Introduction
A thermowell is a great way of enabling accurate temperature measurement of a liquid. It’s sort of like a hollow metal finger that pokes through the walls of a vessel so that it’s in direct contact with the liquid. The temperature inside the thermowell will quickly adjust to the temperature of that liquid. Pushing a temperature sensor along the inside of the thermowell will provide a very easy and accurate means of measuring temperature. The temperature probe doesn’t make contact with the liquid inside the vessel but is effectively submerged in that liquid. In the case of a fermentation vessel this means that the temperature probe doesn’t need to be sanitized or be perfectly clean.
Affluent homebrewers who can afford expensive stainless steel fermenters will probably find a thermowell already installed on this type of product. For those of us leaning towards the more cheapskate way of doing things are probably still using glass carboys or plastic buckets to ferment our beers. Installing a thermowell requires a bit more thought.
The Thermowell Tube
A commercially available thermowell is normally engineered with screwed threads around the probe inlet so that it can be screwed directly into the vessel.
It’s difficult to use this type of thermowell in a carboy or bucket. A better solution is to use a simple stainless steel tube that is sealed at one end. The tube inner diameter must be allow the temperature probe to be pushed along it and its length must be sufficient to dip into the liquid inside the fermentation vessel. I found that a 7mm i.d. tube that was 200mm in length was ideal. Some brewers may want a longer thermowell and these tubes are available in a variety of lengths and diameter. Such tubes are readily available from on-line vendors for a very reasonable price (although you may have to buy several at once). The following photo shows the 5x 200mm tubes I was able to purchase off Ebay for around $16.
Once the tube has been acquired, we need to find some way to mount it into the fermenter. The easiest and most practical options are to push the tube through a hole in a rubber stopper or through a rubber grommet either of which can be mounted on the fermenter. However, before doing that, it’s important to shape the open end of the tube so that it can’t fall out of the stopper or grommet and get lost inn the beer, increase the risk of infection and expose the temperature probe to the beer.
The best way to shape the end of the tube is to flare it with a flaring tool used by plumbers to connect copper and other metal pipes to flared fittings.
Using a Multi-Holed Stopper
When using a glass carboy as a fermentation vessel, the mouth of the carboy is normally sealed with a rubber stopper and an airlock. This stopper provides the only means of introducing a thermowell into the liquid contents inside the carboy. Fortunately this can be easily achieved using a stopper with two holes in it. The first hole, 5/16″ in diameter, will accept the airlock and the second smaller hole, 1/4″ in diameter will accept the thermowell. Such a two-holed stopper can be readily bought from on-line vendors at a high price. It’s also possible to buy a stopper with a thermowell already installed as shown in the picture below.
However, there are a few things about this product that concern me. Firstly, it’s a #7 stopper which I find a bit too big for my carboys (I like using #6.5). Secondly, the tubing is sealed at one end by crushing the tubing and presumably treating it to make a seal. This leaves a wide sharp edge and, with the flair at the other end, makes the thermowell tube difficult to remove without damaging the stopper. Lastly they seem to be expensive.
A better and cheaper solution would be to use the thermowell tubes described above with a two-holed stopper drilled with 5/16″ and 1/4″ holes by yourself as described in another article on this website. The thermowell tube will be inserted into the smaller hole and the airlock into the other. To simplify access to the thermowell for a temperature probe, it’s better to use a 1-piece S-shaped airlock. The pictures below show the assembled parts before and after insertion into the mouth of a 5-gallon glass carboy.
This device can also be used in plastic bucket and large jar lids by drilling out a hole of the right size to accept a stopper.
Using a Grommet
While drilling a hole in the lid of a bucket or large jar to accept a stopper with a thermowell installed in it works well, an alternative would be to feed the thermowell tube through a rubber grommet installed in the lid. A 5/16″ grommet is very slightly too big to properly seal around the thermowell tube which has an outer diameter of 8mm. Fitting the grommet into a slightly smaller hole in the lid than usual will fix this. The picture below shows the thermowell tube fitted to a fermentation bucket lid.
When using a grommet like this to install a thermowell tube, an airlock would have to be inserted into a second grommet fitted elsewhere in the same lid.
Application
Once the thermowell tube is installed into the fermenter using either method of mounting, a temperature probe can be simply pushed into it. This can be a thermocouple, a thermister, a platinum resistance thermometer probe, an electronic probe like the DS18B20, etc.
The thermometer and probe I showed in the earlier photos were designed to display the temperature of an aquarium. It was very cheap and looked ideal for measuring fermentation temperature. Unfortunately, the probe housing was too big to push down the thermowell tube, so I had to break a bit of it off. When you get a digital thermometer, remember to check that the probe will fit down the tube (or can be modified, as I did).
Once, the probe is in place, apply a piece of sticky tape or a cotton wool or rubber plug onto the open end of the thermowell tube. This will help keep the probe in position and stop it falling out. It will also stop air from diffusing in and out of the tube and possibly affect the temperature reading.
Elsewhere in this website, I’ll be (later) looking at different methods to measure, log and control fermentation temperature.
How did you wind up sealing the end of the thermowell so liquid didn’t get in to the probe?
Hi Erc
I’m so sorry for the late reply. I’ve been sidelined into doing other things and rather neglected my website 🙁
If you search on Amazon, Ebay or AliExpress, you can find 1/4″ (6mm) tubing with ends already sealed.
Here’s an example link:
Thermowell Tubing
Good luck,
Andy