This is more of a commercial issue since long-draw isn't common at home, but does anyone with a solid chemistry understanding know why mixed gas partial pressures don't scale linearly according to the BA Draft Quality Manual? My assumption has been that a 60/40 CO2/N2 blend would exert 60% CO2 partial pressure on a keg. So 20psig would be exerting 12psig for carbonation calculations. According to the manual, this is not correct. Their provided equation has you use total pressure, factoring in atmospheric pressure, so 20psig is actually 34.7 psi. Then you do 34.7 x 0.6 = 20.88 CO2 partial pressure, then they have you subtract the 14.7 to get to 6.18psig CO2 partial pressure on the keg. This feels weird to me since you are accounting for the nitrogen in the 14.7psi correction on the way up, but not on the way back down and subtracting 14.7psi of pure CO2 instead of mixed gas. I'm no chemist so if I am missing the reason for this, I hope someone can enlighten me.
Copied and pasted from a post by doug293cz on another forum: ----------------------------------- "You can't do gas ratio calculations using gauge pressure. You have to use absolute, and partial pressures. Gauges measure the difference in vessel pressure to atmospheric pressure - which is 14.7 psi (at sea level.) So, when your gauge reads 10 psi, the absolute pressure is 24.7 psi. Carbonation level is determined by the absolute pressure of just the CO2. If the headspace in the keg is pure CO2, then you can just subtract 14.7 from the required absolute CO2 pressure, to get gauge pressure, and put that on your carbonation chart, to make things simple (i.e. no math) for users. When you have a mix of gases, the absolute pressure of each component (if by itself) is called the partial pressure of that component. Partial pressures are always absolute pressures, never gauge pressures. So, the value that is important for carbonation level when using beer gas (CO2/N2 mix) is the CO2 partial pressure. The partial pressure of any component of a mix is just the volume fraction of that component in the mix times the absolute pressure of all the gas mix. So, if you have 25/75 (CO2/N2) beer gas at 13.3 psi gauge pressure (psig) then the absolute pressure is 14.7 + 13.3 = 28 psia. The CO2 partial pressure is 0.25 * 28 = 7 psi, and the N2 partial pressure is 21 psi. So, if you have beer at 10.1 psig of pure CO2, that is a CO2 partial pressure of 24.8 psi. To drop the carbonation level in half, you need to cut the CO2 partial pressure in half, which would be 12.4 psi. In a pure CO2 environment, that would translate to a gauge pressure of -2.3 psig (not an easy thing to accomplish.) However in a 25/75 beer gas environment, the 12.4 psi CO2 partial pressure would become 49.6 psia total pressure, or a gauge pressure of 49.6 - 14.7 = 34.9 psig..."
