# Correlation between temperature drop and pressure drop...?

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#### Rob2010SS

##### Supporting Member
HBT Supporter
Not sure if what I'm looking for exists or if I'm even calling it the right thing. Is there something out there that details the correlation between temperature drop in a fermenter and the related pressure drop?

I have a NEIPA sitting in the fermenter at 68*F. I want to let that temp drop back down to room temp which will be 61 - 62*F approx. I didn't want it to suck up sanitizer through the blow off tube so I pressurized the fermenter (unitank) to 10 PSI to account for the pressure drop that it's going to see.

What I'm curious about is if 10 PSI was enough to counter the pressure drop from reducing temp 7-8*F. Is there anything that you guys know of that can help me figure this out?

#### Dgallo

##### Instagram: bantam_brews
HBT Supporter
Not sure if what I'm looking for exists or if I'm even calling it the right thing. Is there something out there that details the correlation between temperature drop in a fermenter and the related pressure drop?

I have a NEIPA sitting in the fermenter at 68*F. I want to let that temp drop back down to room temp which will be 61 - 62*F approx. I didn't want it to suck up sanitizer through the blow off tube so I pressurized the fermenter (unitank) to 10 PSI to account for the pressure drop that it's going to see.

What I'm curious about is if 10 PSI was enough to counter the pressure drop from reducing temp 7-8*F. Is there anything that you guys know of that can help me figure this out?
Yes 10 psi certainly enough. I set my to 3 psi dropping from 68 to serving temps and it prevents suckback. Assuming since it’s a unitank you should just be able to completely close off the fv completely so there shouldn’t be suckback regardless.

#### Vale71

##### Well-Known Member
For immediate pressure drop just divide the target temp by the starting temp and use this factor to calculate the final pressure by multiplying current pressure by this factor. Problem is you need to use absolute temperatures (degrees Kelvin) so you 'mericans first need to convert. Twice...
The problem is beer will absorb CO2 given enough time if is not fully carbonated (and even if it is but to a smaller extent) so pressure will keep dropping although much more slowly. Calculating this is a bit more complicated, but unless you have a lot of headspace or can pressurize to a very high pressure then chances are you will probably end up with a partial vacuum in the fermenter which is really, really bad.
Your best bet is to leave the CO2 connected all the time which will also start force carbing the beer.

#### Vale71

##### Well-Known Member
Assuming since it’s a unitank you should just be able to completely close off the fv completely so there shouldn’t be suckback regardless.
No suckback but damage from partial vacuum is a concern.

#### Dgallo

##### Instagram: bantam_brews
HBT Supporter
No suckback but damage from partial vacuum is a concern.
I could be wrong on this but it’s my understanding that most unitanks are also rated to some degree of vacuum stress

#### Vale71

##### Well-Known Member
If it's not in the manufacturer's specs I would not want to test it myself. Even if they are, exceeding the rated vacuum capability will still cause serious and irreparable damage so I think it's best not to take any chances in any case.

#### doug293cz

##### BIABer, Beer Math Nerd, ePanel Designer, Pilot
Staff member
Mod
If you leave the pressure connected, you have nothing to worry about. If you just pressurize, and then remove the CO2, you have a possible problem. The pressure drop due to temp drop is minimal, but the pressure drop due to additional CO2 absorption is a big deal. If you have data on beer volume, headspace volume, starting CO2 partial pressure, starting carbonation level (it is not zero), and starting & ending temps, you can calculate the equilibrium pressure in the headspace after cooling. Check out this thread. The linked post in the thread is where I calculated a specific scenario.

Brew on

OP

#### Rob2010SS

##### Supporting Member
HBT Supporter
Yes 10 psi certainly enough. I set my to 3 psi dropping from 68 to serving temps and it prevents suckback. Assuming since it’s a unitank you should just be able to completely close off the fv completely so there shouldn’t be suckback regardless.
Yep, my concern was creating a vacuum. I don't know if they can handle a vacuum so I didn't want to take a chance.

I do not plan on leaving this long term. Once temp stabilizes which should be tonight I would think, I won't need the pressure anymore. I just didn't want massive suckback and once temp is stable, I'm going to just close off the blow off until I keg on Sunday.

#### balrog

##### Supporting Member
HBT Supporter
CO2 will likely continue to dissolve into beer once temp stabilizes; it is not instantaneous, but a matter of hours if not days. Temp will likely stabilize before CO2 finishes dissolving into the volume of beer.

