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On Wed, 24 May 1995, Ronnie Bell wrote: [RE: PO2 limit of 1.4] > I'll have to register a NAY. > Lets say I am diving a cave that has 90 percent of the passage at 110 and > 10 percent at 160. I am not going to use mix for that dive! For me it's > just not that dangerous . By my calculations, the PO2 of air at 160 is just over 1.2 ATM. Air starts to exceed 1.4 at about 200 feet (OK, 187 feet, but y'all know my feelings about unneccessary precision). > I seem to recall that the limit used to be 1.8, then went to 1.6 now I > hear a call for 1.4. Hmmmmmm? > I think the current limit of 130 feet that PADI (and all the rest) > recognizes is fine for new and recreational divers. Divers with more > experience and training should set their own limits, they, after all, are > the ones who will pay for their mistakes. I haven't gone back and > actually reviewed any logs of incidents but I don't seem to recall ONE > incident where "deep air" (above 225 ft) was a problem UNLESS the diver was > breaking another guideline along with it. Now, you know my feelings about deep air diving, and allowing educated individuals set their own limits (hell, I think my little quote at the bottom says it all!). But I'm still in favor of advocation of 1.4 as a general guideline that ought to be taught to new divers. There aren't going to be any scuba-cops checking your mix *and* your depth (maybe one or the other, but not both), so everyone will always have the freedom to breathe any PO2 they want. I would never attack anyone for chosing a higher O2 mix. Nobody should be prevented from breathing any mixture they want, but I don't see the harm in encouraging people to maintain a good safety margin on the PO2. > Is this depth limitation like the CNS clock? What I mean is, what > happens when your CNS clock hits 100? Exactly what does that mean? What > degree of fit does the CNS clock have to incidents of O2 hits? The > problem with both the CNS clock and the 130 foot limit on air is that > both can be violated my most people with no apparent problem. So the > diver starts to feel that the limit is NOT realistic. I'd "recommend" a > max depth on air, but it sure wouldn't be 130. Didn't that number arise > from the USN and their perception that any work past that limit would be > more effeciently performed by Surface Supported divers? How does that > relate to cave diving? I'm just full of questions aren't I? I look at all these things (decompression schedules, PO2 limits, OTUs, CNS clocks, narcosis, etc. etc.) as REALLY fuzzy guidelines at best. Not all are equally fuzzy. CNS clocks and OTUs are at the fuzziest end of the spectrum, decompression is somewhere in the middle, and O2 toxicity is probably about the least fuzzy (but still very fuzzy nonetheless). On any given dive, all these factors are acting in concert to affect the probability of a problem. Because of the very fuzzy nature of these factors (and the even more fuzzy individual variation component), the probabilities of problems are very fuzzy as well. In fact, absolute probabilities are impossible to determine. Sure we can say that out of 100 test divers under XXX conditions, XX percent had problems; but that doesn't say ANYTHING about a specific person in a specific situation. The best we can do is estimate *relative* probabilities (i.e., the probability of an O2-induced convulsion is likely greater with an inspired PO2 of 3.0, than an inspired PO2 of 1.0). Because PO2 is about the least fuzzy of these variables (based on what I've read and heard from people with first-hand experience), and because few of us have any direct experience with our own bodies and O2 convulsions, and because the consequences of an O2-induced convulsion underwater are not fuzzy at all (unlike the consequences of, say, narcosis or DCI or pulmonary O2 toxicity), I think we all ought to treat PO2 limits with more caution than some of the other variables. Also, PO2 will become a bigger issue when more people start using mixed-gas rebreathers for two primary reasons: 1) Entire dives will be based on the PO2 of the breathing gas, not the FO2 as in open-circuit; and 2) the PO2 of the breathing loop is much more dynamic than the PO2 of open-circuit. Thus, the reasons it's good to keep the PO2 of a rebreather relatively low are that the diver is exposed to the given PO2 throughout the whole dive (i.e., subject to a higher O2 dose than Open-circuit), and accidental spikes can occur. Keeping the PO2 low on a rebreather allows for more margin of error on both of these. > Do I recommend deep air ? NO! > But on the other hand I don't even recommend diving :) > It's your life, feel free to run it the way you want! Resounding agreement on all of the above. Aloha, Rich Richard Pyle deepreef@bi*.bi*.ha*.or* ******************************************************************* "WHATEVER happens to you when you willingly go underwater is COMPLETELY and ENTIRELY your own responsibility! If you cannot accept this responsibility, stay out of the water!" *******************************************************************
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