Beer Gas Mix CO2 Equilibrium Question

[TAK]

This is a question about how the CO2 portion of beer gas mix goes into solution.

I carb any beer that is going on my stout faucet, first with CO2, to something low, say 1.5 vol.

Say I wanted to bottle a few with a beer gun. To do so, I'd release the pressure. Then put a couple PSI on for the beer gun; let's say I do this with the beer gas mix. When done, I'd up the pressure back to stout faucet pressure.

I assume when I first started serving the keg, a portion, or all, of the CO2 that flows into the keg along with the beer gas mix goes into solution, and much less, or none, of the N2 goes into solution. In other words, as I pour a pint, the mix flows in, and the CO2 portion finds it's balance based on gas laws (which I haven't even tried to research or understand).

Here's the question. If the above assumption is true, the headspace at its equilibrium is mostly N2, probably with some CO2, but in less proportion than the mix in the bottle. Well, if/when the pressure is released and then the headspace is refilled, all of a sudden there's a higher proportion of CO2. Does that go back into solution, in search of a new equilibrium, but essentially increasing the carb level in volumes of CO2?

 

[erikpete18]

So the thing about mixed gas equilibrium is that each gas ignores the other gases in the headspace. What that means is that whatever the proportion of gas in the tank will be proportion in the headspace. The CO2 doesn't realize that there's more nitrogen in the headspace, it just seeks to equilibriate itself with the tank, eventually balancing out at whatever the proportion in the tank is. So if you've got the pressure set to a CO2 proportion of 1.5 volumes it will stay fairly close to that. One thing I'd never considered is that if it's true that none of the nitrogen goes into solution, then as you dispense beer you'll be removing the CO2 but not the nitrogen. That should eventually result in a lower proportion of CO2 in the mix. I don't think it would be rapid or dramatic enough to notice anything on a single keg level, but still might occur.

 

[TAK]

That makes sense. Thanks.