Does force carbing work any different when the keg is half-full?

For my latest batch, I bottled half, and kegged the other half in a 5-gallon corny, so the keg is only half-full. I decided I would try the 'set it to a higher pressure for a day' method, so I set it to 30PSI and I'll turn it down in the morning. But will it make a difference that the keg is half-full? The pressure will the same, the area at the top will be the same, but there is half the amount of beer, so it seems reasonable to assume it will carb faster.

To my way of thinking, it won't matter. The same pressure is exerted on the beer in the keg. I would think it would be the same regardless of how much beer is in the keg. While you serve the beer it doesn't become more carbed as you drink it.

I'm going to have to agree. Once the keg equalizes the pressure, the absorbtion rate of CO2 into the beer won't be any quicker.

I'm not talking about the final serving pressure. That should be the same. I'm talking about the SPEED at which the beer becomes carbonated.

If the rate of CO2 absorption is the same (and I agree that it is, because the surface area is the same), then it should take half as long to carb half the amount of beer.

The usual advice for fast carbing is '30psi for 24 hours, then reduce to serving pressure'. Since I have a half of a keg, then I think this might cause me to have overcarbed beer.

The surface area exposed to the gas is what affects rate of carbonation. A full keg (meaning your gas in diptube isn't in the beer, so not 100% filled with beer) standing upright has the same amount of beer exposed to the keg as a half full one. However, if you laid the half-full keg down on its side while carbonating, the surface of the beer exposed to gas would go way up, and carbonation would be faster.

BetterSense said:

I'm not talking about the final serving pressure. That should be the same. I'm talking about the SPEED at which the beer becomes carbonated.

If the rate of CO2 absorption is the same (and I agree that it is, because the surface area is the same), then it should take half as long to carb half the amount of beer.

The usual advice for fast carbing is '30psi for 24 hours, then reduce to serving pressure'. Since I have a half of a keg, then I think this might cause me to have overcarbed beer.

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You're more or less correct. It's not a linear relationship, so it will take more than half as long to carb half the volume of beer, but still much less than the same amount of time. I think you're right about the cause of the overcarbed beer.

Hammy71 said:

To my way of thinking, it won't matter. The same pressure is exerted on the beer in the keg. I would think it would be the same regardless of how much beer is in the keg. While you serve the beer it doesn't become more carbed as you drink it.

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Kealia said:

I'm going to have to agree. Once the keg equalizes the pressure, the absorbtion rate of CO2 into the beer won't be any quicker.

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daksin said:

The surface area exposed to the gas is what affects rate of carbonation. A full keg (meaning your gas in diptube isn't in the beer, so not 100% filled with beer) standing upright has the same amount of beer exposed to the keg as a half full one. However, if you laid the half-full keg down on its side while carbonating, the surface of the beer exposed to gas would go way up, and carbonation would be faster.

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The speed it takes to carb is a relationship between surface area and volume. With the same surface area and less volume, the beer will carbonate faster, just as it would carbonate faster with more surface area and the same volume.

JuanMoore said:

The speed it takes to carb is a relationship between surface area and volume.

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Half right. It' a relationship between surface area and pressure.

If he sets at 30 psi for 24 hours and then purges to serving pressure, it will take the same amount of time regardless of the amount of beer in the keg.

If he sets at 30 psi for 24 hours and does not purge, it will carb faster only because the extra volume in the head space will allow the pressure to remain higher for longer before all the CO2 in the headspace is absorbed.

The trouble with the second option is that you will have a greater chance of over carbing because the amount of time you spend at that elevated surface area to pressure relationship is hard to predict.