent of depth. So it is easy to see why there is no harm in hanging at 20 feet. Conventional theory would say there is no advantage either, as far as pressure gradient and off gassing rate goes. There may of course be an advantage to staying below the worst of the surge, or to staying in a habitat rather than getting out into the water to go up 10 feet. But conventional theory is not the whole story. Although I don't understand all the explanation there is an alternative theory which holds that deeper stops are actually more effective than shallow stops even when breathing air. An article on the BSAC '88 tables, which have the last stop at 6 meters instead of 3 says that according to their model (by Hennesee?) there is no advantage to hanging shallower than 6 meters. According to a rather old book by B.A. Hills, "Decompression sickness : the biophysical basis of prevention and treatment", Wiley, 1977, complex calculations based on thermodynamic theory indicate that the optimimum way to decompress is to stay deeper throughout decompression, with the last stop being at 20 feet or deeper. The total ascent time is less than conventional tables. Empirical evidence from sponge divers who have developed their own schedules by trial and error is said to support this. I have also heard anecdotal evidence from some very experienced deep divers who modify decompression in essentially this way. A very simple partial explanation could be that by staying deeper you keep more inert gas in solution, rather than silent bubbles, and that this helps you off gas faster. I don't know if this is correct, but it is plausible. Or using the theory that it is bubbles that are actually eliminated keeping the bubbles squeezed to smaller volume could help. I am not suggesting that we throw out the tables and computers we now use and experiment with such a radical program. I merely point out that conventional models based on simple diffusion laws are not all there is to it. But note that if we are just willing to decompress longer than may be necessary we can get some of the advantages of both theories. Computers penalize us for staying deep according to conventional theory, and keep us hopefully safe according to that theory, i.e. further from the thresshold of DCS risk, while we actually get any possible advantage to staying deep that alternative theories, or just a conservative application of conventional theory gives. Bill Mayne
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