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>Posted on 19 Jun 1995 at 10:36:27 by /R=IML/R=AM/U=r_larocque/L=IML/TN=( >When I first heard of blending nitrox using the partial pressure >method, I was told that this leads to stratification in the tank. Thermodynamic equilibrium doesn't depend on the initial conditions. In a very careful analysis one would have to look into just how fast a mixture reaches equilibrium but for gasses in a 10 liter bottle I doubt you'll find any *change* in the mixture (gradients) if the fill takes place through one orifice firing down into the common volume, i.e, I assume the mix is near complete when the fill is over (except maybe the last bit that entered as the valve was closed). Now without giving any numbers it is also safe to say that the higher the concentrations the slower the mixing. Possibly by the time we hit 3000 psi things mix a bit slower but on the other hand, the dynamics and details of the filling itself will change as the densities of the gasses increase. I.e., it unlikely that any sheltered pocket can exist. Not only that, the process of compression (filling that is) will force a new and different gas into any virtual gas pocket that might "exist". Working to our advantage (mixing) is the increase in temperature throughout the fill. Stratification will be present in the final state: the densities of the gasses are different and subject to the force of gravity but the problem will be no bigger than that in an ordinary tank of air stored for a few days. >In fact I think it would be very difficult to maintain an O2 and >N2 gradient! You mean without keeping them in separate compartments :-) ? I'd say it is totally impossible at room temperature. >But we still hear this argument quite often and, in Canada at least, >there are many individuals (and maybe even a certification agency) >that believe that stratification can happen and will cause O2 toxicity >problems!? Maybe the concept is used because there is another problem somewhere in this method ? john
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