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> Jody Svendsen writes:
> >
> > Tri-Mix is best to avoid when possible because for all but the longest
> > dive durations, it incurs a larger decompression penalty than Nitrox.
> > This is because you must sum the nitrogen and the helium in
> > the body's tissues when figuring decompression. This is a particular
> > problem in the slow tissues where the M-values are low. The slow
> > tissues rapidly load up with helium, and once a slow tissue is heavily
> > loaded, it takes a long time to get the gas out, particuarly if there
> > is more than one gas in the tissue.
>
> My understanding of Buehlmann's Decompression/Decompression Illness
> (and physics) is that the gases diffuse independently. Therefore,
> although He enters the body 2.65 times as fast as N2, it also
> offgasses 2.65 times as fast. With this in mind, I can't see the
> sense in your last sentence.
That is correct, He in-gasses AND offgasses at 2.65 times the rate of N2.
The problem is that because helium ingasses more rapidly, it tends to fill
the very slow tissues to the point where they require decompression.
Decompresing the slow tissues takes a long time because their M-values are
very low and it is difficult to get a good pressure gradient to push the
gas out of the tissues, particuarly if you are not decompressing on 100%
oxygen. As an example, using bottom gas for decompression, a dive on
normoxic Heliox to 200 feet for 20 mins requires 687 mins of decompression
while a dive on Air akes only 38 mins.
- Jody Svendsen
MiG Technologies
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