#### Vale71

##### Well-Known Member
Once temp stabilizes which should be tonight I would think, I won't need the pressure anymore. I just didn't want massive suckback and once temp is stable, I'm going to just close off the blow off until I keg on Sunday.
You're missing the whole point. Once temp is stabilized CO2 absorption might still go on for a long time and you'll keep seeing pressure dropping. If you wait until Sunday to transfer you might have created a partial vacuum before then. If you want to play it safe you need to either transfer as soon as target temperature is reached or leave the CO2 attached until then or until beer is fully carbed and absorption stops.

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#### Dgallo

##### Instagram: bantam_brews
HBT Supporter
So I was thinking about this topic this morning looking in my refrigerator. I had a half bottle of non carbonated ice tea closed tight. Got me thinking, if the op pressurized the fv to 10 psi, wouldn’t that cover the 7 degree drop in liquid and air volume based on the formula and he is keeping it at that constant temperature, then the pressure would then equalize by any addition pressure from the co2 being absorbed in to the beer. So I’m just having trouble understanding why it would continue to create a vacuum over time because of this were true, wouldn’t the bottle of my ice tea in the be concave

#### balrog

##### Supporting Member
HBT Supporter
Watch this, for instance. CO dissolves into water and sucks vacuum on soda bottle.

Why doesn't ice tea in partially filled bottle suck in any more N2 (primary component of atmosphere) ? Maybe since it's opened, filled, handled, already absorbing all the air already.

Beer on the other hand (or water) has limited CO2 dissolved, so if you fill headspace with CO2, it'll want to dissolve to equilibrium in the closed environment of the bottle.

Thinking out loud here, by the way. I'm no expert and I don't play one on TV either.

#### bucketnative

##### Well-Known Member
Why doesn't ice tea in partially filled bottle suck in any more N2 (primary component of atmosphere) ? Maybe since it's opened, filled, handled, already absorbing all the air already.
The solubility of N2 in water is roughly 100 times less than that of CO2.

This is why kegged wine is typically served with N2.

#### dbayub

##### Member
I don't think you'd run into any problems after temperature has stabilized. At that point any CO2 absorption and pressure will equalize, and wouldn't go negative, otherwise the gas would come out of solution. Carbonation volume and pressure to maintain something like a corny keg seal would be another story.

#### Vale71

##### Well-Known Member
So I was thinking about this topic this morning looking in my refrigerator. I had a half bottle of non carbonated ice tea closed tight. Got me thinking, if the op pressurized the fv to 10 psi, wouldn’t that cover the 7 degree drop in liquid and air volume based on the formula and he is keeping it at that constant temperature, then the pressure would then equalize by any addition pressure from the co2 being absorbed in to the beer. So I’m just having trouble understanding why it would continue to create a vacuum over time because of this were true, wouldn’t the bottle of my ice tea in the be concave
Solubility of O2 and N2 are minimal and the soft drink in your bottle is probably already saturated with them, so there'll be minimal absorbtion even if you drop the tempperature of your bottle to near freezing. Pressure will drop but mostly due to the gas cooling, further reduction in pressure due to absorption will not even remotely be visible to the naked eye as the bottle might contract just a few microns more.
CO2 on the other hand is very soluble and that is what makes carbonated drinks possible. If you have a significant drop in temperature the beer can absorb quite a lot of CO2 per unit of volume until equilibrium is reached and it will do so given enough time. Since it's a sealed vessel it can only absorb CO2 from the headspace and that means that headspace pressure will keep dropping as CO2 concentration in the gas phase drops as a direct consequence of absorption. Since headspace pressure drops the equilibrium will shift until the two changes (CO2 concentration in the beer and CO2 concentration in the headspace) meet at a new equilbrium point, at which point pressure will become stable again. There are analytical formulas to calculate the new equilibrium point given the amount of beer, the amount of headspace, headspace pressure and the CO2 content of the beer for a given temperature change that I won't go in here. I can assure you that given the right conditions headspace pressure can drop below 14.7 PSI absolute which means a partial vacuum on earth's surface (we usually speak of negative pressure although such a thing doesn't really exist) which could be enough to damage an unprotected vessel. The risk is higher the larger the temperature drop and the smaller the headspace to liquid volume ratio or in other words the more beer there is to absorb CO2 and the less excess CO2 there is in the headspace ready to be absorbed.

