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If you are diving in the grey area of depth in which either gas or air could be appropriate (say, 180-220 fsw), and are doing two dives, you can get significantly more bottom time on the second dive by doing the first dive on trimix, and the second on air (assuming that the trimix is a relatively rich mix). This occured to me when planning two dives I did yesterday, which were on a fairly deep (185 fsw) new wreck off New York (the "Big Wreck" found by the R.V. Wahoo). I had some 18/40 trimix I wanted to use in one set, and air in my other set. Because of the boat schedule (5 hours on the wreck), I had to plan both dives carefully to get two in. When planning the dives on DECOM, I found that I could get more BT than expected on the second (air) dive. I thought about this, and realized that this was because the model which the program uses assumes (based on Henry's law) that He and N2 are independent of each other in tissues. On the first (trimix) dive, you absorb He and N2, but significantly less N2 than if diving air. When you do the second (air) dive, you ongass N2 but no He, so from a nitrogen point of view it's as if you did a relatively shallow air dive followed by a deep air dive. Hence more BT for the second dive. In my case, I did 25 min on TX 18/40 for first dive, with 74 min deco on EAN36 and O2 (I spent slightly longer than DECOM required on the O2). Surface interval was 90 minutes, followed by 20 min on air, with 54 min O2-accelerated deco (again, slightly more than required). Now I know that I shouldn't trust neo-Haldanean models, and that counterdiffusion is a concern. It is my understanding that counterdiffusion is a problem when switching from N2 rich mix to He rich mix, however. Doing the reverse (which is what happens when you follow a trimix dive with an air dive) should actually cause better decompression than Haldanean models predict. The bottom line is that it worked yesterday, and has worked for me several times in the past (I also did air dives following gas dives when on the Doria last July). Any comments from those more knowledgable in deco theory would be appreciated. John Heimannj@ma*.nd*.gt*.co*
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