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Rat: the real point should be not whether a nitrox tank has air in or not
(since the MOD will be clearly marked, risk is managed). But rather when
nitrox is used in a unmarked tank...(given the situation outlined in my
previous post). My point is that MOD is the important thing to look for and
many divers regard unmarked tanks (no ugly nitrox stickers) as having a
limitless MOD!

When I dive EANx there are times when the <best mix> is in the low
20s...Cold water, heavy current, paranoia regarding CNS tox...sometimes
this mix is accomplished by adding air to the residue from a previous fill.
This air by the way is ALWAYS O2 compatible -- hydrocarbon-free -- and
that's the case in the air tanks I have too since I dislike the thought of
breathing any PP of oil vapor. When analyising the MOD in some cases 1.3 is
used for O2 dose. (This practice recently gave me a 24% bottom mix for a
dive on the Empress of Ireland.) As always, I marked my tanks CLEARLY with
the MOD. These tanks also have a Nitrox sticker on them. I completed my
dive then had the tanks topped again with air for another deep wreck dive
in a different spot. The resulting mix was virtually air with an MOD far
deeper than I am prepared to dive in local conditions without employing
trimix. The suggestion seems to be that now I should re-vip and perhaps
re-clean these tanks. This is nonsense. They are still suitable for partial
pressure fills. After all a 22% mix is Nitrox...rather lean but nitrox
nevertheless.

The big deal is putting any gas other than air in unmarked tanks that will
be used in an <open> environment, since the other divers in the vicinity
may not have the training/experience/sense of self-preservation to check
for an MOD.

Steve


>I see a "big deal" in putting *air*, hydrocarbon free or otherwise into a tank
>marked for nitrox.
>
>Mis-use is a mislabeled tank with regard to its contents, not the purity of its
>contents.  It's mis-use if the tank is filled with *non* hydrocarbon-free
>nitrox
>or hydrocarbon free air.  I do not see the problem as "what could happen"
>or how
>dangerous is this...   The tank should contain what its identification says.
>That only makes sense.

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