You probably know that temperature control is a key way for brewers to ensure consistent, quality beer from one batch to the next. Professionals enjoy the benefit of glycol-cooled, jacketed cylindroconical fermentation tanks, but we homebrewers typically improvise a temperature-regulated fermentation chamber or even an ice bath. Just measure the temperature of the beer and let a controller turn on the cold as needed.
Alas, as with human beings, there are multiple places at which one may take a beer’s temperature. Ideally you’d just sanitize the probe and drop it directly into the fermenting beer. In practice, however, this is probably a bad idea unless said probe is specifically rated for submersion in liquid. Here, then, are the two of the most common ways to measure temperature without voiding your controller’s warranty.
Within the Beer Itself via a Thermowell
This is the best place to measure because the beer in the middle of the fermentor can be much warmer than that near the wall, perhaps by as much as 10°F (5.6°C). Measuring your evolving beer at its warmest spot ensures that things stay nice and cool during the most active phase of fermentation.
The easiest way to measure internal temperature is to use a thermowell (pictured above), which is a long, closed metal tube that extends into the fermentor. If you use a carboy or a plastic bucket, a drilled stopper can hold a thermowell alongside your airlock. Many stainless fermentors already have a thermowell. If not, it’s easy to install one yourself. Once the thermowell is in, simply drop the probe into it and you’re on your way!
Attached to the Outside of the Fermentor
In the absence of a thermowell, taping the probe directly to the outside of the fermentor is your next best bet. This measurement will track the internal temperature fairly closely, but thanks to the insulating properties of the vessel and the aforementioned thermal gradient, you may wish to adjust the setpoint temperature a few degrees lower than your target. As fermentation slows, the internal temperature will more closely match that read at the edge.
It’s also worth taping some insulation around the probe to ensure that it measures the beer temperature and not an average of the beer and the surrounding air. Any kind of cheap insulation will work, including bits salvaged from old camping mattresses and yoga mats. Some brewers even use good, old-fashioned bubble wrap.
Whatever you do, don’t just hang the temperature probe in the chest freezer or refrigerator without attaching it to a fermentation vessel in some way. The mass of the fermenting beer is much greater than that of the surrounding air, and by the time the probe senses a warming of the air, the internal temperature of the wort is likely to have gotten well out of hand.