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From: ddrake01
I have to backup a few of the statements and put my $.02 in.
I agree with some of what Richard P., John, William E. Sadler have said,
but I don't think everything came out real clear.

First of all, He2 is not nearly as heavy as O2 or N2 which are nearly the
same atomic weight.  The densities of the gasses follow suit.  Therefore,
N2 and O2 mix readily (given time and agitation), while He2 will need a
little more effort, but will not stratify.  Argon being very heavy relative
to the above gasses will tend to slide to the bottom of the mix tank initially.
Theses gasses are similar to like liquids with respect to flow and mixing.
Ever added some water or whiskey to cola?  Ever notice the swirls as the
alcohol and cola mix?  I don't know as there is much difference between
"shaken, not stired", but time and agitation helps the "blending" process.
Fast fills will blend the gasses better due to the velocity of the gas
going into the tank and the increased movement due to higher temps (energy).
"like" liquids refer to liquids that mix vs. oil and water which do stratify.

Rolling tanks for 5 minutes does help.  Is it always needed? How long do
you need to do it? I don't know, but I do it, especially with large tanks
that had slow fills like I do mine as advised from the list.  If you have a
gas that is at say 1000 psi, it will tend to compact in the bottom of the tank
as you raise the pressure towards 3500 psi or whatever you're filling to.
If the velocity of the gas going in is sufficient to create enough turbulence
to mix the gases well, then you've got a homogeneous mix. If you fill slowly,
then you have very little turbulence and mixing due to physical factors.  The
chemistry of gasses then comes into play which says that they will mix on
their own time schedule if you leave 'em alone. Agitate, and the girls dance
with the boys sooner. With tall tanks, you will not have as much success with
blasting the top off mix into it and have it get all the way down to the bot-
tom to blend with the first gas(es).

As has been pointed out, He2  may leave the scene altogether over time...
If you really want to get fancy, you can put a magnetic rod coated with
teflon in the non-magnetic tanks and use a rotating magnet under the tank
to stir the mix effortlessly.  This is commonly used in chemitry labs to
mix fluids in a big beaker when it is important to maintain an even mix.

Of course, you will be cutting down on your volume for gas and you'll most
likely be the first to "experiment" with your life support gas in this
manner...I wonder what people would say and look like when they said,"
your tank has something rolling around in it" , and you reply "yea, I know,
don't worry about it!"

Oh well, I'll stop rambling now.  There simply is not a big mystery to
this mixing/blending thing.

David B. Drake       ahdnn1a.ddrake01@ed*.co*
Romulus, Michigan, USA EDS/GM Powertrain 8-375-5467 (GM NET)

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