Keg pressure not reacting as expected when cold crashing

[MagicMatt]

Hey hopefully someone can clue me in to what's going on here. I understand how pressure changes with temperature, or so I thought......


I've had 3 full, carbed kegs aging at room temp (~75°F). I am serving these kegs at an event in 7 days, and I need to bottle a few beers of each for a competition. Last night around 8pm, I checked the pressure on one of my kegs and it was at 32psi as expected. I put it in my keezer to chill (not connected to anything, just sitting in the freezer).

This morning I just checked the keg, and it is at 36°F but it's still at 31psi. How is that possible? I expected it to go down to ~11psi, but that didn't happen. It seemed to not change at all. How is this possible?


I ended up purging a bit of pressure and got it to drop to about 12psi. I'll leave it there and see if it creeps back up, but seriously, what gives?


Edit: P.S. I am using one of these to check the pressure.

 

[Subdivisions]

Gas pressure changes with temperature but not a whole lot between 75 and 35. If you'd have left it alone for 3 or 4 days you'd have noticed that the beer absorbed the CO2 and equalized a little higher than what you were aiming for. I did something similar where I had the beer carbonating and chilled it with 30 lbs in the headspace and no gas hooked up and it overcarbonated a bit.

With CO2 carbonating a liquid, it's not that the gas pressure drops from temperature, it's that the chilled liquid much more readily absorbs the CO2 and drops the pressure until the liquid can't absorb any more.

 

[MagicMatt]

With CO2 carbonating a liquid, it's not that the gas pressure drops from temperature, it's that the chilled liquid much more readily absorbs the CO2 and drops the pressure until the liquid can't absorb any more.

That goes against everything I've learned. Pressure absolutely decreases due to temperature decrease. Blow up a balloon and stick it in the freezer. It'll come out way smaller than it went in. It's not because it lost air, but because the pressure decreased in the cold environment. If you let the 'frozen' balloon warm up to room temp, it'll regain it's original size/pressure.

I figured the same should be happening with my keg. If I use the carbonation chart to set the pressure for the temp it was at (75°=33psi), then when changing temps, the pressure would change to reflect so according to the chart (i.e. 50°F=18psi, 40°=12psi, 30=7psi, etc) .

If you'd have left it alone for 3 or 4 days you'd have noticed that the beer absorbed the CO2 and equalized a little higher than what you were aiming for.

How is that possible? If I carbed it to the desired levels, and just change the temp of the keg, the volumes of CO2 should be equal. There should be no way to overcarb without introducing more gas.

Gas pressure changes with temperature but not a whole lot between 75 and 35.

According to this chart, a keg going from 65°F to 30°F will have a pressure change of ~20-25 psi.

 

[rawlus]

That force carb chart does not tell you what will happen when you change beer temps. It's for calculating the effect when starting with uncarbonated beer.

The chart is to tell you the pressure to use based on the temp of flat beer and the desired final volumes of co2.

 

[MagicMatt]

That force carb chart does not tell you what will happen when you change beer temps. It's for calculating the effect when starting with uncarbonated beer.

The chart is to tell you the pressure to use based on the temp of flat beer and the desired final volumes of co2.

Yes it does. That's it's whole purpose. Hence the y-axis.

It tells you that if you want a given carb level (volume of CO2) - say 2.5 volumes - for a given beer temp - say 65°F, it would take about 29 pounds of pressure. I mean, maybe I'm the one that way off base here, but I don't think so.

Perhaps it would help to think about what exactly a "volume of CO2" is. It is the amount of CO2 that would be contained in a given volume (i.e a 5 gallon keg) at 1 atmosphere of pressure. So if you have 2.5 volumes of CO2 in a keg, regardless of whether there is liquid in it or not, that means you have 2.5 times the amount of CO2 molecules in the keg as you would have at a single atmosphere of pressure (i.e not under pressure at all, but filled with CO2 and not 'air').

Once there is a given amount of CO2 ('volumes') in the keg and the gas is disconnected, the level of CO2 ('volumes') cannot change. It is physically impossible (barring any leaks or active fermentation). If you fill an empty 65°F keg with 29psi of CO2, it has 2.5 volumes of CO2 in it. If you chill that keg to 30°F, it will still have 2.5 volumes of CO2 in it, but the pressure will be about 7.5psi. The chart above directly tells us this.

