What's the best method to use a temperature controller during fermentation? If you're using a freezer (or warming apparatus) connected to the controller, how do you compensate for the fact that the wort's interior temperature can be several degrees higher than the air temperature around the fermentor?
In answering a recent question about temperature control inside a freezer, mdma stated:
Finally, if you want to be really sure what your fermentation temperature is, it's best to add a thermowell and a separate thermometer. Don't be tempted to use the controller thermometer to both measure and control the wort temperature - the wort changes temperature very slowly, and the controller will overshoot massively, causing everything else in the freezer to be frozen.
This was enlightening to me. I just bought a temp controller and until reading this post I had been planning to put the controller probe into the thermowell. If I understand what mdma is saying, because the relatively dense mass of the wort will not change temperature as quickly as the surrounding air, by the time the probe in the thermowell has reached the target temperature, the air surrounding the fermentor will have dropped considerably farther. Then the ambient temp will continue to chill the wort itself, resulting in a lower-than-expected wort temp and wider swings in temp as the controller cycles cooling on and off, thus defeating the goal of maintaining a consistent temperature.
(Side note: I suspect that when fermenting ales at room temperatures, the rest of the freezer is unlikely to get all the way down to freezing temperatures, but it would still be a significant variance.)
So how do you balance between air temp and wort temp? Do you just keep an eye on it and adjust the controller if the difference is significant? How much do you compensate between air temp and wort temp?