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> >Suppose one wanted to dive trimix followed by an air repet a few hours later >using a computer. I believe the following approach works, in the sense that >it is conservative, although may not get you the maximum bottom time on the >second dive. At any rate I would be interested in comments. > >Here's the method - bring the computer along for the first (gas) dive, but >dive the gas tables. The computer will get bent, but can be lowered to 20 >fsw and recompressed (I typically do this anyway just to shut up the >computer's audio alarm). Wait surface interval, and then dive air according >to the computer. > >Here's why this should work - when diving 18/40 mix on some UAC-generated >tables (Buhlmann-based) I have noticed that my computer (Aladin Pro, also >Buhlmann-based) typically stays happy until I hit the 10 fsw stop. The mix >table stops for 10 fsw on O2 are shorter than what my Aladin wants, and it >gets bent when I leave the water. I imagine that for longer or deeper dives >the computer might get bent at the 20 fsw stop as well, but in either event I >should be able to recompress it at 20 fsw. > >Now when I pull the computer from the water, it will think I have >significantly more nitrogen in my tissues than I actually have. This is >because 1) I have been diving a mix with only 42% N2, not the 79% N2 that my >computer assumes, 2) I will have been offgassing on the surface faster than I >would have been if I were actually hanging with the computer at 20 fsw. This >means that my computer will be quite conservative on the repetitive dive. >Note that although I have He in my tissues as well as N2 from my gas dive, we >can ignore the He on the second dive since there isn't any in the air I will >be breathing. > >Comments? John, Have you considered the bubbles that you will almost certainly have after the dive? These will be composed of helium, nitrogen, oxygen, carbon dioxide and water vapour. Diving on air will create a diffusion gradient into the bubbles for nitrogen and a diffusion gradient out for helium. Which gradient is greater will depend on and the relative N2 and He diffusion coefficients and what their actual gas composition is, these will be determined by where the bubbles are ( diffusion coefficients and perfusion), what mix was breathed, and for how long. If inward diffussion exceeds outward you may get sick. It has been demonstrated that air bubbles in adipose tissue or spinal cord shrink during 20/80 heliox breathing in rats, presumably because nitrogen diffuses out faster than helium diffuses in. I wonder if the reverse is true, would heliox bubbles grow with air breathing? I don't think the Aladin computer models this. When he published his ZH-L16 model Buhlmann made it quite clear that, although theoretically possible, his models had never been validated for trimix. I would not do repetitive trimix dives using a Buhlmann algorithm; however, I am a coward. regards, David Doolette ddoolett@me*.ad*.ed*.au*
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