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First off, I'm not suggesting this is better than what's being done now, nor
that it is problem free.  This is something that I have been thinking of
for a while now, and seems to make sense.  I thought I'd bring it up here,
and let The Experts(tm) rip it up and add their comments.

Caveat Emptor:  I am not a technical diver, and I don't play one on the net.

Mixed gas is, from what I have seen, traditionally mixed by pressure.  To do
this well, one needs a fairly accurate (and fairly expensive) gauge, and one
also needs to worry about compressibility of the various component gases,
and the speed at which you fill them, which will affect the temperature of
the mix gas in the tank, and the resulting percentages of the mixture.
Overall, there are several variables that can make it tricky (though far
from rocket science) to do accurately.

But why mix by pressure at all?  Why not mix by weight?

I can get at the local office supply store, for CDN$100, a postal scale that
will measure to 0.1 pound accuracy, with a max weight of 200 pounds.  A
more accurate scale is probably fairly easy to find.  If we assume that a
standard 80 holds 6 pounds of air, then a twin 95 set will hold 14.25 pounds
of air.  If those tanks are filled to 2250psi, the the "guage error" of the
scale is:

	(error)x (pressure) / range == (.1) x (2250) / 14.25  == 15.8

Which is +/- about 15psi, which is more accurate than a digital gauge, at
+/- around 50psi.  

This of course changes for smaller tanks.  A 40cuft stage, for example, holds
about 3 pounds of air at 3000psi, resulting in an error of +/- 100psi, but
this is still quite good. 

From here, calculating mix percentages is easy.  For a 50N/25He/25O mix in
the aforementioned twin 95s.

190 cuft of gas / .79 cuft per mol of gas == 240 mols of gas to fill 
the tank.  That means, we need:

	120 mols of Nitrogen == 120 * 28 grams = 7.4 pounds of N2
	60 mols of Helium == 60 * 4 grams = .5 pounds of He
	60 mols of Oxygen == 60 * 32 grams = 4.2 pounds of O2

This works out to 9.6 pounds of Air (7.4 pounds of N2 + 2.2 pounds of O2,
which is 79% and 21% *by* *molecular count*) blown onto 2 pounds of O2, and
half a pound of He.

Similarly, if you had 1200psi of this gas left in the tank, then the
fractions are:  
	Total weight of gas ==  6.5  (This assumes the tank is full at 2250.
					normally, you'd just subtract the
					empty weight of the tank from the
					current weight)
	6.5 pounds = 2950 grams
	Number of mols remaining = 2950 / (%N2 * 28 + %O2 * 32 + %He * 4)
				  = 2950 / (.5 * 28 + .25 * 32 + .25 * 4)
				  = 2950 / 23
				  = 128 mols

	Nitrogen == 128 * .5 * 28 = 4.0 pounds
	Oxygen == 128 *.25 * 32 = 2.3 pounds
	He == 128 * .25 * 4 = .3 pounds.

It would take a little re-learning, and you would still want to analyse your
gas before diving it, but it seems to make a lot of sense to me...  As I see
it, the pros are:

	1)  Cheaper equipment needed
	2)  Gas temperature is irrelevant 
	3)  Smaller range of measurement results in smaller "gauge error"
		(ie:  you're measuring between 80-100 pounds, not 200 - 2500 psi

Cons:

	1)  % percent of gas == %psi of tank pressure in old system, % gas
		is *NOT* % weight in this system.  May be confusing.
	2)  % error increases as tank volume decreases
	

Comments?

Brian Greenberg
Armchair diver  :)
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