The only thing adding liquid will do is slow down how long it takes for the keg to equalize at 2.5 volumes. Since it takes longer for CO2 to dissolve in liquid than in air (i.e. an empty keg), you have to leave constant pressure on it. After a week or so, you'll have 2.5 vols of CO2 in the keg.

If an empty keg and a full keg both have 2.5 volumes of CO2 in them, regardless of their temperature, that means they have the same exact amount of CO2 molecules in them. Even if one is at 65°F (29psi) and one is at 30°F (7.5psi).

This is basic molecular science. Temperature directly affects gas density.

 

[PapaO]

Was the keg equalized at 75 ? Or was there more co2 in the head space and needed a day or two to go into solution at 36 ?

 

[Qhrumphf]

I would assume they just hadn't reached equilibrium yet.

 

[MagicMatt]

Was the keg equalized at 75 ? Or was there more co2 in the head space and needed a day or two to go into solution at 36 ?

Yes, I think. It's a bit more complicated than that.....

This beer is a Saison that took 8 weeks to ferment at 94°F. It finally seemed to halt at 1.013, which was off of the expected FG of 1.006. But it was stable for 10 days, so I racked into the keg and force carbed at 32°F and 9psi for 2 weeks.

I tapped the keg and had about 5 pints off of it, then pulled it out of the freezer and let it sit at room temp to age a bit. It was carbed to where I wanted it (~2.7 vols), and I left it disconnected from the gas for a couple of months.

The next time I check on it, the pressure is around 52psi. I'm guessing it finally fermented out, even though it was in my keezer at near freezing temps for 3 or 4 weeks. So for the next 10 days or so, I purged the keg daily to get it around 32psi. Eventually, it didn't creep up any longer from 32psi (over 2 weeks), so I assumed the pressure had equalized. Perhaps this is my mistake, but I had no reason to suspect otherwise, as CO2 would have come out of solution if there was more in the beer than in the headspace.

So that is the story leading up to me putting the 32psi 75°F keg into the 35°F freezer. Maybe it's true that it just needed more time to come down in pressure, not sure. I have 2 other kegs in the same condition (75°F, 32psi), and I just added one to my keezer. I'll let it go a couple of days and see what's the haps.

 

[doug293cz]

75˚F = 297.0˚K, and 35˚F = 274.8˚K. Converting gauge pressure to absolute pressure, 32 psig = 46.7 psia. So, the absolute pressure going from 75˚F to 35˚F is:

46.7 psia *274.8˚K / 297.0˚K = 43.2 psia = 28.5 psig​

So gas pressure only drops 3.5 psi due to the temperature change.

As far as the beer reabsorbing the excess CO2 and bringing the pressure down to the equilibrium value for the carbonation level, that will probably take more than a week, as it is similar to the timing for set and forget force carbonation.

Brew on

 

[doug293cz]

...

If an empty keg and a full keg both have 2.5 volumes of CO2 in them, regardless of their temperature, that means they have the same exact amount of CO2 molecules in them. Even if one is at 65°F (29psi) and one is at 30°F (7.5psi).

Not quite right. If you have 1 gal of beer in a keg with 2.5 volumes of carb, the beer contains the same mass (# of molecules) of CO2 as does a 2.5 gal container of CO2 at atmospheric pressure. If you have 5 gal of beer in a keg with 2.5 volumes of carb, the beer contains the same mass of CO2 as does a 12.5 gal container of CO2 at atmospheric pressure.

Brew on

 

[ba-brewer]

I naturally carb some of my kegs, it normally takes a couples days for the kegs to drop from 30psi to 12psi.

 

[MagicMatt]

75˚F = 297.0˚K, and 35˚F = 274.8˚K. Converting gauge pressure to absolute pressure, 32 psig = 46.7 psia. So, the absolute pressure going from 75˚F to 35˚F is:

46.7 psia *274.8˚K / 297.0˚K = 43.2 psia = 28.5 psig​

So gas pressure only drops 3.5 psi due to the temperature change.

As far as the beer reabsorbing the excess CO2 and bringing the pressure down to the equilibrium value for the carbonation level, that will probably take more than a week, as it is similar to the timing for set and forget force carbonation.

Brew on

Thanks for the math check. That's interesting to say the least, as I always assumed the pressure would drop equally.


So now that I'm thinking about this, let's say I carbed a keg to 2.5vols at 34°F, and keg pressure (disconnected from gas) was a stable 9 psi for several days. If I then took that keg out of the keezer and let rise to room temp, following the same logic the pressure shoud only rise a few psi, correct?

