Tom Horne wrote:

Is there any difference between a tank of welding oxygen vs
medical oxygen (...)

And just to be clear -

Welding oxygen is more (way more) than just compressed "air".
And what I mean by "air" is the stuff that's all around us
right now.

Yes?

To be precise it is way less. The air we breath is roughly twenty
percent oxygen.

And what I meant by "way more" was that welding oxygen has a higher
oxygen content (or oxygen concentration) vs ordinary air. So I don't
know why you'd say it's "way less".

Medical oxygen is nearly one hundred percent oxygen.

And likewise for welding oxygen - yes?

Harry K wrote:

Yes there is a difference according to my first aid training (years
ago). You can use in the case of emergency. It is IIRC too dry to
use for extended periods (I should have paid more attention to that
discussion).

From what I've been reading tonight, ALL forms of compressed oxygen
(Aviation, Medical, Welding) come from the SAME source (a tank of Liquid
Oxygen - LOX) and are transfered to variously labelled tanks and charged
various prices based on the label on the tank.

My guess is that the price differential is caused by liability insurance
and the need to recoup that cost based on the end-use of the gas. The
insurance industry might perceive that aviation oxygen (as a product)
carries the highest risk to the producer / seller, with medical oxygen
less risky, and welding oxygen the lowest risk. Risk in this context
means what sort of incident could happen if the wrong gas is
accidentially sold to the end user, or could happen if the tank fails.

The humidity of compressed oxygen seems to be a red herring. In medical
situations such as the hospital bedside, oxygen supply lines are passed
through a bubbler or some other humidification device to add humidity to
the air. This is a stationary situation where the person is likely to
be on the air supply for an extended period, and humidification is done
more for comfort or to prevent airway irritation than anything else. In
other medical situations (EMS O2 respirator tanks) the air is dry -
because it simply can't supply O2 for an extended period anyways.

And you don't want to get water in your high-pressure tanks anyways - if
only so they don't rust.

Aviation air also can't contain a lot of humidity because (or so the
story goes) the water could freeze at high altitudes and mess up the
supply and metering lines.

So the bottom line is that if you walk into a welding supply store to
buy an oxygen tank, don't let on that you intend to use it to fill your
plane's on-board tank, or you want to make an oxygen tent for your sick
pet. The guy behind the counter will most likely go ape-**** and either
deny your purchase, or force you to buy the more expensive tank -
probably because their insurance company forces them to do that.

The insurance industry plays a far larger role behind the scenes in our
daily lives than we realize. The products we can buy, the services we
use, the way they are delivered or sold to us, etc, exist because the
manufacterers, retails or providers have reached a stable (perhaps even
strained) coexistance with the insurance industry.