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John Heimann (jheimann@sc*.dn*) wrote: > Last night I was thinking about Bill Mayne's note on using the Navy model for > comparing O2 vs. air deco, and thumbing through my handy copy of Wienke's book. > At lunch today I set up a spreadsheet to compare decompression times for each > Navy tissue compartment on air vs. O2 at the 10 fsw stop... Bravo! This is just the kind of analysis I had in mind but did not have time to complete. Besides being busy I am not a spread sheet user and would have had to write a program for it. > They are still a little back-of-the-envelope, but some > might find them interesting: >... > Trem = (1/L) * ln [(P-Pa)/(M0 - Pa)] > > Just to make things easy, I assumed in my spreadsheet that each > tissue compartment was as full of N2 as it could be at the 10 fsw stop. That > is, for each tissue, I assumed P=M0+10*deltaM... And the results for the compartments are: > 5 10 20 40 80 120 > 1.42 1.54 1.74 2.12 2.43 2.51 > > What this means is that if a diver has done a dive where the slowest tissue is > the dominant factor for decompression at the 10 fsw stop, then decompressing on > O2 is 2.51 times as fast as air, whereas if the fastest tissue is dominant > then O2 is 1.42 times as fast. Intermediate tissues have intermediate > advantages for O2. I concur with your methods. I would just add that these are worst-case or conservative estimates. I would not use any other kind. If the controlling tissue at the 10 foot stop is not as loaded as it can be the theoretical advantage of O2 will be more - to the upper of bound of 3.0 for short stop controlled by the 120 minute compartment, as I previously posted. Admittedly that is an unlikely, maybe impossible scenario. I once calculated the longest 10 foot stop which could be controlled by each compartment. This would be the time to decompress from M0+10*deltaM to M0. I don't have the means to easily reproduce that at hand. But as I recall any 10 foot stop of 20 minutes (or some not very long time - Don't hold me to that number) on an air schedule will be controlled by the 40 minute compartment or one of the slower compartments. Hence, strictly theoretically according to the USN model, one minute on O2 at 10 feet is worth at least two minutes on air for all stops long enough to worry about. Caveat: This is THEORETICALLY, and using a model I would not trust. But come to think of it a more conservative model would predict a bigger advantage to O2, since it would have lower permitted tensions of N2 in tissues.) Actual physiology is more complicated. My degrees are in computer science, not physiology or medicine, by the way. So take this as a layman's speculation - from someone too lazy to learn to use a spread sheet, at that :-). Bill Mayne
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