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On Fri, 21 Apr 1995 Cvdvr@ao*.co* wrote: > I have a question regarding PO2 toxicity. When diving a mix, whether it be > trimix or nitrox, the operating depth is extremely important. But the same > consideration is not given when diving compressed air deeper than 218' (PO2 > 1.6). Why does it seem that diving compressed air at high PO2's seems much > more forgivable than a mix with a comparable PO2? > Any feed back to this question (I hope that this was not beat to death > before my recent arrival on the net)would be appreciated. The simple answer is that diving compressed air below a depth of 218fsw leads to just as great a concern for O2 toxicity as diving any gas mixture with a PO2 greater than 1.6ATA. You just don't see it discussed much among technical divers because diving to such depths on air is seldom endorsed by people who are trained in the use of trimix. Also, the narcosis is often as great, if not greater concern than the O2 toxicity at any given depth on a deep air dive. I've never heard anyone suggest that CNS O2 toxicity is less likely when diving air at a PO2 greater than 1.6 than it is diving any special mix with a similar PO2. The less-simple answer is that CNS O2 toxicity may be *GREATER* for air than almost any other technical diver's breathing mixture at a given PO2. The reason for this is that a deep air dive combines high PO2 with high gas density, which means exertion of breathing is greater, which means CO2 levels may be higher, and increased CO2 levels appears to increase the probability of a CNS O2 toxicity problem. Enriched-air nitrox mixtures achieve a given PO2 at depths shallower than the same PO2 for air, and O2 is a lighter molecule than N2, so enriched air will always have a lower density than air at any given PO2. Almost any practical mixture with helium in it will similarly have a lower gas density than air at any given PO2. Not to mention the added complexities if there is truth to the trend that CNS O2 toxicity is more likely for gas mixtures than for 100% O2 (i.e., which gases affect toxicty problems and how)...... 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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