I plan on ussing a bag/balloon filled with CO2 attached to the fermenter when cold crashing. I want to calculate how big this bag needs to be.


  • Fermenter Volume: 30l (23l beer)
  • Room Temp: 20° Celsius
  • Beer Temp before Cold Crash: 20° Celsius
  • Beer Temp after Cold Crash: 3° Celsius

How can I calculate the volume CO2 that is sucked into the fermenter?

My approach was to look at the Density of CO2 at the different temperatures:
1.815 g/l at 20° C
1.923 g/l at 3° C
So at 20°C there are 54,5g CO2 inside the fermenter at 3°C there should be 57,7g CO2 in the fermenter, the difference would be 1.8l CO2 at 20°C. I feel that I'm missing something, probably something about the CO2 in the beer.

  • 1
    You might be able to look at the co2 absorption of beer at x temperature. The capacity is different at different temperatures, and is calculable, so you could figure out what volume is in the beer at 20C, subtract from that what will be dissolved at 3C at saturation at atmospheric pressure, and that should be your volume. Assuming you calculate your dissolved co2 in volumes that is. Does that help at all, or have I missed something? ETA: the total volume of the headspace and ballon would need to match or exceed the volume of co2 to be made up.
    – Frazbro
    Commented Jan 13, 2020 at 3:22
  • As another note, I'm assuming that you're just hoping to reduce air ingress on cold crash? If you have a gas cylinder, you could put an on demand valve (cask breather) on, which would negate the need for a vessel with volume to hold the gas.
    – Frazbro
    Commented Jan 13, 2020 at 3:25
  • This makes sense @Frazbro. I added an answer with according calculations. Commented Jan 13, 2020 at 13:01

1 Answer 1


Per the suggestion of @Frazbro

At 0 psi there are 0.85 volumes (1.68g/l) of CO2 in the beer at room temp and 1.47 volumes (2.91g/l) at cold crash temperature. So the difference is 1.23g/l. For 23l 28.29g CO2 are required, which will translate to 15.56 liters CO2 at room temperature. This number seems more reasonable.

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