"Pete C." wrote:

What is the compressed gas supplier doing differently that would
result in that very slight (but consistent?) difference between
those two products?

They aren't doing anything different, those are the *standards*,
not the actual product spec. The reality is that both grades are
filled from the same cryo tanks and both exceed the 99.99% welding
grade standard.

So how is it known that these gasses meet this 99.99% spec?

Are they tested?

Or are they compressed with equipment that is known to NOT inject
contaminents during pressurization?

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.