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This is all speculation based on freshman physics:

1) Mixed gas will not "unmix" or stratify in a tank.

This is true for a number of reasons: 
(i)Entropy (=S). The gas "prefers" to be in a DISORDERED, mixed state. 
Just as you don't see a broken (disordered) piece of china reassemble 
(ordered)itself in your wreck-bag, gas will not ummix in a tank.

(ii) Internal energy (=U). The gravitational potential energy (gravitational 
chemical potential for you p.chemists...) of a molecule 
differs so slightly from the top to bottom of a tank that the thermal energy
of the molecules overwhelms any tendency to stratify (this is not true if 
you look at the earth's upper atmosphere, though. In an (oversimplified) 
isothermal atmosphere in diffusive equilibrium, the light molecules will 
escape the earth's gravity. this is why martians talk like donald duck. the
process requires 10s of kilometers of height to be noticeable--no tanks are 
that big).

(iii) put i and ii together for the free energy: F  = U - TS. F is minimized
(see your IAND thermodynamics text) for a given molecule when S is as 
large as possible--disorder.

2)I'll bet the tank tumbling (or leaving your tank at the shop over nite 
for "molecular mixing") is bs.

Look at the diffusivity of a typical gas at scuba pressures to estimate 
the propagation time of a molecule from one side of a tank to the other. you 
will find that it takes ~1-5 minutes for say helium to diffuse through 
air. diffusion is a relatively "slow," thermal process, however this is 
NOT how scuba gas gets mixed. 

To do a good mix job, start with a tank of thermally equilibrated O2 at 
the desired pressure. "blast" (1-10 cfm)
in the 2nd gas right up to the calculated fill pressure, then wait for 
the gas to cool from the "dieseling" compression and top off to the 
correct pressure. repeat for the 3rd gas (air in a 3mix fill). This is 
not diffusion here...big convection. Once the blast stirs things up, 
the different molecules come into thermal equilibrium with each 
other and the walls and diffusion takes over to rapidly finish the mix 
job.

Gas will mix thermally (by itself!) in just a few minutes (max).

The big sources of mixing errors are likely:

1)RULE NUMBER ONE
2)Temperature control
3)bad O2 analysis (re #1)
4)Compressability for ppHe>500psi, ppO2 & ppN2 > 4000psi.

Regards, em
 


_____________________________________________________________.sig
Eric Maiken                    email: eapg243@ea*.oa*.uc*.ed*              
Dept. of Physics                   o: 714 824-6621   
U of California                  fax: 714 824  2175
Irvine, CA 92715-4575

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