Robert M. Gary wrote:
When they fill an
aviation bottle they must test it for use in aviation. That test
results in a certification of the O2 in the bottle. I'm not sure if
the FBO's do that or if they are somehow able to certify their main
bottles.

-Robert

The test is done at the gas plant where the bottles are filled. If the
whole batch passes, the tanks pass too. The FBO's involvement is getting
the bottles delivered by the vendor, using the bottles, and having the
bottles picked back up by the vendor.

Medical Grade handling changed about 10 years ago. Now exhausted medical
bottles are sucked to vacuum to remove any possible contamination. Then
they are filled at the gas plant. The bottles have serial numbers. There
is record keeping. This adds to the cost. From $2/month bottle rental to
$10/month bottle rental.

Before 10 years ago, filling a medical bottle was no different than
filling a welding bottle or filling an aviation bottle. You hooked up,
you filled, you went and used it.

If you NEVER let the small bottle go completely empty, and keep what the
medical industry called a "safe residual" pressure (200 psi)in it,
contaminants would never enter the system. The whole change in medical
bottle handling was to fix a "problem" that was caused by misuse in the
first place, and really wasn't a problem in the first place if you
didn't leave an empty bottle OPEN to atmosphere for days on end.

You can buy a little two bottle cart at a welding shop, put two M
cylinders of medical or welding grade.. your choice.. put a little
manifold and filling pigtail.. and stand it in the corner of your
garage, and chain it upright for safety. You will have at least as safe
transfilling as was acceptable in the medical industry for YEARS until
recently.