But wouldn't that conflict with the table that says that a keg at room temp should be 30-32psi for 2.5 vols? Do you see what I'm saying? Maybe a scenairo would help show what I'm getting at and where my confusion stems from:

I have 2 kegs:
- Keg A was carbed for 2 weeks at 34°F at 2.5 vols (thus, ~9psi)
- Keg B was carbed for 2 weeks at 70°F at 2.5 vols (thus, ~30psi)

I then take Keg A out of the keezer and place next to keg 2 for a few days. Now both kegs are at 70°F. However if I check the pressure, Keg A will be around 15psi (based on your explanation above) while Keg B will be at 30psi. Yet they're supposed to be carbed to the same level? Keg A would seem to be in contradiction with the carbonation chart. Why wouldn't it rise to 30psi as well? Is it just a matter of time, say a week or more, for this to happen?

 

[doug293cz]

Thanks for the math check. That's interesting to say the least, as I always assumed the pressure would drop equally.


So now that I'm thinking about this, let's say I carbed a keg to 2.5vols at 34°F, and keg pressure (disconnected from gas) was a stable 9 psi for several days. If I then took that keg out of the keezer and let rise to room temp, following the same logic the pressure shoud only rise a few psi, correct?

There are two things going on in a pressurized keg when the temperature changes.

The first is the change in gas pressure with a change in temperature. According to the gas law: PV = nRT, where:

P = absolute pressure (not gauge pressure)
V = Volume occupied by the gas
n = number of gas molecules (usually expresses as moles - 6.02E23)
R = constant
T = Absolute temperature (not ˚C or ˚F)​

So with "n" and "V" constant, the absolute pressure is proportional to absolute temperature (and 75˚F to 35˚F is a small change on the absolute temp scale.)

Now in a keg you also have the "true gas" vs. "dissolved gas" equilibrium going on. If the head space pressure is higher than the equilibrium pressure for the current amount of dissolved gas at the current temperature, then more gas will dissolve in the liquid, increasing the amount of dissolved gas and decreasing the amount of gas in the head space (the "n" in the head space goes down), so the pressure in the head space goes down. If the pressure is lower than than the equilibrium pressure, then gas comes out of solution into the head space (the "n" in the head space increases), so the pressure in the head space increases. The thing to note is that "n" changes, so the simple "pressure proportional to temperature" no longer applies.

The pressure change in the gas phase is instantaneous with the temperature change. There is no lag time. But, the pressure change due to no longer having the gas phase and dissolved phase in equilibrium does not change instantaneously. It occurs over a period of time until the equilibrium is finally restored. The rate at which the pressure changes is approximately proportional to how far out of equilibrium the system is. The implication of rate being proportional to offset is that the closer you are to equilibrium, the slower things change. The rate of pressure change is also approximately proportional to the volume of liquid in the system (assuming surface cross sectional are is constant with volume changes.) The time constant for equilibrium changes is several days for corny kegs.

So, when you change the temp of an isolated keg, you get a small pressure change instantly (changes as fast as the temperature changes), and a larger pressure change over several days, as the gas/dissolved equilibrium is restored.

The carbonation charts only tell you what the pressures and carb levels are when in equilibrium. They tell you nothing about what is happening when you are in a dynamic state.

Edit:

But wouldn't that conflict with the table that says that a keg at room temp should be 30-32psi for 2.5 vols? Do you see what I'm saying? Maybe a scenairo would help show what I'm getting at and where my confusion stems from:

I have 2 kegs:
- Keg A was carbed for 2 weeks at 34°F at 2.5 vols (thus, ~9psi)
- Keg B was carbed for 2 weeks at 70°F at 2.5 vols (thus, ~30psi)

I then take Keg A out of the keezer and place next to keg 2 for a few days. Now both kegs are at 70°F. However if I check the pressure, Keg A will be around 15psi (based on your explanation above) while Keg B will be at 30psi. Yet they're supposed to be carbed to the same level? Keg A would seem to be in contradiction with the carbonation chart. Why wouldn't it rise to 30psi as well? Is it just a matter of time, say a week or more, for this to happen?

Keg A will rise to 30 psi eventually, but it will take considerably more than two days. So, yes it is just a matter of time.

Brew on

 

[MagicMatt]

Keg A will rise to 30 psi eventually, but it will take considerably more than two days. So, yes it is just a matter of time.

Brew on

Great explanation, thanks a ton.


