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> While causually reading the CDG manual the other day, I came across Reverse
> Gas Diffusion - the claim was that when diving specifically on Heliox, it was
> unacceptable to decompress on Nitrox mixes because incoming Nitrogen would
> inhibit the release of Helium already in the body - you should decompress on
> Heliox until shallow enough to use O2.
I was unable to find this in the latest (1990) CDG manual - which page number?
This is exactly the opposite of what really happens. Helium is a 'fast' gas
which is taken up and released by tissues much faster than nitrogen.
Therefore, if, without changing depth, a diver switches from a helium mix
to a nitrogen mix, the helium will be released faster than the nitrogen is
taken up, and therefore the total inert gas loading of the tissues will
fall. This phenomenon was used to dramatic effect by Keller and Buhlmann in
the 1960's, who used the _subsaturation_ effect of switching from a 'fast'
gas to a 'slow' gas to offset the _supersaturation_ caused by decreasing
ambient pressure (i.e. a fast ascent) and therefore shorten decompression
time. They amplified the effect by using argon instead of nitrogen: an
example was a 300ft, 60 minute bottom time dive with a total decompression
time of 85 minutes!
The problems occur when a switch to a 'fast' gas occurs at a constant or
decreasing ambient pressure, which creates a state of supersaturation in
the tissues. If you fill your drysuit with heliox or trimix whilst at depth
while breathing air, helium can diffuse into the skin faster than nitrogen
can diffuse out and the resulting supersaturation causes characteristic
lesions in the skin, as well as bubbles in the venous blood coming from
them.
The name of this effect is 'isobaric inert gas counterdiffusion'. It can be
useful or harmful, and unfortunately a lot of misunderstanding surrounds
the subject. Here are some simple rules:
1. Fast (helium) to slow (nitrogen) is good (especially on ascent)
2. Slow (nitrogen) to fast (helium) is bad (try to be descending)
3. Get off the bottom mix as soon as possible
Hope this helps.
Dr Andrew Pitkin
apitkin@ad*.de*.co*.uk*
apitkin@ci*.co*.co*.uk*
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