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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?

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Regardless of whether you are heating or cooling you simply need to equilibrate the wort to the temp of the freezer (in your example). Meaning chilling the wort to the temp you expect to ferment at. I have never had the problems that mdma describes in the quote you provide.

Don't put 80F wort in a carboy into a freezer unit and set it to 65F, that will create the problem mdma describes. Chill to 63-67F first. IF you can't do that with your chilling, method, then put the fermentor in there first without the probe on it. Let it equilibrate, then attach (or insert) the temp probe to the vessel/wort.

If you start with just a degree or two of difference between the ambient temp and the wort temp things will work out fine. I tend to tape the probe to the side of my fermentor. I tape a small hand towel that's been folded over a couple times against the fermentor to make an insulated pocket. Then slide the probe in there against the carboy/bucket directly.

You also can to play with the differential settings on the controller too. That will ensure the device cuts out prior to things getting too far away from your set point. Say you are trying to cool to a set-point of 50F, setting the differential to cut out at 55F will let the carboy "catch up" to the chilled air before the freezer comes on again. If the differential is set to 51F you are likely to overshoot.

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I'm curious about why you tape the probe to the exterior of the fermentor instead of using a thermowell. Wouldn't the reading be more accurate from the middle of the vessel with a thermowell? I've read in several places opinions from people who don't like thermowells, but I haven't yet read a good explanation of why. –  Galapagos Jim Feb 7 '12 at 0:13
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In theory measuring in the center might be more accurate, but in reality the temperature monitored at the side is the same. If you've ever used a carboy for fermentation and see how much its stirring itself around the temperature is pretty homogenous throughout the vessel. That and I don't want to clean and sanitize another piece of stuff going into the carboy. –  brewchez Feb 7 '12 at 1:34
    
+1 to taping the thermometer's probe to the outside of a fermentation vessel. From Yeast by Chris White and Jamil Zainasheff: "[One] option is just taping the controller probe to the outside of the fermentor. If you do this, you must cover the probe with some sort of insulation. ... As long as there is any fermentation activity in the fermentor, the temperature will be the same throughout the beer." p. 103 -- Hope this helps. –  Dustin Rasener Feb 7 '12 at 3:08
    
+1 good to hear you're not getting these problems, brewchez, but how do you chill the wort to the desired temperature to avoid the large fluctuations - isn't that what the temp the controller is for? I find taping on the outside is the best compromise and allows the freezer to still be used for storing other things. I had written a paragraph about taping the probe to the outside in my quoted answer, but it was already getting too long so I pulled it. –  mdma Feb 7 '12 at 8:05
    
I have a temp controller as well, and I don't use it to cool just-boiled wort unless I have to. I have an immersion chiller, and I always try to pitch yeast a degree or two below my target fermentation temperature. The only time I have trouble getting the wort to ~65 is in the summertime, when the cold-water spigot is too warm (lagering is a similar problem). In these cases, I chill the wort as far as I can with the chiller, then put the un-pitched wort in the temp controller to get down to fermentation temp. See: homebrew.stackexchange.com/questions/4629/… –  Dustin Rasener Feb 7 '12 at 12:28
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