"Pete C." wrote:

Much of what is speculated here is not with the "purity" of the
source gas, or the compentency of the compression equipment to
maintain that purity as a tank is being pressurized, but with
what *might* happen with these tanks when in the hands of end-
users as they reach their empty state prior to being returned
to be reused.

Seems that some people here are hung up on that point, and we
are all speculating as to just what the gas retailer does
behind the scenes with these returned tanks prior to refilling
them.

The gas suppliers are not about to fill 2,000 PSI+ of pure O2
on top of some amount of mystery gas in the "welding" cylinder,

While that sounds logical, just saying it is not going to satisfy others
unless more concrete proof is posted to show that the average compressed
gas retailer or supplier performs the same processes and proceedures on
all types of returned cylinders (evacuation, internal cleaning, etc)
prior to refilling.

But I still submit that many of their arguments as to just how these
tanks can become contaminated in the first place is implausible.