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From filling a lot of nitrox tanks, I am also inclined to believe that
partial pressure mixing does not produce an instantaneous homogenous mix. I
have found you can get a change of up to about 2% if you test a cylinder
straight off the whip to a few hours later.

Nitrox mixes seem stable after about an hour maximum in reasonably warm
ambient temperatures. A good guideline is to touch the tank: if it is still
warm from filling, wait a while before testing wherever possible, or allow
for possible error in your dive plan. Trimixes can take significantly
longer, especially if it is cold.

You can really see the speed of mixing when you fill a nitrox bank with
standard anti-debris tubes fitted to the valves. It can take hours for final
mixing to occur. This effect can be reduced by fitting long, perforated
tubes, which drop down the full length of the bank cylinders.

Of course, as has been stated elsewhere, once it is mixed it stays mixed,
which is just as well, or skiing and mountain climbing would probably be far
less popular.


Mark Caney
Scubex, Cyprus
Tel + 357-6-246680 (GMT + 2 hrs)

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