#### Vale71

##### Well-Known Member
I don't think you'd run into any problems after temperature has stabilized. At that point any CO2 absorption and pressure will equalize, and wouldn't go negative, otherwise the gas would come out of solution. Carbonation volume and pressure to maintain something like a corny keg seal would be another story.
Yes, it can go negative (meaning less than 14.7 PSI absolute pressure) and that because of the gas going into solution. A corny keg will lose its seal exactly because of that as the lid by its design cannot take any reverse pressure (from the outside to the inside) and will start leaking air into the keg.

#### doug293cz

##### BIABer, Beer Math Nerd, ePanel Designer, Pilot
Staff member
Mod
Solubility of O2 and N2 are minimal and the soft drink in your bottle is probably already saturated with them, so there'll be minimal absorbtion even if you drop the tempperature of your bottle to near freezing. Pressure will drop but mostly due to the gas cooling, further reduction in pressure due to absorption will not even remotely be visible to the naked eye as the bottle might contract just a few microns more.
CO2 on the other hand is very soluble and that is what makes carbonated drinks possible. If you have a significant drop in temperature the beer can absorb quite a lot of CO2 per unit of volume until equilibrium is reached and it will do so given enough time. Since it's a sealed vessel it can only absorb CO2 from the headspace and that means that headspace pressure will keep dropping as CO2 concentration in the gas phase drops as a direct consequence of absorption. Since headspace pressure drops the equilibrium will shift until the two changes (CO2 concentration in the beer and CO2 concentration in the headspace) meet at a new equilbrium point, at which point pressure will become stable again. There are analytical formulas to calculate the new equilibrium point given the amount of beer, the amount of headspace, headspace pressure and the CO2 content of the beer for a given temperature change that I won't go in here. I can assure you that given the right conditions headspace pressure can drop below 14.7 PSI absolute which means a partial vacuum on earth's surface (we usually speak of negative pressure although such a thing doesn't really exist) which could be enough to damage an unprotected vessel. The risk is higher the larger the temperature drop and the smaller the headspace to liquid volume ratio or in other words the more beer there is to absorb CO2 and the less excess CO2 there is in the headspace ready to be absorbed.
This is exactly correct. I linked an analysis I did for a certain set of starting parameters, but will repeat the information here. The net is that headspace pressure dropped from 30 psi to -5.56 psi.

"Ok, I was a little slow getting to the answers. Had to find and modify an old spreadsheet. You can access the spreadsheet here. The sheet should be downloaded to Excel or LibreOffice Calc so that you can use the "Goal Seek" tool (required to get the answer for your inputs.) Inputs cells are all blue, and the calculations work on volume in liters and temps in °C. There are some conversion calcs available on the top of the sheet. (The default sheet has suitable volumes for 5 gal of beer in a standard ball lock corny.) The spreadsheet assumes that the initial temperature is the fermentation temp, and calculates the initial carbonation level for an unpressurized fermentation.

So, I ran a test case for 5 gal of beer in a standard 5 gal ball lock corny, where the initial temp is 20°C (68°F) and the cold crash temp is 0°F (32°F), and the headspace in the keg is completely purged of air and pressurized to 30 psi. The headspace pressure ends up at -5.56 psi (i.e. a partial vacuum.) You can use the spreadsheet to run other sets of initial conditions.

Brew on

#### doug293cz

##### BIABer, Beer Math Nerd, ePanel Designer, Pilot
Staff member
Mod
I don't think you'd run into any problems after temperature has stabilized. At that point any CO2 absorption and pressure will equalize, and wouldn't go negative, otherwise the gas would come out of solution. Carbonation volume and pressure to maintain something like a corny keg seal would be another story.
Nope, the pressure can and will go negative for some conditions. See above. The CO2 absorption and pressure will equalize, but that can be at "negative" (less than 14.7 psi absolute) CO2 pressure.

Brew on

OP

#### Rob2010SS

##### Supporting Member
HBT Supporter
You're missing the whole point. Once temp is stabilized CO2 absorption might still go on for a long time and you'll keep seeing pressure dropping. If you wait until Sunday to transfer you might have created a partial vacuum before then. If you want to play it safe you need to either transfer as soon as target temperature is reached or leave the CO2 attached until then or until beer is fully carbed and absorption stops.
To be safe, I'll just reconnect the gas and keep positive pressure in the headspace until Sunday. When I said my previous comment, I thought once temp stabilized, the beer would absorb the CO2 until the headspace hit 0 PSI and then it would hold steady there. I didn't know (yet) that pressure would go negative.

Staff member
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