On Monday, February 13, 2017 at 12:14:04 PM UTC-5, pyotr filipivich wrote:
Martin Eastburn on Sun, 12 Feb 2017
21:20:21 -0600 typed in rec.crafts.metalworking the following:
The impurity is 'natural' from pumps and valves. In medical oxygen that
might feed through machines that are very expensive or feed a person
with 20% of their lung left, the air is FILTERED heavily.

At 10 or 20,000 feet with flights up to 80,000 one does not want
moisture in the air line. Simple as that.

I'm still confused.

What sort of 'impurities" can be in an oxygen environment? Okay,
water / humidity I can understand - it is "inert". But how did it get
in their in the first place? One would think that distilling Oxygen
out of the atmosphere would first remove the water.
Part of my confusion come from having dealt with the specs for
manufacturing medical equipment which would be part of the oxygen
system. "Not oil at all." Not before, not after, not during "Thou
shallt have no oil in the presence of the metal. Neither shall it be
on the tools thou useth. On the finished part is straight off."

For one thing, improperly cleaned and/or purged tanks / lines / valves / connectors could have all sorts of impurities. just opening the empty tanks valve to ambient air will introduce impurities (notably, in this case, water in the form of atmospheric humidity).