But one final question if you don't mind. So the 'Keg A' scenario just happened. I had a carbed, cold keg and I pulled it out of my keezer this morning. When I got home from work, the pressure was at about 13psi, up from 9. I then made the decision that I wanted a bit more carbonation in it, so I attached the gas and set to 30psi, which is the carbonation level for 72°F and 2.5 vols.

Did I royally mess up the carbonation by doing this? I thought it would work just like if the keg wasn't carbed at all and add just a touch of carbonation over the next couple of days. I need to serve this keg on Saturday, and felt like it was just barely under carbed for my taste.

 

[doug293cz]

Great explanation, thanks a ton.


But one final question if you don't mind. So the 'Keg A' scenario just happened. I had a carbed, cold keg and I pulled it out of my keezer this morning. When I got home from work, the pressure was at about 13psi, up from 9. I then made the decision that I wanted a bit more carbonation in it, so I attached the gas and set to 30psi, which is the carbonation level for 72°F and 2.5 vols.

Did I royally mess up the carbonation by doing this? I thought it would work just like if the keg wasn't carbed at all and add just a touch of carbonation over the next couple of days. I need to serve this keg on Saturday, and felt like it was just barely under carbed for my taste.

I think you should be okay. The keg couldn't have been much below 72˚F, so it shouldn't have picked up too much carb before it actually got to 72˚F. You cannot over shoot the carb volumes at the equilibrium pressure for the temperature.

Brew on

 

[MagicMatt]

Sweet, thanks! The help is much appreciated!

 

[sae220261]

Hello. I have a kegging pressure question that goes right along with this so I figured I would just jump on board.

I kegged a Belgian dubbel at room temperature. I set the pressure at 36 psi at a temperature of 70 to get 3.0 volumes. The keg was just sitting for a few weeks so it had definitely stabilizer.

I put the keg in the fridge to cool when I was ready to tap, hooked up the co2 at around 10 for tapping and blew off the excess pressure. The next day when it was cool, and tapped, it foamed excessively. I would have thought the carbonation would have been proper at this point and should have poured fine.

Any ideas what I did wrong?

 

[Subdivisions]

What length and inside diameter are your serving lines? They may be too short to serve a 3 volume beer without foaming.

Also, if you want your beer to stay at 3 volumes then you need to set the serving pressure according to the temperature your keg is chilled to. For example - for 3 volumes at 38F you'd need to keep the serving pressure at 16.5 psi. Use this calculator to figure it out

http://www.brewersfriend.com/keg-carbonation-calculator/

At 16.5 psi you'll need about that many feet of 3/16 ID beer line to keep it from foaming.

 

[doug293cz]

Sweet, thanks! The help is much appreciated!

Glad I could help.

Brew on

 

[doug293cz]

What length and inside diameter are your serving lines? They may be too short to serve a 3 volume beer without foaming.

Also, if you want your beer to stay at 3 volumes then you need to set the serving pressure according to the temperature your keg is chilled to. For example - for 3 volumes at 38F you'd need to keep the serving pressure at 16.5 psi. Use this calculator to figure it out

http://www.brewersfriend.com/keg-carbonation-calculator/

At 16.5 psi you'll need about that many feet of 3/16 ID beer line to keep it from foaming.

Word

Brew on

 

[sae220261]

So, if I understand correctly, if I carbonate for a volume of 3 but push at a much lower pressure I will have the foaming issues .... Without excessively long tap lines ... Is that correct.

So .... How do commercial breweries and bars handle it. I'm sure they don't have 10 different regulators setting the push pressure on each keg .... Or do they?

How do you handle different beers with different volumes but coming off the same push regulator?

Thank you for your help.

 

[chickypad]

So, if I understand correctly, if I carbonate for a volume of 3 but push at a much lower pressure I will have the foaming issues .... Without excessively long tap lines ... Is that correct.

Likely yes, the pressure is not high enough to maintain that carb level and the CO2 is coming out of solution. The beer will slowly decarb to the new equilibrium level if you pour enough off.

So .... How do commercial breweries and bars handle it. I'm sure they don't have 10 different regulators setting the push pressure on each keg .... Or do they?

They do if they want to serve beers at different carb levels. Usually banks of secondary regulators within the fridges or cold rooms - see pics below.

How do you handle different beers with different volumes but coming off the same push regulator?

You don't if you want the different kegs to stay at the correct volumes, you need to get a dual primary or some secondary regulators.

And now we've totally derailed this thread.

 

[sae220261]

Thank you very much ... That makes a lot more sense to me now. Sorry about getting off subject.