# CO2 absortion while cold crashing

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#### emerinohdz

##### Member
Hi!

We've been (partially) naturally carbonating our beer in our SSB unitank. We set our spunding valve to 12psi when we are a few gravity points away from our FG and leave it on for about 5 days at 66F before cold crashing to 38F.

After a few of days of cold crashing, the pressure gauge reads 6psi on the fermenter.

According to the carbonation charts, when the beer was at 66F with 12psi of pressure, it should had only ~1.5 volumes of co2, however after cold crashing at 38F and with the gauge reading 6psi we should have ~2 volumes of co2. I was actually expecting the pressure to reach closer to 1psi to match the original volumes of co2.

I'm guessing the extra volumes of co2 are being absorbed from the head space, is this correct and if so, is there a way to calculate these extra volumes of co2 that'll be absorbed at cold crash?

Thanks!

#### McMullan

##### wort maker
HBT Supporter
Are you sure you're actually getting ~2 volumes CO2 after cooling? My observations indicate naturally conditioning under spunding - with so many active yeast cells present - doesn't take more than a day or two. Therefore, when you cool the beer, after 5 days, it's unlikely sufficient CO2 is still being produced to increase carbonation levels much. The drop in pressure is more associated with shrinkage than anything. Don't the calculations assume there's a constant CO2 pressure being applied? What happens when the beer temperature increases to serving levels?

OP
OP

#### emerinohdz

##### Member
Are you sure you're actually getting ~2 volumes CO2 after cooling?
I'm not sure about having ~2 volumes of co2, however I'm assuming so based on the amount of pressure I see in the fermenter.
Don't the calculations assume there's a constant CO2 pressure being applied?
I'm not sure about the calculations either, but I'm also assuming when you carbonate using the "set and forget" method, that co2 will be added up yo the desired psi, the regulator will stop once desired pressure is reached and as the beer absorbs co2 more co2 will be released from the co2 tank until volumes of co2 are reached, at which point there will be no more co2 being applied from the tank.
The drop in pressure is more associated with shrinkage than anything.
I thought the drop in pressure had to do with both shrinkage and co2 absorption, is this not true?

What happens when the beer temperature increases to serving levels?
The beer does not feel carbonated enough, I always finish carbonation using a carbonations stone.

#### McMullan

##### wort maker
HBT Supporter
I'm assuming, based on 1.5 vol CO2, you're brewing ales and possibly English ales? I cold crash lagers, but not ales. Try cranking up the spunding valve to about 15 psi and start conditioning/spunding sooner, say with about 10 points left. When finished, cool to serving temperature until ready/bright. This way you can assess it and tweak if needed.

OP
OP

#### emerinohdz

##### Member
Thanks for the suggestion, McMullan. Just to clarify, the desired final volumes of co2 for this beer are 2.5, the 1.5 volumes I mentioned are what I'm assuming are there when spunding based on the pressure, temperature and the info from the carbonation charts.

My assumption is that even if fermentation is very active when you cap the fermenter, since you are releasing the excess pressure with the spunding valve you won't get more co2 volumes added once equilibrium is reached.

Up until now I haven't really been too concerned about this since I finish carbing with the carbonation stone after cold crashing in our SSB unitank, but now I have a small batch being fermented in a keg and I spunded all the way to 25psi (which according to the carbonation chart, at 66F I should end up with ~2.5 volumes of co2). I want to understand the absorption of co2 while cold crashing since I might end up over carbonating beer.

I would probably lower the pressure for now and after cold crashing see if I'm missing carbonation and adjust with the co2 tank, it'll be great if there's an actual way to calculate this though so I can lower my margin error for subsequent batches.

#### DavidWood2115

##### Well-Known Member
Hi!

We've been (partially) naturally carbonating our beer in our SSB unitank. We set our spunding valve to 12psi when we are a few gravity points away from our FG and leave it on for about 5 days at 66F before cold crashing to 38F.

After a few of days of cold crashing, the pressure gauge reads 6psi on the fermenter.

According to the carbonation charts, when the beer was at 66F with 12psi of pressure, it should had only ~1.5 volumes of co2, however after cold crashing at 38F and with the gauge reading 6psi we should have ~2 volumes of co2. I was actually expecting the pressure to reach closer to 1psi to match the original volumes of co2.

I'm guessing the extra volumes of co2 are being absorbed from the head space, is this correct and if so, is there a way to calculate these extra volumes of co2 that'll be absorbed at cold crash?

Thanks!
When you cold crash beer in a fixed volume, sealed fermenter, there are two effects that reduce the headspace pressure. The first, contraction of the beer as it cools, is actually a very small effect: for 5.5 gallons of beer the shrinkage will be only a little over an ounce when crashing from 66F to 38F. The ideal gas law predicts that this would result in a drop of gauge pressure from 12 psig to about 10.5 psig. The second, much larger factor is, as you correctly surmise, the absorption of additional CO2 into the beer. There are a fixed number of CO2 molecules inside the fermenter, some of the molecules in the headspace will be absorbed into the now colder beer, increasing the volumes of CO2 and decreasing the head pressure. Assuming you have 5.5 gallons of beer in a 7 gallon fermenter with an extra gallon of headspace in the lid, my spreadsheet predicts a final gauge pressure of 5.96 psig (including the beer shrinkage) which is stupidly close to what you observe.

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