Does a given pressure and temperature determine the number of volumes of CO2?

Question

I have seen "force carbonation tables" used by folks who use keg systems. They apply the tables to getting CO2 forced into a beer, and have an ending number of CO2 volumes in mind, then set the regulator to a certain pressure. But is it valid to measure temperature and pressure and derive how many volumes of CO2 are in a bottle of beer?

Going from primary to secondary I had a little too much to fit into the secondary, so I put it in this 3 liter bottle to get an early taste. I squeezed out all the air, put some CO2 on top of it and shook it for a minute or more. I let it sit in the fridge for a few hours, shook it again. Then afer another hour, I did this measurement (53F and 22.5 PSI). Does is this tell me that this beer has about 2.7 (or so) volumes CO2?

Answer 1

If you repeat the measurement a little later or after more shaking and the head pressure is still the same then you have achieved equilibrium and you can assumption about the volumes based on measured temp and PSI are indeed correct.

If the pressure has dropped then the beer is still absorbing CO2, add more and keep going.

BTW: This is the coolest thing I have seen in a long time. I love it when people think out of the box on things I take for granted every day like the carb temp chart. I had never thought to use it that way. This certainly the coolest thing in this site IMO. Cheers. Upvoted and favorited.

Answer 2

A table like that will show you the volumes of CO2 when the beer and headspace have reached equilibrium. If you only had the beer pressurized for a couple hours, the CO2 may not have fully dissolved.

If you check the pressure later and it's stable at 22.5 PSI, that means it's at equilibrium (and has about 2.7 volumes of CO2). If the pressure has gone down, it means that more CO2 has dissolved in the beer in the meantime.

(When you force carbonate using a regulator to apply a constant pressure to the beer, the pressure will never drop and the amount of CO2 in solution will rise until the system reaches equilibrium. But you're doing something different — a fixed amount of CO2, with the pressures equalizing.)