Is it possible to know how many volumes of CO2 end up in a keg after force carbonating?

Question

There are two methods used to carbonate a keg:

1. Set and forget
2. Force carbonation

In the first method, the keg's PSI is set to a value determined by a kegging chart and is left at that PSI for the duration of the beers life. Some time after 1-2 weeks, the beer is fully carbonated and the exact volumes of CO2 in the beer is known.

In the second method, the keg's PSI is set to around 30 and left there for 24-36 hours, before being brought down back to "serving pressure" which is typically the same value as set in the first method. Some people may purge the keg before dropping the PSI, and others may shake the keg.

I have heard that when we force carbonate, it is not possible to know the exact volumes of CO2 in the beer. Is this true? I would think that the beer would eventually equalize out to the PSI at the serving temperature. If this is not true - why isn't it? If I set the PSI to 30 for only 1 hour (instead of 48), does the same apply? How about 1 minute?

Answer 1

Its not impossible to know how much CO2 is dissolved in the beer. There are tools for measuring it. Many professional breweries use these tools because have an exact carbonation level is important to them. As homebrewers we don't normally buy these tools so we can make random measurements.

That said. It is well known how many volumes of CO2 are dissolved at equilibrium when the temperature and pressure are known. This is why the set and forget method works. You decide what volume of CO2 you want, pick a PSI rating based on your fridge temp and given enough time the beer reaches equilibrium at the predicted CO2 volume.

The High PSI method, or rapid method, is a little more tricky. Without using the previously mentioned dissolved CO2 tools you don't really know the PSI. What people are relying on there is that they can quickly get the volumes of CO2 up to an acceptable taste/feel level in the beer. Then once they set the system back to serving pressures, given time, the beer equilibrates to the predicted volumes of CO2 from a temp/PSI chart.

If you leave beer on at 30PSI for too long, it will equilibrate to that volume of CO2 and be over carbed. The higher the PSI the quicker it will go, but 1 minute isn't likely enough time for that to happen.

And a word on terminology. Both methods you ask about are forced carbonation. The high PSI method is just a bit faster and less predictable.

Answer 2

I've adopted a force carb method at serving pressure that carbs to desired volume in 15-20 minutes. This way you can control the volumes using the easy temp/pressure charts for a desired co2 volume.

Set your tank to serving pressure, in my case 12 psi. With the keg and beer at serving temp (38°F), attach gas to the out lock (dip tube) so the co2 enters the bottom of the tank. Then in the upright position I tip it slightly on its edge and make a swirling motion to get the beer moving, after 15 minutes or so you won't hear bubbles anymore and your beer is carbonated.

While this isn't total saturation of the co2 it seems to be within 0.2 volumes as I've compared the pours immediately and after a week with little difference.

While there is math to calc exact volumes it requires many variables just not available to a home brewer, exact submerged pressure (factoring beer depth and gravity), co2 diffusion on contact with beer (tube/stone/surface), duration.

I've overcarbed too many beers using over the serving psi, to do anything different.