I am new to kegging and I'm having trouble getting my stout faucet/beer gas mix right. I carbonated using beer gas at 20 psi at a ratio of 25/75 gas which would give me 5 psi Co2 right?
The gas laws work with respect to absolute pressure, not gauge. 20 psig is 34.7 psia. 25% of that is 8.7 psia which is -6 psig and about 0.6 atm and would result in about 0.6 vol dissolved at 51 Â°F (see below as to why I used that temperature) . For 1.2 volume you want about 15.7 psia (at 51 Â°F) CO2. At 25% CO2 that implies a total (mix) pressure 4*15.7 = 62.8 psia. That's about 48 psig.
The carb chart said 5 psi would give me 1.5 vols at my temp
That implies a temperature of 51 Â°F but that would be for a partial pressure of CO2 of 5 psig. You have -6.
I noticed a funky after taste after it had fully carbonated that I believe to be carbonic acid. Does the stout faucet have anything to do with this? I have thought about going with lower pressure and cutting my beer out line;
Given that you are under carbed I doubt that the funny taste is carbonic acid (which has a sour, sharp taste).
but nitrogenated beer is still supposed to have 1- 1.5 vols of Co2 right?
Yes. Your setup (20 psig) would give about 0.8 vol. 48 psig would give 1.2. For 1.5 you need 64.1 (at 50 Â°F).
If you want to do some calculations of your own type
(Pressure_cell+ 14.695)*(0.01821 + 0.090115*exp(-(Temperature_cell - 32)/43.11)) -0.003342
into a spreadsheet and enter the the Fahrenheit temperature into the temperature cell. Then put trial values of gauge pressure into the pressure cell until you get the volumes you want (or use the Excel Solver if you know how). Then convert to absolute pressure by subtracting 14.695 (14.7 is close enough). Then multiply by 4 to get the total pressure of the mix required to give the partial pressure calculated by the spreadheet and subtract 14.7 to get the gauge pressure of the mix.