Yes, when purging an “empty“ keg, you want to connect the fermenter outlet to the liquid post of the keg, and connect the gas post to some kind of airlock, but not for the reason stated in the quoted post.
Pushing CO2 into the bottom of the keg will actually minimize mixing of the CO2 and air, which will enhance to the CO2’s ability to push air (and O2) out of the keg.
CO2 is more dense than O2 or N2, so will tend to fall to the bottom of the keg if fed from the top, and stay pooled in the bottom if fed from the bottom. Falling from the top will encourage mixing, thus reducing any stratification. Any stratification is only temporary however, as over time (a matter of minutes) gas mixtures homogenize completely due to diffusion.
In my original analysis I estimated how fast CO2 would be pushed into a keg vs. how fast the CO2 would mix with the gas already in the keg (assuming mixing only due to diffusion, no convection currents, or turbulent flow to speed mixing.). Turns out the rates are similar which means that some of the gas in the keg gets pushed out before it mixes completely with the incoming CO2. To the extent the existing headspace gas gets pushed out before mixing, more of the preexisting O2 will be pushed out, than for the case of complete mixing.
The actual case of partial mixing in the keg being purged was too difficult to analyze, so instead I did a worst case analysis. For purging, the worst case is when the incoming CO2 completely mixes with the preexisting headspace gas, before any of the existing headspace gas is displaced from the keg. The worst case analysis came out at 5 ppb residual O2, which means the actual should be even less. The worst case analysis showed the fermentation gas purging method was more than adequate to purge O2 to insignificant levels.
Brew on
