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Peter PO2 limits vary with application. Most of us in technical diving try to remain below a po2 of 1.4 ata based on our persoonal experiences and the fact that we spike the po2 when we switch to decompression gases. I beleive both the US and British navies have adopted 1.3 for use with rebreathers as the po2 is constant. Of course they make an exception when o2 rebreathers are used and go back to high po2's. Commercial companies vary with Comex using as high as 1.8 but remember they are in surface supplied equipment and thus do not have the drowning risk of an open circuit of rebreather diver. Hope this did not add more cinfussion but it is the variability that is applied to oxygen hits. To me on technical dives there is no advantage of using a po2 for bottom mix above 1.4 ata. On short recreational dives with no gas switches, and limted exertion I would allow a slightly higher po2. Again for most practical applications there is limted gains in running a po2 above 1.4 ata. even in recreational diving. In tech diving it is a risk that simply is not justified as it reduces your ability to have an efficient deco . There have been o2 hits in tech diving at 1.6 ata on deco. two that I'm aware of last year. In both cases it was the total exposure that led to the problem. I do not know of any oxygen hits in pure no stop recreational diving at 1.6 or less, however as we have no way of determining all persons tolerance to oxygen I would highly recommend that even in recreational diving 1.6 be used only as a maximum OBTAINABLE (note! not operational) depth. In other words a wreck dive planned for a 1.4 exposure but with a bottom depth that is at 1.6 and the safety planned that in case for some unexplained reason the diver had to momentairly drop to the real bottom then his limit would not exceed 1.6 however the planned or target operational depth remains at 1.4 ata. Tom You wrote: > >Dennis & George, > >There have now been several posts pointing out that *safe* not an >appropriate scientific term to use in the above context, as it depends on >what degree of risk you are prepared to assume. I agree with the authors >on that and that "local standards" seem to prevail for pPO2s as they do >for depth limits for sport divers. > >Nontheless, I do believe that better estimates of risk can be made when >they are based on cases rather than opinion. If indeed there has only been >one episode of O2 toxicity at 1.3 ATA, then most people would say >"interesting, but probably not relevant to my risk calculations". On the >other hand, if someone says "every time we push the limits. someone takes >a hit", then one is likely to be more conservative. > >Risk estimates take at least two forms: (1) Where you get a percent risk >over the population; say what's my risk for getting lung cancer if I >smoke?. Most of us are comfortable with this kind of calculation, but >also hold back by saying "Well, it still may not happen to ME". The better >way of estimating risk has to do with calculating them for an individual >by looking at co-factors that likely contribute to the 100% hit you take >if the event happens to you. > >Certain factors, notably CO2 retention (due say to exercise loading) and >rapid change/increase in FI O2 are believed to contribute to O2 hits - at >least at high (? >1.3 ATA) pPO2s. This is from the US Navy Divers >Handbook and most dive medicine texts. So this is a logical way of >estimating your own risk potential: Several people have posted or called me >to say "In cold water, or in high effort situations, I'll plan to limit my >pPO2 to 1.2 (even), with 1.6 ATA for deco." > >But I'm left with an empiric issue: Brett says no-way, no-how, *anyone* >has ever experience an O2 hit below 1.6 ATA unless they had already >exceeded their CNS clock. > >If that's true, then *no one* need worry until either they have exceeded >their clock or get to a pPO2 of 1.6. But it's clear that others in the >dive community think that there is enough risk for them to limit their >pPO2 as noted in other posts. > >Other than opinion - do we have facts? George IV, you've been very >silent. Can you direct me to someone(s) in the commercial diving industry >who have facts or are willing to share their opinion on this? If anyone >has extensive experience under high work load and high pPO2's related to >depth (not just breathing O2 at 50 fsw), it must be commercial divers. > >- ph > >-- >Send mail for the `techdiver' mailing list to `techdiver@terra.net'. >Send subscription/archive requests to `techdiver-request@terra.net'. >
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