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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? I put hypothetical dive profile through the Buhlmann ZH-L16 algorith using this procedure. I chose the profile at random and did not do any others (enough goofing off for one day). The imaginary scenario is as follows. First dive 40%He/47%N2/13%O2 (40% He in air) to 90 metres for 20 minutes, air deco from 36 metres to 9 metres, 36 minutes total and a 30 minute O2 stop at 6 metres. 4 hour surface interval. Air dive to 50 metres for 20 minutes. The procedure outline above for using an aladin did not violate decompression requirements for the second dive; however, the calculated tissue tensions in the slower tissue compartments, half-times 109-635, were greater for the actual dive than those calculated assuming the first dive was on air and a sufficiently prolonged (76 minute) 6 metre stop on air. Therefore it is likely that the procedure will not work for all dive profiles. If you don't leave your computer at 6 metres until it allows surfacing the the calculated second dive violated decompression obligations. Of course with an aladin you can't do this because it locks you out for 24 hours. Regarding repetitive trimix diving in general. The type of dives that are done on trimix are of sufficient exposure that organizations that have a greater experience with decompression practice than me, eg. US Navy, Royal Navy and DCIEM, would not permit repetitive diving. It would seem unwise to ignore that experience. regards, David Doolette ddoolett@me*.ad*.ed*.au*
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