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Hi Pete: A number of people wrote me about their experiences with unmixed gas. Being susceptible to reason and evidence, i'll outline a simple "theory" of estimated mix time (good to within a factor of 10, lets say). This is just a scratch calculation (really just dimensional analysis) to make a reasonable guess on how long it takes one gas species to diffuse into another. Reality is complicated for binary and 3-nary mixtures--the diffusion coefficients of each species depend on local composition and may be independent of other species. (if you want to flog yourself look into: "Diffusion of Carbon Dioxide into a Helium Nitrogen Mixture," J. Chem. Phys. 32,1314 (1960).) for these purposes, consider nitrox. O2 and N2 are about the same size and mass, so it's reasonable to assume one diffusion coefficient. now, a diffusion coefficient (D) measures two things: how fast a molecule travels (v) and how far it travels before colliding with another molecule (d). A large diffusion coefficient leads to rapid mixing. D = v*d [cm^2/sec] a characteristic time can be obtained by L^2/D [sec], where L is a dimension characteristic of the system (say a tank, L ~ 10 cm). at 1 ata: v~ 5*10E+04 cm/sec; d ~ 2*10E-05 cm, so D ~ 1 cm^2/sec for an empty tank. The characteristic mix time would be about: ((10cm)^2)/(1cm^2/sec) = 100 sec ~about a minute. at 200 ata: v~ 5*10E+04 cm/sec; d ~ 10E-07 cm, and D ~ 5*10E-03 for a full tank. The characteristic mix time would be about 20,000 sec or 5.5 hours. These values are just rough guesses. In some senses they represent an overestimate of time to mix because they don't account for the convection and turbulence that occurs due to gas flow during filling. These processes should reduce the characteristic length L over which diffusion occurs. 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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