[Joe Sr.]

I'm not talking about diffusion, the idea is that the o2 molecules are forced out when you open the PRV but why would that happen? the o2 pressue inside the keg is at atmopheric. there wouldn't be any exchange of o2 because there wouldn't be a pressure differential inside and outside. when you open the PRV the partial pressure of co2 inside and outside the keg rapidly equalizes. But with the o2, because we have not added any additional o2, there is no equalization so no o2 leaves.

I'm no engineer, but my assumption is that when you depressurize the keg so quickly everything in there comes blasting out.  The o2 inside the keg gets blown out along with the co2 and whatever small amount of beer comes with it.

The law of partial pressures, which I have not researched, probably assumes ideal conditions.

Blasting 30psi of gas out of a keg rapidly is likely a very different condition.

[Philbrew]

I'm not talking about diffusion, the idea is that the o2 molecules are forced out when you open the PRV but why would that happen? the o2 pressue inside the keg is at atmopheric. there wouldn't be any exchange of o2 because there wouldn't be a pressure differential inside and outside. when you open the PRV the partial pressure of co2 inside and outside the keg rapidly equalizes. But with the o2, because we have not added any additional o2, there is no equalization so no o2 leaves.

I'm no engineer, but my assumption is that when you depressurize the keg so quickly everything in there comes blasting out.  The o2 inside the keg gets blown out along with the co2 and whatever small amount of beer comes with it.

The law of partial pressures, which I have not researched, probably assumes ideal conditions.

Blasting 30psi of gas out of a keg rapidly is likely a very different condition.

My gut says you're correct.  Ideal conditions need time for phenomenon to occur.  The O2 is going to get swept out in the rush as part of a mixture of gasses.  But co2 settling (if it actually happens) would also take time.  Probably hours or days for a significant co2 layer to form on top of the beer.

[erockrph]

My gut says you're correct.  Ideal conditions need time for phenomenon to occur.  The O2 is going to get swept out in the rush as part of a mixture of gasses.  But co2 settling (if it actually happens) would also take time.  Probably hours or days for a significant co2 layer to form on top of the beer.

Stratification of CO2 is a myth at the scales we're dealing with. You need a perfectly calm column of gas on the scale of kilometers for gravitational effects to have a significant effect on a gas blend.

[BrewArk]

Time is required for diffusion.  An open beer will go flat, but not instantly.

[Jimmy K]

Because the partial pressure stuff really only applies to diffusion. Gases mix and behave as a single solution in a situation like purging a keg. Partial pressure forces are tiny compared to the energy of releasing a valve.

[Phil_M]

Venting a keg doesn't fit the "ideal gas" ideal at all, hence why the partial gas law doesn't make much difference.

My physics is rusty, I'll admit. I work with electronics, not gases.

[hopfenundmalz]

Because the partial pressure stuff really only applies to diffusion. Gases mix and behave as a single solution in a situation like purging a keg. Partial pressure forces are tiny compared to the energy of releasing a valve.

This is very true. Diffusion due to partial pressure may take hours. It can cause O2 to get under a bottle cap through the liner, given enough time. When we purge the keg, it all flows out.

[HoosierBrew]

Because the partial pressure stuff really only applies to diffusion. Gases mix and behave as a single solution in a situation like purging a keg. Partial pressure forces are tiny compared to the energy of releasing a valve.

This is very true. Diffusion due to partial pressure may take hours. It can cause O2 to get under a bottle cap through the liner, given enough time. When we purge the keg, it all flows out.

My thoughts, too.