Force carbonating with mixed beer gas [Archive]

PFDarkside

03-06-2005, 02:28 AM

I'm in the process of adding to my kegerator to be able to dispense nitro stouts and ales, and looking down the road I'd like to homebrew some of the same and dispense with the N2/CO2 mix and stout faucet. Is there anything special to consider when it comes to force carbonating when you intend to dispense like this? Should it be forced with straight CO2 or the mix? Any difference in the target PSI when using mixed gas?

Steve16823

05-25-2005, 11:38 AM

After taking the winter off of homebrewing i was catching up on my posts and noticed nobody answered this one. It's over a month old but I thought it was an interesting question.

When you have a mixture of N2 and CO2 (or any two gasses) each gas behaves independently, each exerting it's own pressure as the molecules bounce around. This is called the "partial pressure" of each gas in the mixture. The total pressure would be the sum of the partial pressures:

P(total) = P(CO2) + P(N2)

And the partial pressures of each gas would be proportional to the mole-fraction of each gas in the mixture (1 mole = 6.02x10^23 molecules which weigh 28g for N2 and 44g for CO2)

P(CO2) = P(total)*(number of moles CO2)/(moles of CO2 + moles of N2)

P(N2) = P(total)*(number of moles of N2)/(moles of CO2 + moles of N2)

Now, I think that the percentages given for mixed beer gas are mole fractions already, so you can calculate the partial pressure of each gas using the percentage directly.

In summary -- to correctly force carbonate, you have to adjust the pressure of the mixed gas so that the partial pressure of CO2 matches the pressure you would use to force carbonate using CO2 alone.

To further complicate things, also note that you must use ACTUAL pressure for these calculations, and not gage pressure (Actual Pressure = Gage Pressure + 14.7 psi.

EDIT: Here's a link I found that explains the same thing, only with pictures and examples!

http://www.mcdantim.com/beergas.htm