This has been nibbling away at my brain for a while now. I purge kegs by filling them to the brim with star san and pushing it out with co2 but I don’t transfer under pressure because I don’t have pressurized fermenters. I know a lot of folks that just fill and purge the headspace and I’ve done that as well and I’ve never had a problem with oxidation. But I suspect this is because of active yeast more than having actually purged the keg effectively. So the other day I say down and flexed my Google-fu. The law of partial pressures states that the sum of the pressures of each distinct gas in a given volume equals the total pressure in that volume. I interpret this to mean that if I were to pressurize a keg to 10psi with normal air there would be about 2psi of o2 pressure in the keg.
Assuming this is correct, and I’m not a scientist so it may be wrong already, if I have a keg with normal air at 2 psi or whatever normal atmospheric pressure was at the time I have about 0.4 psi of o2. Now I can pressurize the keg with pure co2 to 10 or 20 or even 40 psi but there will still be 0.4 psi of o2 there.
The accepted wisdom is that if you then dump that pressure you end up with a significantly reduced amount of o2 because the 20% in normal air was diluted by all the pure co2 and when you released the pressure the o2 left at a rates proportional to the total pressure inside the keg.
But the partial pressure of o2 outside the keg and inside the keg will equalize no matter what so you will always have 0.4psi of o2 inside the keg. Am I missing something? Or is purging the keg completely useless?
If you purge many times you can get to a very low level of O2.
The amount that will go back in due to the partial pressure depends on the diffusion trough the rubber gaskets, not zero but pretty low I think.
On bottles there are better crown seal liners that let less O2 diffuse in. Sierra Nevada changed back around 2008 to those liners, but to be effective they had to go with pry off as that gave a higher clamp load to make it all work.
but when you open the PRV it’s no longer a case of how much enters past the gaskets the pressures will equalize very quickly between outside and inside through the PRV. because the pressure imbalance inside the keg is only the co2 (39PSI inside vs whatever tiny pressure outside) but the o2 won’t travel because the partial pressure inside the keg is equal to that outside.
The air (oxygen, nitrogen, co2, etc) in the keg before filling should be at local atmospheric pressure. When you close the lid and charge the keg to 10 PSI that is at gauge pressure. Gauge pressure disregards the local atmospheric pressure. That is you will be 10 PSI above atmospheric pressure.
When purging, I think the idea is that oxygen is lighter than co2 so oxygen should rise to the top of the keg. Then when you pull the relief valve the oxygen escapes. IMHO, for that to work you should pause bit between pulls of the release valve to allow the gas to reorganize.
co2 is heavier then o2. I put a out connector on the co2 line and connect to the keg. I leave the top unlatched and fill for about 10-15 seconds and then close the latch. This should purge any o2 out the top. Then when filling the keg it should displace any left over o2.
Just my process.
Opening the PRV equalizes the total pressure very rapidly. Equalizing concentrations at the same pressure takes a very long time because that is diffusion driven.
Something else to consider alongside the law of partial pressures is the ideal gas law. Thus, it doesn’t account much for things like striation due to mass/temperature differences.
Has anyone ever done an exbeeriment comparing the effects purging versus not purging? If O2 is lighter than CO2 then maybe all the O2 (a relatively small amount) will slowly make its way to the top of the keg and there will always be a layer of CO2 between the O2 and the beer. If the O2 is always isolated from the beer maybe purging is not even necessary.
I’m not worried about O2 just sitting in a layer above the CO2 while the keg in use. I’m worried about the CO2/O2 mixing and contacting the beer as it sloshes around whenever the keg is moved.
Also I’m not sure how much time it would take for such layers to develop. Also, I’d wager that there aren’t two completely distinct layers, but rather a slow shift from mostly CO2 to mostly O2.
I’m not worried at all. as i said, I’ve never had a problem. I suspect, as Mark (SCerv) would point out, unless we are filtering our beer there is plenty of yeast still active to clean up any introduced o2. I’m just curious.
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.
It should be testable actually. put an o2 sensor in a keg, off the bat it should read = atmospheric concentrations. Pressurize the keg with co2. there are now the same number of o2 molecules as there was before but lots more co2. when you pull the PRV until it stops hissing the o2 sensor will either read the same as at the begining or much lower. I don’t know that it matters but it’s interesting.
I always looked at it as more of a volume thing. The keg is full of O2, by adding CO2 at some pressure above ambient forcing some CO2 into the keg. Then allowing the CO2 to settle and O2 to rise, purging the keg removes O2. Multiple purges removes more O2. Higher pressure with a sealed keg only allows more CO2 to enter, thus reducing the amount of purging steps required. I “purge” at lower pressures 10#-15#, then fill and seat the lids at 30# to seal it up and check for leaks. I’m definitely no scientist, just my process.
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.
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.
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.
