How to Test & Calibrate Your Brewing Thermometer

No single tool can have as much of an effect on your home brew quality as your thermometer. Whether you are an extract brewer cooling wort to pitching temp or an all grain brewer trying to hit your mash temps, having a reliable and accurate thermometer is one of the primary keys to success.

Unfortunately, thermometers are often one of the most neglected brewing tools. Brewers have a tendency to think they can simply buy a thermometer and that it will be accurate (and stay accurate) for the rest of their lifetime. Sadly, this is not true. Thermometers must be regularly tested and calibrated to ensure accurate readings.

With this information in mind, let’s discuss what kind of thermometer to buy and how to test and calibrate your thermometer to ensure a consistent brew every time.

What Kind of Thermometer Should I Buy?

What kind of thermometer you buy is dependent upon your budget and how you intend to use it. There are essentially four types of thermometers common in home brewing, and each has its advantages and disadvantages.

Floating Thermometer:

The floating thermometer is a common addition to find in homebrewing starter kits, and they definitely have a few advantages. For one, they are an enclosed system and are rarely inaccurate.

Unfortunately, if your thermometer is, indeed, inaccurate you can only adjust the temperature by doing a little math after the fact.

For instance, if your test shows the thermometer reads the ice water at 36 degrees, you need to subtract 4 degrees from any reading you perform with that thermometer. This ensures that your readings are accurate, even though your floating thermometer may not be.

Floating thermometers are also typically pretty inexpensive, though they can also be pretty fragile.

Bi-Metal Dial Thermometer:

The bi-metal dial thermometer is a very common upgrade to a home brewer’s equipment, and it is a very good investment to make. In my opinion, an instant-read thermometer is your best bet, as it gives a very quick response time when you crack open your mash tun to check temperatures.

Dial thermometers are also easy to calibrate, but can become inaccurate easily, especially when you use them regularly or have dropped it once or twice. So, it is important to test them regularly. While bi-metal dial thermometers tend to be a bit more expensive than floating thermometers, they are also a bit tougher and an overall great investment.

Digital Instant Read Thermometer:

The digital instant read thermometer is another great upgrade for any brewer, as well. Though, please note that there are some risks involved with having electronics so close to so much water.

Digital thermostats can vary greatly in price, and there are even some great ones that rival dial thermometers in price. Do some research and find a reputable model, and make sure it is easy to calibrate.

Also, if you do buy a digital thermostat, make sure that you can easily order replacement thermowells, as those parts tend to wear out with heavy use.

Vessel-Mounted Dial Thermometer:

The last type is not really a separate type, but a variation on the bimetal dial thermometer. This variation is a vessel-mounted dial thermometer. You see these built into many homebrew kettles, and they are great to use. They also tend to be very easy to calibrate, and they afford the brewer the ability to read the temperature on a vessel instantly and without opening the vessel.

There are a few disadvantages, including the fact that you will have to have more than one if you intend to use one on your mash kettle, hot liquor tank, and kettle. The need for multiple thermometers increases the investment somewhat.

In addition, they can provide unwanted nooks and crannies which enable bacteria or wild yeast to set up shop. To counteract this possibility, it is a good idea to regularly remove and clean these thoroughly.

What Happens If You Use A Bad Thermometer

So, my thermometer’s off a little bit. What’s the worst that could happen? This is a common question amongst beginning brewers.

A bad thermometer can cause everything from minor off-flavors to a completely ruined batch of beer. Being a few degrees off when soaking specialty grain as an extract brewer won’t hurt much, but an all grain brewer needs to hit their mash temperatures right on, and pitching temperatures are vital to yeast health. If you are using a thermometer to monitor your fermentation temperature (and you should be), a few degrees of inaccuracy can have an astounding effect on the beer your yeast makes for you.

Take, for instance, mashing temperature. A low mashing temperature may cause the mash to take a very long time to complete and your beer may be dire and have less body, while a temperature that skews too high can make your beer too sweet. In the worst case scenario, a bad thermometer can completely disable your enzymes, resulting in a mash that never converts at all.

When cooling and pitching, a bad thermometer can cause anything from stressed yeast (which causes a wide range of off flavors) or, if the temperature is too high, it can kill your yeast altogether, opening a very wide window for contamination with wild yeast or mold.

Testing Your Thermometer

As you can see, having an accurate thermometer is incredibly important. But, can you ever trust your thermometer again? Of course.

The most important thing a homebrewer can do is test their thermometer regularly. At the very least, you should test your thermometer if it has been dropped or has undergone another mishap. Some brewers elect to test theirs before every brew day. This is a good habit to develop if you plan to brew your own beer regularly.

How to Test & Calibrate Your Thermometer

To test and calibrate your thermometer, follow these instructions:

1. Fill a 2 quart container or pitcher with ice.
2. Add water almost to the top of the container.
3. Stir the mixture to even out the temperature.
4. Allow it to rest for a minute or two to finish equalizing.
5. Place your thermometer in the container. Make sure the sensing area is completely submerged, but is not touching the sides or bottom.
6. Let the thermometer sit in the ice water for 30 seconds (or until the dial stops moving).

Adjust your thermometer accordingly:

• Dial Thermometers:

  Use a calibration tool on the hex adjusting nut (located behind the dial, typically) to rotate the dial until it reads 32° F while still holding it in the ice water.

• Digital Thermometer:

  You may need to press the reset button instead. Follow the instructions that came with your thermometer.

• Non-Adjustable Floating Thermometers:

  You will simply need to remember how far the thermometer is off and adjust whenever you take a reading rather than attempting to adjust the thermometer.

Please note: You can also follow these steps using boiling water, but instead of adjusting to 32° fahrenheit, you would adjust to 212°.

Always keep a backup thermometer around just in case. You never want to be caught without a thermometer when you are attempting to cool your wort to pitching temps.

From grain to glass, temperature plays a key role in the consistency and quality of your brew. Choosing the right thermometer and taking great care of it is an investment which pays you back with fantastic beer.

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