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Name: Jessica D.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 7/15/2004


Question:
Companies that manufacture gas mixing systems claim that, over time, gas mixtures such as nitrox and heliox will stratify into their original components. I have read that scuba divers traditionally roll their tanks before using them in order to mix the contents and disturb the uneven distribution of gases. However, I have also read conflicting articles that claim that compressed gas mixtures will not separate once properly mixed. Who is right? Do homogenized gas mixtures stratify over time?


Replies:
Bad Science!! Mixed non-reacting gases don't "unmix" upon standing assuming one or more don't condense. The decrease in (free) energy of the mixing of ideal gases (all gases at normal conditions are essentially ideal) is: [Energy Decrease] = -RT * {SUM[Xi * ln(Xi)]} where R = gas constant = 1.987 cal /k *mol, T = absolute temperature in kelvins, SUM means add, Xi is the mole fraction of gas "i" = Ni/{SUM[Ni]} where the SUM is over all "i" gases, Ni = number of moles of gas "i". Since Xi < 1 always, there is a decrease in the energy. There is no "unmixing". Same is true for mutually soluble non-reacting liquids as well.

Vince Calder


Jessica,

Once mixed, gases will not "un-mix" or stratify if allowed to stand undisturbed. This is true even if the gases are of different molecular masses. All that is required is that the gaseous components be in the gaseous state at the temperature and pressure extant inside the cylinder. If that condition is met, they will not stratify no matter how long they're allowed to stand. Rolling scuba tanks around to mix their contents is a tradition ungrounded in scientific fact.

Regards,
ProfHoff 875



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