On 6/20/2010 7:54 AM, Some Guy wrote:
Ed Pawlowski wrote:

You typically never allow the pressure to drop to zero because
higher pressure in the tank is the only way to keep ambient
moisture from entering the tank. Ultimately it's to keep the
tank from rusting on the inside.

That is exactly why medical oxygen thanks have to be evacuated.
Rare that one would come back with any pressure at all.

I would think the contrary.

O2 tanks used by hospitals are more likely to be part of a manifold
system, and as such will always be maintained at some positive pressure
by virtue of the fact that at least one of their "gang-mates" is likely
to have enough excess pressure to keep them partially pressurized.

If a gang of O2 tanks at a hospital collectively fall below some
acceptible level of pressure, then they're no longer useful as an air
souce and MUST be changed out. So again the argument here is that
medical O2 tanks are MORE likely to be returned while still containing
some positive pressure charge.

If the strongest argument so far is that a "medical-grade" tank of O2
has lived it's life with minimal to zero infiltration of atmospheric
humidity (or even nitrogen) compared to a welding tank, then that's a
pretty weak argument to say that a tank of welding O2 is unhealthy to
breath. Last I checked, we all take in some some water vapor and
nitrogen when we breath standard air.

In other words, a lack of "purity" does not equal unhealthy or hazardous
for human breathing. A lack of purity (it seems) will degrade welding
performance, maybe mess up equipment, etc.

And so far, it's just been pure speculation here that tanks of welding
O2 are *not* evacuated prior to filling, just as supposedly medical
tanks are.

I can see a huge LOX tank next to a main traffic artery in the Southside
neighborhood of Birmingham where UAB Hospital is located. Tanker trucks
pull up next to the thing and fill it on a regular basis. The
maintenance guys who work for the complex tell me there are tunnels all
around under the place filled with all sorts of conduits and pipes that
distribute various electrons, liquids and gases that keep the hospital
alive. I imagine that LOX tank supplies O2 to the whole hospital and
perhaps a couple of different hospitals in the same general area. The
hospitals share doctors, why not oxygen?

TDD