Bottle priming -- temp vs. residual co2.
Here's the issue. The various charts and calculators make it a simple job to figure out the residual co2 in a beer at various temperatures. But here's the issue: short term changes in temp. How does this affect residual CO2?
Scenario: Suppose I have a beer at 50F. It will have a residual CO2 of [X vol.] If I warm that beer to 68F, it will have less residual CO2 [y volumes]. If I bottle at 68F, I will calculate residual CO2 based on the 68F level.
BUT, what happens if I then chill that beer back to 50F for a few days before bottling? What temperature do I use? 68F or 50F? Since fermentation is over, it is not going to produce very much additional CO2 to replace what was lost out of solution when the temp increased. But there will be some more than at 68F, simply because it will absorb some from the atmosphere.
I have found plenty of conflicting sources about this. Some say that lowering a beer's temp post fermentation will not lead to an increase in residual CO2, only the highest temp post fermentation matters. Other sources that that yes, lowering the temp increases the residual CO2 but by small amounts. Finally, some say that increasing the temps lowers residual CO2 but it happens slowly. So if you take a beer from 76F down to 50F and hold it for several weeks that the beer will have the correct CO2 of 50F but increasing it a short time to 68F will lead to a reduction in residual CO2 but not by the amount for 68F. It will be some amount in between.
This is as much art as science, and there's no hard and fast rules. But what's a good general rule? I'm shooting for a higher carbonation level of about 3.25-3.5 volumes of CO2 which makes a proper calculation important.