[Mikey]

The shrinkage of the air/CO2 is only about 4% when you go from 20C to 0C. The shrinkage of the beer is much smaller. This means it is the increased solubility of the CO2 that is the problem here.

Kai

Yes, but you still get a vacuum in your keg.

[kerneldustjacket]

This is what I've been trying to explain in another thread. If you only put 20 pounds on the head, that will be absorbed by the beer and, coupled with the temperature drop, will leave you with a low pressure and maybe a vacuum in your keg. I know this from "experience".

Aha! So sort of like leaving helium balloons tied to a mail box on a cold day...the cold "shrinks" them a bit, plus the helium starts to diffuse out of the balloon?

I had this happen with a keg and all along I thought the keg had a small leak. Thanks Mikey!

[tschmidlin]

The shrinkage of the air/CO2 is only about 4% when you go from 20C to 0C. The shrinkage of the beer is much smaller. This means it is the increased solubility of the CO2 that is the problem here.

Kai

Yes the increased solubility of the wort is the problem, but that doesn't mean the fullness of the keg isn't important.  If the keg is 50% full you'll have a lot more CO2 in it at 20 psi than you will if it is 99% full.  The increased solubility of the wort will only absorb so much.

But as I said, if your keg is very full, as in very full, (perhaps I should have used more emphasis) even a small amount of shrinkage can have a huge effect.  Am I the only one who tries to fit 6 gallons into a 5 gallon keg?

[Mikey]

Just spend about 5 minutes rocking and rolling your keg at 15-20 psi and your lagering worries will be over. You'll have near carbonated beer ready for drinking, It will also have had plenty of time to settle and won't be disturbed by force carbing later.