On Tue, 07 Feb 2006 10:09:06 -0800, Eric Greenwell
wrote:

I didn't realize dive shops could/would refill oxygen cylinders, because
I thought they used air in the scuba tanks. Do they start out with
oxygen, and mix when they fill?

As another poster mentioned, there is, in the high end, mixed gas
diving. In the last 10 or so years, NITROX has become more
mainstream; my understanding, but with nothing to backit up, is that a
tank is filled to several hunderd psi with O2 then topped off to the
normal pressure (usually 3000) with air. As an approximation, if we
started with 79-21 N2-O2, you might get (TOTAL SWAG) 70-30. I've not
done any scuba since recreational diving began the NITROX stuff, so
don't know what the improvements are; I'd guess if one could slow
their breathing, the benefits would be longer time.

George