Sat Aug 11, 2012 11:47 am
If you put CO2 pressure into the headspace over uncarbonated beer there will be a huge chemical potential difference across the interface. Thermodynamics says gas will flow across the interface until the chemical potential is the same on both sides. Once there is some CO2 is the beer there is chemical potential difference between the beer at the interface and the beer at the bottom of the keg. Again, thermodynamics says gas will flow across that gradient untill the chemical potential of CO2 is the same at the bottom of the keg as at the top. What thermodynamics does not say is how long this will take (nor really whether it will happen at all). It can take a long time. I usually find that it takes 3 - 4 weeks for a full size (half bbl) keg to come to equilibrium. A recent experience similar to yours and yours lead me to believe that the reason carbonation takes place as quickly as it does is because temperature non uniformities in the cold box set up temperature gradients in the keg and this causes circulation of the beer which mixes it. I suspect that in your (and my) recent experiences these cold/warm gradients did not exist because of the placement of the kegs in the cold box, shielding of the kegs from the fan coil air or some such. But this is a guess.
