When bottling a kegged beer, how much carbonation is lost to the headspace?

It seems that when bottling a beer from a keg, you'd want to (theoretically) over carbonate beforehand to compensate for the carbonation that goes to pressurizing the bottle and also the carb lost in your bottling process. The carb lost to the headspace is a matter of calculation, so I'm wondering if anyone has run the numbers.

From a broader perspective, is there a consensus on how much carb is typically lost when bottling, assuming a relatively foam-free process?

I just weighed a bottle full to the brim and the water weighed 371 grams. So if you put 355 mL of beer into a 371 mL space the carbonation would be .957 of what it was before, barring losses in handling.

I think most people will raise the pressure by 10 or 20% to bottle. I say gas it. Too much pressure will cost you a point, but too low is ten or twenty.

Wynne-R said:

I just weighed a bottle full to the brim and the water weighed 371 grams. So if you put 355 mL of beer into a 371 mL space the carbonation would be .957 of what it was before, barring losses in handling.

I think most people will raise the pressure by 10 or 20% to bottle. I say gas it. Too much pressure will cost you a point, but too low is ten or twenty.

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Seems you may be using the equation PV=nRT, and since nRT should be constant then P1V1 = P2V2. I figured this system with gas and liquid together would be much more complicated than that. You're either way ahead of me, or way behind, and I can't figure out which. LOL!

Oh darn, you want to get fancy. OK you got me, I ignored the air pressure of the head space. Also hopefully the yeast will eat the oxygen, eliminating it’s partial pressure. The correct number is probably a little more than 97%.

The point I was trying to make is the volume of the headspace is pretty much nothing compared to the loss you will get from bottling. My experience in judging is that most under-carbonated beers were bottled from a keg. You can tell by the lack of a yeast ring.

I think maybe this is what you are looking for:

For the CO2 dissolved in the beer, the situation is considerably more complicated because of gas and liquid interactions. Fortunately a relationship called Henry’s Law applies to this situation. It states that the concentration of a slightly soluble gas (e.g. CO2) is directly proportional to the partial pressure of the gas.

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p163 “Principles of Brewing Science” George Fix

I leave no headspace when filling from keg. Top off all the way up and have no CO2 loss or O2 in the bottle.

Of everything ive read, most people fill it like regular and then give one quick burst to cause a foam over. Then cap it like that so the foam over purges all oxygen out of the bottle. At that point, ur not gonna lose a lot of CO2. I don't have a crazy equation for it, just read a bunch of forums of people bottling from keg and even a month later the beer tastes good and has proper carb level.

Pratzie said:

Of everything ive read, most people fill it like regular and then give one quick burst to cause a foam over. Then cap it like that so the foam over purges all oxygen out of the bottle. At that point, ur not gonna lose a lot of CO2. I don't have a crazy equation for it, just read a bunch of forums of people bottling from keg and even a month later the beer tastes good and has proper carb level.

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Yep, for sure!! purge, fill and cap over foam!

Cheers
Jay

I usually go with a couple of PSI higher than the normal serving pressure if bottling from the keg. Mostly I bottle a entire batch at once. If it was just a few to take to a mates I wouldn't bother. Also be vary careful on capping on the foam - I just got oxidation picked up in a few of my beers using the slow fill with picnic tap/ racking cane method and capping on the foam (maybe I need to be quicker with the capping!)

Freezing the bottles and chilling the beer extra cold beforehand also helps conserve some CO2.