A thermometer is only helpful if it's accurate
A kitchen thermometer is one of the best tools to have on hand to ensure quality and safety when cooking and preserving. They come in a variety of types, from dial-stem meat thermometers to glass candy thermometers to digital thermometers with wireless remotes. Since even a few degrees can make the difference between over- or underdone meat or overcooked jams or candies, testing for accuracy is important. Fortunately, it’s easy to test a thermometer, and some – such as stem thermometers with hex nuts under the dial or digital thermometers with reset buttons – can even be physically recalibrated so that they read accurately.
There are two calibration testing methods: the ice water method (used for cold processes) and the boiling water method (used for hot processes). In general, it’s most accurate to calibrate your thermometer closest to the temperature for which it will be used. So if, for instance, you’re checking the temperature of a pot of boiling jam, using the boiling water method is preferable. That said, many people prefer to use the ice water method because it’s easier and you don’t need to worry about altitude adjustments – water freezes at 32°F regardless of elevation. If you find that your readings are getting drastically off, it’s probably time for a new thermometer.
Test your thermometer regularly for accuracy
Boiling Water Method
Bring a pot filled with clean water to a full rolling boil. Immerse the stem of the thermometer into the boiling water at least 2”, making sure that it does not touch the bottom or sides of the pot. Wait at least 30 seconds or until the temperature indicator stabilizes, then check the temperature reading on the thermometer (take care to read at eye level and not at an angle if you’re using a candy- type thermometer). If the reading corresponds with the temperature of boiling water at your elevation, it’s accurate! If it’s off, you’ll need to physically recalibrate it if you can, or take the temperature difference into account.
To account for a temperature difference, just add or subtract the variance to the temperature required by your recipe. For example, if your thermometer registers 214°F in boiling water at sea level, it reads 2° hotter than it should. If your recipe calls for a temp of 220°F, then you’d need to cook to 222°F.
Another way to put it: If your thermometer reads higher than it should, add the difference to your recipe’s indicated temperature. If it reads lower than it should, subtract the difference from the recipe’s temperature. To adjust for the gelling point for jams and jellies, simply add 8° to the boiling temperature reading from your thermometer.
Ice Water Method
Fill a large glass with crushed ice, add clean water to the top of the ice, and stir well. Insert the stem of the thermometer at least 2” into the water, making sure that it does not touch the bottom or sides of the glass. Wait 30 seconds or until the temperature indicator stabilizes, then check the reading on the thermometer. If it does not read 32°F, physically adjust the thermometer if possible, or account for the difference like in the boiling water method described above.
For further information on canning visit the National Center for Home Food Preservation (NCHFP) at nchfp.uga.edu or contact your local Cooperative Extension office.
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Brought to you by the UCCE Master Food Preservers of El Dorado County
Last updated: July 2025