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>
>
> On Sun, 28 Aug 1994, Jason Rogers wrote:
> snip
> > Richard Pyle writes that he uses industrial grade oxygen.
> > I've always used medical grade [B.P. - US equivalent is U.S.P grade I think]
[Snip...]
> In certain parts of the U.S.A. you need prescription to buy Medical
> Grade O2. I believe it is by state law but it was done so that people
> who need it for their health could get reimbursed by their insurance
> companies. Over the counter items are not reimbursable. It helps out the
> older people on fixed incomes.
> I think the procedures set up are mainly to protect idiots from themselves.
> I think most of us don't fit that classification. Aviation grade might be
> considered a better choice than welding O2 and you don't need the doctors
> permission.
> Ronnie Bell
> just back from a week cave diving in Florida!
>
To clarify the differences between grades of oxygen in the U.S.A.;
At a recent DAN O2 course, it was pointed out that welding,
medical and aviator oxygen differ only in the water content of the gas
and the certification of the gas. Medical and welding grade oxygen
have the same water content, but medical grade gas is tracked as Jason
Rogers and Frank Deutschmann have already pointed out. Aviator oxygen
is ultra low water content, so that at -80(at 50,000'), the oxygen
regulators won't freeze due to the water in the oxygen. There is a
fourth grade of oxygen, but it isn't available for sale in the US and
I would have to check my notes for its specifications-my recollection
is that it is wetter than all of the above.
In the US, medical oxygen is usually sold in indexed pin
tanks, which use regulators with a matching pins- this is to prevent
doctors from inadvertently supplying a patient with pure halothane
anethesic instead of oxygen. Depending on where you get your oxygen,
this may complicate things.
As an aside, oxygen for sale is usually manufactured by
cryogenic means. Air is compressed, cooled, drained of water, passed
over catalytic scrubbers to remove CO and methane/hydrocarbon gases
and then expanded at about -80, which removes the CO2, recompressed
and recooled, expanded to give liquid air. The liquid air is then
differentially boiled to give gaseous nitrogen (boiling point 77.4K),
argon(87.5K), then oxygen(90.1K), then methane(109.1K), then
krypton(120K) and so on. So, there are two ways to get a second gas
into your oxygen tank. The first is that the stream contains some
argon and traces of methane, because the boiling tower was not run
properly. (Hard to do, but conceivable.) In that case, you would have
traces of argon and methane in your tank. More likely, someone can
contaminate the gas at filling, either by leaving another gas in the
pipes before filling your tank (very improbable) or by filling oxygen
into a tank that contains something else (good suppliers will vent and
purge tanks, for their own safety if nothing else). If you buy your
own tank, and have it refilled, the chance that you will get someone
else's tank that contains back flowed gas Y into an oxygen tank will
be diminished.
If the oxygen is being made at a small plant, they may use
pressure swing absorbtion to boost the O2 content to 80-90% before
starting the process. Here, most large welding shops use pressure
swing absorbers to supply the oxygen for the welding rigs.
I hope this helps,
Peter David
david@la*.st*.ed*
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