Keg carbonation mystery

[gr8shandini]

I've got an odd "problem" and I was wondering if anyone else noticed the same thing. When I get down to about a gallon left in the keg, the beer starts to get over carbonated. At first I thought it was the beer slowly reaching equilibrium at a higher carb level than I expected, but except for draining a keg in one night at a party, it doesn't seem to matter how quickly I drink it. It seems to be based solely on the volume remaining.

So any chemists here know of a reason for this? Is the equilibrium based on a surface area to volume ratio in any way?

 

[JoshuaWhite5522]

Thats strange, carbonation level is a product of temp and applied pressure, so as long as your regulator is working you should maintain the same pressure. At least that is how I understand it.

Do you use one of those carbonation stones at the bottom of your keg? I suppose that could cause a sharp incease in the amount of disolved CO2 especially when there is less beer left in the keg. Still the extra co2 should work it way out over a period of time.

I could be wrong, hopefully someone will correct me if I am.

 

[audger]

what is your definition of, or the indication that its "overcarbonated"?

Is the equilibrium based on a surface area to volume ratio in any way?

nope. its a function of temperature of the beer and pressure of the gas. if those two are constant, carbonation level will remain constant.

 

[gr8shandini]

For me, "overcarbonated" depends on the style. Right now, I'm fininshing up a keg of stout that I set to about 1.8 volumes, but it's now "fizzy" like a standard lager would be. If I were to guess, I'd say it's at about 2.7ish.

Temperature could be another culprit. My kegerator has a pretty wide hysteresis on the thermostat. It kicks in around 48 degrees and doesn't cut out until 36 or so. When I have a full keg in there, it stabilizes around 45 degrees, but maybe with the smaller mass it's following the swings more. I wonder if beer absorbs CO2 faster than it off-gasses when it warms up?

 

[spokaniac]

Having the same issue and posted nearly the same question. The answer on my thread is that it is normal for them to be more foamy toward the end.

 

[audger]

When I have a full keg in there, it stabilizes around 45 degrees, but maybe with the smaller mass it's following the swings more. I wonder if beer absorbs CO2 faster than it off-gasses when it warms up?

that is a pretty good guess. if the carb level equilizes at a lower temperature, and then warms up a few degrees, that would cause a slight increase in keg pressure (above the set pressure on the regulator) and extra foaming. the gas would be forced out of soloution by the warmer temperature, but it cant go back into the CO2 bottle, so the pressure would just build in the keg.

though we are talking about very small amounts, it might have some small effect.