dpb wrote:

The bottom line is how confident do you want to be that what comes
out of that refilled tank is, indeed, fit for breathing purposes
and hasn't been contaminated since that point?

I presume that the first time that any brand-new O2 bottle is filled
with it's first batch of O2, that it has been cleaned and vacuum
evacuated first.

After that point, unless the air pressure in that tank ever falls below
ambient atmospheric pressure, it's hard to see how anything other than
pure O2 could ever re-enter it - even if it was ever connected to a
manifold system where other bottles of similarly-clean O2 are also
connected.

The scenario in the posting link later of a single large bottle
refilling known smaller ones is reasonably well controlled; just
taking the next random welding bottle returned from who knows
where...errr, not so much.

I understand that I can buy, or rent, oxy-acetelene tanks. If I buy,
I'm not sure if I can have my bought-tank re-filled and returned to me,
or if I simply exchange it for filled (but used) tank.

If I buy a brand new tank, and if I keep refilling that same tank when I
need more, then I am removing the uncertainty of what could have been in
the tank before it was filled.

And when it comes to refilling returned tanks, is it normal practice to
at least let the tank fully depressurize itself before it's refilled?
Wouldn't that dillute any potential non-oxygen gas or even particulate
contaminent that it *may* have once the tank has been refilled with
known-pure O2?