Installing a Thermowell into a Plastic Fermentor

Most homebrewers ferment in a cylindrical vessel whether it’s a glass carboy, plastic Better-Bottle or a plastic fermentation vessel. The problem is that the walls of these fermentors are made of (relatively thick) thermal insulators (plastic or glass) and taping a probe to the side of them is not going to get a sufficient response to the heat generated during the height of fermentation. Coupled with the relatively small contact area between your probe and bucket wall, this can create enough of a lag to generate off flavors in your beer. This is especially important in higher gravity beers with a vigorous fermentation and in lagers where off flavors are difficult to hide.

In order to properly measure the real wort temperature and respond to it with your heating/cooling cycle, we ideally would like a reading inside the wort. The problem results in how to do this effectively without infection. The answer is a simple device called a thermowell. In essence, it’s a (stainless steel) tube that is inserted deep inside the wort and sufficiently far away from the walls to minimize the influence of the outside walls (the source of the cooling). Considering that a fermenting beer is always churning and moving and that water is a good thermal conductor, the cross-sectional temperature profile should be relatively uniform once you move away from the walls a few inches. Just to be sure we try to get the probe of the thermowell in the center of the vessel.

Let’s see how it’s done.


16″ Stainless steel thermowell .304″ ID
Note: you could also use the .250″ ID thermowell depending on size of your temperature probe. My probe had a rubber sleeve around the electrical connections so I opted for the larger ID above.
5/16 ID x 5/8 OD Rubber Grommet
10ml of Mineral oil (optional)
(This assumes you already have the lidded plastic fermentation bucket)


Drill (or drill press)
7/16″ bit
utility knife (optional)
fine grit sandpaper (600 grit or better) (optional)


About 5 minutes from start to finish


(1) Thermowell @ $12 (plus shipping)
(1) Grommet @ $1.07 for 2
TOTAL: ~$15


1. After sourcing the materials, test the fit of all parts!
– Does the thermowell fit inside the grommet tightly? It should. If not, get a smaller ID grommet.
– Does the temperature probe fit inside the thermowell? If the probe is a little tight, you can sand it down with some fine sandpaper (600 grit). It is best to have the probe as close to the same size as the thermowell; the more contact the better. This way the temperature will conduct directly from the thermowell walls into the probe. If you are worried, get the bigger ID thermowell; don’t worry, we can adjust for that at the end.

2. Decide were on the lid to insert the thermowell. Take note that the grommet  that is already installed for your blow off tube or airlock is about the same size as is necessary here so you may be able to use that to test fit your installation before you drill. You may need to adjust the size of the drill bit for your grommet!
You are going to want to install the thermowell close to the middle of the lid. Since there was already a depression in the middle that was sufficiently sized, I chose dead-center.


3. Drill the hole. Be careful! Plastic is soft and normal twist bits may catch as they push through. If you have a drill press, you may want to use it here to help with this. Also, you may want to clamp the lid to a piece of wood as a backer.
Note: There may be some burrs on the edge of hole you just drilled. so gently use a utility knife (or any other blade) to remove them. Don’t get too overzealous… plastic is SOFT!
IMG_6152 IMG_6149

4. Press the grommet into the hole with your fingers and test the fit. There should be enough space so that it can freely rotate without binding. Keep in mind that the thermowell will spread the rubber outwards when inserted.

5. Slowly insert the thermowell through the grommet in the lid (from the top). If you go too fast, you will push out the grommet. It may help to hold the grommet on the back side with your fingers as you push or or use some water/oil as lubricant.

6. Place your temperature sensor inside the thermowell during fermentation and connect it to your favorite temperature controller.
Note: if you bought an oversized thermowell, you can improve it’s performance by adding a small bit (capful) of mineral oil to the bottom of the thermowell. Mineral oil is not electrically conductive, so it won’t mess with your temperature probe wiring. It’s also a good thermal conductor, odorless, and cheap to boot. Just don’t forget it’s in there when you turn your lid over next time or it will spill out!
IMG_1411 IMG_1413


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