Just to add - the keg's carbonation levels change as the volume of beer relative to pressurized CO2 volume decreases. Neglecting time to carb up, your beer will have higher CO2 volumes towards the end of the keg if you don't decrease the pressure. That's why you might not have seen this foam issue initially. Most of us just deal with this as we don't have multiple regulators...
That's not how the physics of vapor pressure vs. dissolved gas works. If your CO2 pressure and carbonation level (dissolved CO2 volumes) are at equilibrium with a full keg (carbonation is fully complete), and the CO2 pressure is kept the same as the keg empties, then the carbonation level will not change.
If your serving pressure is higher than the equilibrium vapor pressure for your original level of carbonation, then the carb level will increase over time (until equilibrium is reached again.) If your serving pressure is lower than the equilibrium vapor pressure for your original carb level, then carb level will decrease over time. The previous two things will happen no matter how full the keg is.
The common carbonation charts:
Show the equilibrium carb levels vs. CO2 gauge pressure, assuming the gas in the keg is 100% CO2. The CO2 vapor pressure is given in psig, which means that the absolute vapor pressure is psig + 14.7 (assuming sea level), and carb level actually depends on absolute vapor pressure.
IF you don't purge the air from the keg, then the starting CO2 vapor pressure will be less than you expect (14.7 psi less), and will increase as the keg empties. This happens because you are filling the newly created headspace with CO2, so the partial pressure of the air will go down, and the partial pressure of CO2 will go up. In this case, your beer would start out with less carbonation than you intended, and increase as the keg empties. But, this only occurs if you are doing things wrong.
Brew on
