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On Mon, 6 Feb 1995, David Giddy wrote: > Has anyone tried an alternative of Reduced air Nitrox ? By this I mean air > to which *Nitrogen* has been *added* to reduce the FO2. The interesting thing about air (Nitrox-21) is that it happens to be about the optimum blend for deep diving of any nitrogen and oxygen mixture. The reason for this is that both oxygen toxicity and narcotic limits for most people are achieved at about the same depth when breathing air. Many people prefer to limit their oxygen exposure to about 1.4 ATA, which on air occurs at about 190fsw, at which depth most people are feeling strong effects of narcosis. A few people set their PO2 limit at 1.6ATA, which is ~220fsw on air, a depth where only a very few individuals are not seriously incapacitated by narcosis. In other words, O2 toxicity and narcosis are almost equally limiting factors for depth when breathing air. Nobody really knows for sure what the relative contributions of nitrogen and oxygen are for narcotic intensity. One paper suggested they are approximately equipotent. If you look at lipid-solubility, oxygen should be about twice as narcotic as nitrogen. Some people with a lot of nitrox experience feel very confident that narcosis is *less* intense on nitrox at 130 fsw than air at 130fsw. I have one anecdotal experience which suggests very strongly to me that nitrox-29 at 200fsw is *more* narcotic than air at 200fsw. The process, I am certain, is much more complex than simple partial pressure math, and likely involves a lot of body chemistry. I actually believe that breathing Nitrox-10 at, say 240fsw, would be slightly less narcotic than breathing air at 240fsw (nothing to back this up...just a hunch). But what would the point be? Any reduction in narcosis would be subtle, at best, and would likely be dwarfed by other variables affecting narcosis intensity (water temp, CO2 buildup, water clarity, etc). It's true that the risk of acute O2 toxicity might be reduced somewhat, but that becomes unimportant if narcosis is incapacitating. Although a few individuals like Bret Gilliam, Dan Manion, and others have pushed beyond 400 or even 500 fsw and lived to tell about it, even they wouldn't advocate nitrogen/oxygen bimixes as a *practical* breathing gas for deep water. I'm sorry if this doesn't satisfactorily answer the question...I'd sum up by saying that: any advantages gained by increasing the nitrogen content of air at deeper depths are tiny compared with the advantages of diluting air with helium (even when considering the relative price of the two gases). So, except for some highly unusual situations, it would be better to use air, trimix, or heliox for deeper dives. Aloha, Rich deepreef@bi*.bi*.ha*.or*
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