In the Dec.2015 issue of Brew Your Own in the Help Me Mr. Wizard column by Ashton Lewis, there is an explanation of how to calculate priming sugar mathematically supposedly without recourse to the online calculators.
He creates a scenario starting with 20L of beer with 3.2 grams of CO2/liter and then demonstrates the calculations needed. The math is simple once you’ve seen it, but I don’t know how a brewer would be able to determine how much CO2 would be in his or her fermented beer to use as the starting point for the calculations. Advise please.
Are you asking how to empirically measure dissolved CO2 in your homebrew? It’s possible to do with some special equipment, but I don’t think it would be any more accurate than simply looking up the saturation concentration of CO2 in liquid at whatever temp your beer is at. I’m guessing that’s how the wizard arrived at 3.2 g/L.
Can a brewer assume that once the beer is fully fermented that the CO2 has reached the saturation point for any given temperature? Are beers with a higher proportion of dextrins different from other recipes regarding amount of CO2 in solution? Beers with more alcohol should produce more CO2, but does that mean more CO2 in solution? I don’t know.
Can a brewer assume that once the beer is fully fermented that the CO2 has reached the saturation point for any given temperature? Yes. This is why the online calculators ask for the beer temp. Are beers with a higher proportion of dextrins different from other recipes regarding amount of CO2 in solution? No. Beers with more alcohol should produce more CO2, but does that mean more CO2 in solution? No.
The concentration of CO2 dissolved in beer at atmospheric pressure is, for all intents and purposes, a function only of temp. The other independent variables have a negligible effect, if any.
If you’re seeing gas coming out of your airlock, then that means that your beer is holding the maximum amount of CO2 it can at that temperature and pressure, and the rest is coming out of solution and pushing its way out the airlock. That is why you can make a somewhat universal claim about how much CO2 is in the beer, assuming room temp and 1 atmosphere of pressure.