There are two ways to check the accuracy of a food thermometer: ice water and boiling water. Many food thermometers have a calibration nut under the dial that can be adjusted. Check the package for instructions.

Ice water. To use the ice water method, fill a large glass with finely crushed ice. Add clean tap water to the top of the ice and stir well. Immerse the food thermometer stem a minimum of 2 inches into the mixture, touching neither the sides nor the bottom of the glass. Wait a minimum of 30 seconds before adjusting. (For ease in handling, place the stem of the food thermometer through the clip section of the stem sheath and, holding the sheath horizontally, lower it into the water.) Without removing the stem from the ice, hold the adjusting nut under the head of the thermometer with a suitable tool and turn the head so the pointer reads 32°F.

Boiling water.To use the boiling water method, bring a pot of clean tap water to a full rolling boil. Immerse the stem of a food thermometer in boiling water a minimum of 2 inches and wait at least 30 seconds. (For ease in handling, place the stem of the food thermometer through the clip section of the stem sheath and, holding the sheath horizontally, lower it into the boiling water.) Without removing the stem from the pan, hold the adjusting nut under the head of the food thermometer with a suitable tool and turn the head so the thermometer reads 212°F.

For true accuracy, distilled water must be used and the atmospheric pressure must be one atmosphere (29.921 inches of mercury). A consumer using tap water in unknown atmospheric conditions would probably not measure water boiling at 212°F. Most likely it would boil at least 2°F, and perhaps as much as 5°F, lower. Remember that water boils at a lower temperature in a high altitude area.

Even if the food thermometer cannot be calibrated, it should still be checked for accuracy using either method. Any inaccuracies can be taken into consideration when using the food thermometer, or the food thermometer can be replaced. For example, water boils at 212°F. If the food thermometer reads 214°F in boiling water, then it is reading 2°F too high. Therefore, 2°F must be subtracted from the temperature displayed when taking a reading in food to find out the foods true temperature. For another example, ground beef patties must reach 160°F for safety. In this instance, if the thermometer is reading 2°F too high, 2°F would be added to the desired temperature, meaning the hamburger patties must be cooked to 162°F.

If you have questions, contact Dr. Angela Fraser, Associate Professor/Food Safety Education Specialist, Clemson University, Clemson, SC.

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