Physics of keeping positive keg pressure

[reuliss]

Folks, there's a concept I just don't understand. Why, after a beer is carbonated, does the keg maintain positive pressure even if you take the keg off the gas, but if the beer is not carbonated and you remove from the gas with, say 20 psi, the beer will absorb all CO2 in the head space and reach equilibrium (rather than keep positive pressure)?

I ask this because there are uncarbonated beers that I'd like to store in kegs and have confidence that no oxygen is getting in there. Everytime I've done this, though, I've come back to the keg only to find that there is no positive pressure. Can anyone help me with this?

 

[snowtires]

Because carbonated beer has co2 dissolved into the liquid so if the beer was carved at 11 psi then there will probably be about 11 psi in the headspace. The headspace has less pressure so some co2 leaves the dissolved form back into the gas form.
If the beer was carved at say 11 psi then there is as much co2 absorbed into that beer as possible at that pressure which is called the volume. The beer is essentially full.

If there is no co2 in the beer then it works the opposite way. Say there's 11 psi in the headspace well there's no co2 in the beer. Its an easier path to dissolve into the beer then stay on top under pressure. The beer will keep taking co2 until it is full at that psi. Then the pressure to go into the beer and the pressure to come back into the headspace is equal.

 

[MrFoodScientist]

Simply put:
Carbonated beer is at equilibrium and will stay that way unless there is a change in pressure or temperature.
Uncarbonated beer under pressure is not at equilibrium, and will absorb CO2 until it reaches equilibrium. If it is not hooked up to gas, there is nothing to replenish the CO2 absorbed from the headspace so pressure will drop as it reaches equilibrium.

If you want to keep uncarbonated beer stored in a keg, probably your best bet is to pressurize with something other than CO2 or O2, maybe nitrogen? And probably best to store it (for pressure purposes anyhow) unrefrigerated.

 

[LLBeanJ]

I ask this because there are uncarbonated beers that I'd like to store in kegs and have confidence that no oxygen is getting in there. Everytime I've done this, though, I've come back to the keg only to find that there is no positive pressure. Can anyone help me with this?

If you pressurize the keg and find later that there is no longer any pressure, then the keg must have a leak. As the other posters have stated, when you pressurize a keg, some of the gas will be absorbed by the beer and the remainder will occupy the headspace. If you want to store uncarbed beer in a keg for an indefinite period of time, be sure to fill and purge the headspace with CO2 a few times, then hit it one last time with 30 psi to seal the lid. Do that and, even if the CO2 escapes from a bad seal, the likelihood of oxygen entering the keg while in storage is pretty much zero.

 

[FredTheNuke]

This is my take based on my understanding of Henry's Law:

It's not that your keg is going to zero psi - it is going to the equilibrium pressure for a very very small amount of CO2 in solution. In the case of 5 gallons of beer with typical fermentation levels of CO2 (0.8 volumes average) the beer will absorb the CO2 when the headspace is pressurized - say to 30psi. As the CO2 at 30psi is absorbed the headspace pressure will decrease. It will not go to complete zero because even at 0.8 volumes should the headspace be at complete zero some CO2 will diffuse out of the beer and pressurize the headspace - maybe it only goes down to 0.1 psi - so low that you don't here that hiss when you pull the relief valve so you assume there was zero psi of pressure. That fraction of a pound of pressure is all that is present at such low equilibrium. As you add more CO2 and it reaches equilibrium (such as carbonating to 2.5 volumes) the headspace pressure increases - for this case at 40F the headspace would pressurize to about 12.5 psi. At 0.8 volumes at 40F you would only have about 0.5 psi of headspace pressure.

 

[reuliss]

This is my take based on my understanding of Henry's Law:



It's not that your keg is going to zero psi - it is going to the equilibrium pressure for a very very small amount of CO2 in solution. In the case of 5 gallons of beer with typical fermentation levels of CO2 (0.8 volumes average) the beer will absorb the CO2 when the headspace is pressurized - say to 30psi. As the CO2 at 30psi is absorbed the headspace pressure will decrease. It will not go to complete zero because even at 0.8 volumes should the headspace be at complete zero some CO2 will diffuse out of the beer and pressurize the headspace - maybe it only goes down to 0.1 psi - so low that you don't here that hiss when you pull the relief valve so you assume there was zero psi of pressure. That fraction of a pound of pressure is all that is present at such low equilibrium. As you add more CO2 and it reaches equilibrium (such as carbonating to 2.5 volumes) the headspace pressure increases - for this case at 40F the headspace would pressurize to about 12.5 psi. At 0.8 volumes at 40F you would only have about 0.5 psi of headspace pressure.

That makes sense and have theorized the same. If that is correct, that's great.

 

[JuanMoore]

This is my take based on my understanding of Henry's Law:

It's not that your keg is going to zero psi - it is going to the equilibrium pressure for a very very small amount of CO2 in solution. In the case of 5 gallons of beer with typical fermentation levels of CO2 (0.8 volumes average) the beer will absorb the CO2 when the headspace is pressurized - say to 30psi. As the CO2 at 30psi is absorbed the headspace pressure will decrease. It will not go to complete zero because even at 0.8 volumes should the headspace be at complete zero some CO2 will diffuse out of the beer and pressurize the headspace - maybe it only goes down to 0.1 psi - so low that you don't here that hiss when you pull the relief valve so you assume there was zero psi of pressure. That fraction of a pound of pressure is all that is present at such low equilibrium. As you add more CO2 and it reaches equilibrium (such as carbonating to 2.5 volumes) the headspace pressure increases - for this case at 40F the headspace would pressurize to about 12.5 psi. At 0.8 volumes at 40F you would only have about 0.5 psi of headspace pressure.

This. Well said. I'd only add that a drop in beer temperature can cause the pressure to decrease significantly, which can result in a vacuum and loss of seal. If you keg the beer at fermentation temp and then chill it to serving temp, the pressure drop created will be more than the tiny residual pressure after the CO2 is absorbed, and can actually create a vacuum in the keg.