My current method of fermentation temperature management is submerging my carboy into a large bucket of water. I regulate the temperature of my water with ice packs/frozen water bottles to keep it at the desired temperature. I use a pretty large volume of water so temperature fluctuations are minimal, maybe a degree or two over 24 hours.

My concern is that I am setting the temperature of the water to the desired fermentation temperature but the actual temperature inside the carboy is somewhat higher.

My question(s) are,

First, what is the best way to get an accurate temperature of a fermenting wort? I have read some responses to the question How many degrees will my carboy heat up as a result of fermentation activity? and after further research am uncertain that a stick on thermometer will do the job since it is positioned on the glass and likely to be affected by outside temperatures.

Second, is there a way to calculate the appropriate temperature to set the environment, in my case bucket of water but could also be a fridge, to in order to appropriately compensate for the added heat of fermentation?

3 Answers 3


Your carboy is I imagine not huge, and therefore the temperature in the carboy will not be wildly different from the outside, but a small difference will exist.

I suggest buying an aquarium thermometer and dropping it in the top through a bung or down the side of the bung. You can then get an idea of the difference between edge and central temp of your carboy.

  • Good call on the aquarium thermometer.
    – Kingfisher
    Jul 11, 2018 at 14:02

I put one of the stick on thermometers on the fermenter and adjust according to what it tells me. They're very accurate in reflecting fermentation temp.


This is what I did.

The ales I’ve been making have an optimal range fermentation temp. of between 18C and 22C. However I have found that leaving my wort to it’s own devices will see it’s temp going well up to 26C. Which is on the high side.

My environment is relatively warm as such I have use a chest freezer as a brew cupboard, but I built a rig similar to this :


This guy built it for an aquarium, but I built vertically the same thing for my brew cupboard (aka chest freezer). I set the temp to about 20 and put the thermometer in the freezer but not connected to the wall.

This works perfectly and using the stick-on temperate palate on the side of the carboy shows that it’s sitting perfectly as expected.

This is an inexpensive method (except for the freezer) but works very well and there is little manual intervention during the initial fermentation.

Happy brewing.

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