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---- Microsoft Mail "VMS Mail" message ---- From: HeimannJ on Fri, Feb 19, 1993 10:16 AM Subject: RE: Deco stops in open ocean To: techdiver From: HeimannJ on Fri, Feb 19, 1993 10:07 AM Subject: RE: Deco stops in open ocean To: HARVEY::"story@be*.wp*.sg*.co*"; techdiver David Story writes, in response to my note: >> >> [...] use a reel of some form and a liftbag. In NY and NJ, people >> tend to use a long spool with about 250 ft of sisal rope; you can see this in >> typical underwater pictures of people on the Doria. I find it easier to just >> carry one or more Dive Rite reels, which double as penetration lines. > >Of course the reason they use sisal is that it decays, and doesn't >leave nondecaying nylon ropes all over the wreck. I agree that the biodegradable property of sisal malkes it the best choice if you often decompress on an upline intentionally - there is already too much damn synthetic line on most wrecks!. I used to carry a sisal upline myself. The problem is that if you only use it for emergency hangs, you are likely to discover that it has started to rot just when you need it (Gentile also tells a story about this in "Ultimate Wreck Diving Guide"). Nylon is much more reliable if it is just there as a backup. Also: >>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. > >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? I'm not a Doctor, Jim, I'm an engineer! But I'll make a stab at an explanation: Henry's law states that the rate transfer of a particular gas into or out of solution is proportional to the partial pressure difference between solvent and surrounding gas. Assume you have just completed a dive, have done your 20 fsw stop, and are contemplating moving up to the 10 fsw stop. Your tissues now have some level of N2 in them. Assuming circulatory and respiratory efficiency don't change for some reason, the partial pressure of N2 (PN2) in the capillaries surrounding in your alveoli should be the same at 20 fsw or 10 fsw. This is because (unless bubbles form) gas partial pressure is purely dependent on the amount of gas dissolved in your tissues, not on ambient pressure. Now if you breath pure O2, the PN2 in your alveoli should be essentially zero, both at 10 fsw and 20 fsw. Hence, one would expect, the N2 gas transfer rate should be identical when breathing O2 at 10fsw or 20fsw. Now, let's factor in some real gas behavior. On most extended dives, there is some bubbling which occurs in venous blood. If one decompresses properly, bubbles are trapped in the capillaries surrounding the alveoli and dissipate harmlessly. Well, almost harmlessly - they do reduce the efficiency of offgassing, and deco tables have to take this phenomena into account either explicitly or empirically. Here's where decompressing at 20 fsw makes a difference, because increased absolute pressure does reduce the size and tendency of bubbles to form in the blood. Thus decompressing on O2 at 20 fsw should be more efficient than decompressing at 10 fsw, since fewer bubbles form. N.B. this argument doesn't apply to decompressing on air, which is mostly N2.
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