# External fermentation temps vs. actual internal temps

this question probably has a lot of variables that affect the outcome, but is there an average temperature difference between the ambient temperature of a room (let’s say 68F) and the temperature in the wort where the yeast are working?
I just read a post that said at 68 degrees (ambient room temp) that the actual internal temp could be as high as 76, is that true? is it that much higher?
Is there a standard difference? or is it all situational, to the yeast, grain and other factors?

I’m no chemist of physicist but if you are looking for a “true” maximum it all depends on lots of variables like the volume of beer, the thermal conductivity of the vessel its in, The amount of surface area of the vessel it’s in, if it’s in a swamp cooler, etc. And im sure there are plenty of variables that deal with the chemical makeup of the beer and if its allowance of thermal transfer throughout.

Basically there isn’t a hard fast rule but I could see a variance of 8*F being within the realm of possiblity

FWIW, I have a thermowell directly in the middle of the carboy, with temperature probe, AND also an LCD thermometer stuck to the outside of the carboy. The entire thing is in a small temperature-controlled fridge.

I regularly see the reading on the LCD is off by 2 degrees compared to the thermowell temperature probe. Could be actual difference, or could be the poor accuracy of the LCD. Something to consider.

I have no doubt that an 8* difference is possible. But as mentioned above, there is no way of knowing the exact change that will take place.

When I used to ferment without contols I think I was fine a lot of the time. But all you need is that one beer to get way out of control to realize you need to be an active player in the process.

I have seen situations at high fermentation where the beer added 2* within an hour. Had I not been using a swamp cooler, who knows how quickly that beer would have gone over to the dark side.

I have found that with regards to my larger batch sizes (46L), when their temperatures start building momentum, they can be more likely to get way out of control than my 23l batches.

I can only go by my temp probe and fermometer on the outside of my fermentors. I’ve read that fermometers are pretty accurate, but I’m sure there is some room for error here. Having said that, during active fermentation (24-48hrs from the start of visual fermentation) I notice a temp difference of around 2-5f. I don’t doubt a 8-10f difference is possible, but wouldn’t expect that to be the norm.