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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?

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