Beer gas question

[Izzie1701]

Can I push an already carbonated keg with beer gas on a standard faucet using 25psi and then have the same pressure on another keg with a stout faucet and have both pour correctly? I have tried to find this answer but cannot. I have read lots of bars use beer gas to push there regular kegs through long runs and not risk over carbonation but then have read that using beer gas will cause the beer to go flat and have no head on a standard faucet. If I cannot do this with on a single reg can I get duel regs and push an IPA with a lower pressure beer gas through a standard faucet and set my stout higher through a stout faucet and have both pour correctly?

Edit: already carbonated keg meaning force carbed with CO2 or primed with sugar in the keg.

 

[doug293cz]

75/25 beer gas at 25 psi is equivalent to 6.25 psi CO2 as far as equilibrium carbonation level. A standard beer at ~2.5 volumes of carb would loss carb (down to around 2 volumes) if stored on beer gas.

Brew on

 

[physics911]

Doug293cz, I have no real world experience with beer gas, so help me understand how the CO2 in the beer would come out of solution and equalize at around 2 volumes. In this scenario, the head space is only 25% CO2, but it is also at 25 psi. Regardless of the gas mix, why would the CO2 absorbed at say 12 psi come out of solution in a 25 psi environment?

 

[doug293cz]

Doug293cz, I have no real world experience with beer gas, so help me understand how the CO2 in the beer would come out of solution and equalize at around 2 volumes. In this scenario, the head space is only 25% CO2, but it is also at 25 psi. Regardless of the gas mix, why would the CO2 absorbed at say 12 psi come out of solution in a 25 psi environment?

CO2 is absorbed or desorbed based on the partial pressure of CO2 in the headspace. The CO2 partial pressure is equal to the volume fraction of CO2 in the gas mix times the total pressure (total pressure is atmospheric pressure + the gauge pressure.) The nitrogen, and the pressure due to it, plays absolutely no role in CO2 absorption/desorption.

Also, my brain was already slowing for the evening when I posted my original reply. I forgot to add in the effect of atmospheric pressure. 25 psi gauge pressure (psig) of beer gas will give a total pressure of about 40 psi (39.7 actually, but can't do the math in my head with that.) So, the partial pressure of the CO2 would be 0.25 * 40 = 10 psi. 10 psi total pressure is equivalent to -4.7 psi gauge pressure. At 38°F, -4.7 psig gives an equilibrium carbonation level of about 1.0 volume. Much lower than my previous, incorrect estimate.

Brew on

 

[physics911]

Doug, you are, of course, correct. Thank you for the reminder. See what happens when I try to think too early in the morning? I forget my basic physics. I probably even still have some old notebooks in the bottom of the closet with Dalton's Law calculations in them. That whole O2 exchange in our lungs wouldn't work so well without partial pressure.

 

[Izzie1701]

Ok so then is there an issue using beer gas? I think I have gathered it would go flatter then I wanted even if it was pre carbed and a standard faucet would work? Or would I require 2 stout faucets? Im thinking I would still get a bit of head on a standard faucet and a lower carbonation then I wanted. If I remember correct when we toured Smithwicks they used a 50/50 mix on a standard tap and had some carbonation and a creamy head. Not as creamy as Guinness but close. It was a flow control standard faucet.

 

[doug293cz]

Ok so then is there an issue using beer gas? I think I have gathered it would go flatter then I wanted even if it was pre carbed and a standard faucet would work? Or would I require 2 stout faucets? Im thinking I would still get a bit of head on a standard faucet and a lower carbonation then I wanted. If I remember correct when we toured Smithwicks they used a 50/50 mix on a standard tap and had some carbonation and a creamy head. Not as creamy as Guinness but close. It was a flow control standard faucet.

If you put beer gas on a fresh keg that is fully carbonated, the CO2 partial pressure will be a little higher than optimal due to the CO2 already in the headspace, but as you serve, and the headspace increases, the CO2 partial pressure will drop. So, as the keg goes down the carb level will drop. If you finish a keg in a week, you might not notice it. If it takes a couple of months, you will definitely notice.

Pre-mixed beer gas at more than 25/75 can get problematic. With 25/75 and a cylinder pressure of 3000 psi, you only have 750 psi CO2 partial pressure, and the CO2 stays as a gas above 60°F. A 50/50 mix would only be able to have a cylinder pressure of 1500 psi to keep the CO2 as gas above 60°F, cutting the total amount of gas in a fresh cylinder in half. If the CO2 partial pressure is too high in the cylinder, then some of the CO2 will condense to liquid, upsetting the CO2/N2 ratio. Smithwicks probably has an on-demand gas mixing system to get around this issue.

I'm not a faucet expert, so won't comment on standard vs. stout taps.

Brew on