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Don't DCIEM tables include schedules for O2 decompression? If so can they be compared directly to DCIEM air-only schedules to see if the "1 minute on O2 = 2 minutes on air" rule holds up? I don't have DCIEM tables to check. I have run some simulations using the Buhlmann model (various versions). My vague recollection is that the 20 and 10 foot stop times were not always cut in half. But I wouldn't swear to that and don't have time at the moment to reproduce the experiment. By the way some Florida cave divers are using 50% nitrox instead of O2 as a safer decompression gas when using air schedules. They claim it is almost as good as pure O2 for washing out nitrogen. It can be started deeper (on the first stop of all but the most extreme air dives) and is cheaper and safer as far as non-DCS risks go. Again I don't have specific calculated schedules at hand to evaluate that quantitatively. To get some bounds, consider that the advantage of one gas over another for offgassing a given tissue at any given instant is the ratio of the difference between the PN2 in the tissue and the ambient PN2 for the two gases. At 10 feet the latter figures are 0 for O2, 21.5 for EAN50, and 34 for air. Using the USN model, where the least loaded compartment at the end of a 10 foot stop has at least 51 fsw PN2 I get a lower limit on pressure differential of 51 fsw for O2, 41-21.5=29.5 for EAN50, and 51-34=17 for air. This is rather off the cuff, but unless I'm mistaken this indicates that, according to this model, a minute on O2 is worth at most 51/17=3 minutes on air and EAN50 is worth at most 29.5/17=1.74 times the equivalent air time at 10 feet. This is for the last part of the 10 foot stop controlled by the 120 minute compartment. The advantage would be less for more loaded compartments earlier in the stop. For a lower limit the highest pressure you could have in a compartment at 10 feet should be 126.7 fsw (in the 5 minute compartment). Hence the lower limits on the O2 and EAN50 advantages (relative to air at 10 feet) are 126.7/92.7 and 105.2/92.7, respectively. But these lower limits won't contribute much to the average for realistic schedules of interest, since it is unlikely the 5 minute compartment will be controlling at that point, and if it is the stop will be so short it is not worth worrying about. This is an interesting discussion. I wish had time to contribute more to it, but my current work load limits this. Bill Mayne
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