BitterSweetBrews
Tim Trabold
I'm surprised to hear you over carbed the beer using this system, that should be impossible to do if you're at serving temperature and pressure. Unless you weren't using serving temp/pressure, in which case you should be.
Also your math is a little off. First, atmospheric pressure is about 14.7 psi, not % The pressure gauges we use shows relative pressure (or gauge pressure), which is zero referenced to atmospheric pressure. What this means if that if I pressurise a vessel to 14.7 psi, that has an absolute pressure of 29.4 psi. Absolute pressure measures from 0 psi (perfect vacuum).
So if you pressurise to 30 psi you're roughly tripling the moles of gas in the keg, not doubling them. This drops you to 9 purges to get rid of the oxygen, though only if you assume perfect mixing and distribution of the gases.
Well, I figured out why I over carbed it. The CO2 valve on my spare CO2 tank is a welding one from Harbor Freight. It has two scales, one for flow and the other for pressure (I think). The numbers on the outside, flow scale are half the values of the inside, pressure scale. I verified that the inside scale was the correct one using an air pressure gauge in line. I had done this when I first got the gauge, but forgot which one was correct (I have noted it on the gauge for the future). I was using the outside scale and setting it at 14lbs was actually priming it at about 30lbs. The last two times I have used it I have had much better results.