I'm planning a fermentation monitor based on an arduino board. The temperature sensors comprise a 1/4" OD stainless tube 8" long, which will be used with buckets and carboys.

There are stainless thermowells for housing temperature sensors. These are 3/8" OD, 16" long. Do I need a thermowell in this case for measuring wort temperature, or can I put the temperature sensor directly into the wort without the thermowell? Here's why I'm unsure:

  • the thermowell page talks about protecting the temperature sensor, yet the stainless tube on the temperature sensor is made using the same process as the thermowell, so it's ok to immerse the stainless part of the sensor directly in the wort?
  • most grommets for airlocks are 3/8" size, making the thermowell a perfect fit. Are there 1/4" grommets that would fit the temperature probe? A quick search didn't find anything.

Just some background info - I will also be measuring ambient temperatures as well as wort temperature, and understand the issues involved with temperature control based on the wort temperature due to the large thermal mass. I'm curious to see in practice how much fermentation temperatures rise above ambient over the entire fermentation period.

  • Check this out: elcojacobs.com/uberfridge if you want to see a cool arduino powered temp controller. Also, I would be very interested in seeing the solution you come up with.
    – tomcocca
    Mar 5, 2012 at 12:12

1 Answer 1


As long as the stainless part of the probe is the only part that will be in the liquid, I believe you will not need a thermowell.

A thermowell is generally used to protect a non-submersible probe or thermocouple.

enter image description here Image source.

It's also good for getting deeper into the carboy than your probe will allow. E.g. I use a 24" thermowell in my conical to reach the center of 10 gallons of liquid.

enter image description here

As for grommets, head to your local big box store. You can normally find a bag of assorted grommets for a buck or two. Check in the electrical stuff aisle.

  • 2
    mmmm...look at the slopes on that conical!
    – mdma
    Mar 6, 2012 at 14:51

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