To ensure that your poultry is cooked to a safe temperature without drying it out, or to accurately grill a steak to a medium-rare or well-done order, it is important to take a precise temperature reading. This can be accomplished with a properly calibrated food thermometer. A thermometer calibration can be done in a matter of minutes with a few supplies found in every kitchen. You should always calibrate a thermometer after it is purchased, if it is dropped, if it is being used between hot and cold temperature extremes, or on a routine and monthly basis.
Food thermometers have a hex screw that allows them to be calibrated appropriately. Two primary calibration methods exist: one allows you to calibrate to the boiling point of water, and the other allows you to calibrate to the freezing point of water.
Use the boiling water method if you usually use the thermometer for hot temperature readings and the ice-water method if you usually use the thermometer for cool temperatures.
Freezing or Ice Point Method
To calibrate using the ice point method, fill a glass with as much crushed ice as you can and fill the remainder of the space with distilled or filtered water. If you do not have distilled or filtered water, use clean tap water. Stir the ice chips and water.
Insert the thermometer's probe or stem into the middle of the glass so that it is not touching the bottom or sides of the glass. Be sure to submerge at least 2 inches of the probe into the ice bath. Hold the stem in the ice bath for 30 to 60 seconds.
Adjust the hex nut under the head or dial of the thermometer so that the temperature reads 32 degrees F, which is the temperature of ice water, also known as the freezing point. If you have a digital thermometer, press the reset button to adjust the temperature. To check the calibration, you can repeat this method with a fresh ice water bath. In this second round, the thermometer should read 32 degrees F without requiring any adjustment.
Boiling Water Method
Instead of using ice water, you can use boiling water, especially if you plan to use the thermometer primarily for hot foods. To calibrate your thermometer with boiling water, pour tap water into a cooking pot and place it on the heat of a stove top element. Bring the water to a rolling boil.
Place the stem or probe into the boiling water so that at least 2 inches of the stem are immersed; the stem should not be touching the sides or the bottom of the pot, as this will affect the reading by making it too high.
Hold the stem in the water for 30 to 60 seconds. Adjust the hex nut under the head or dial of the thermometer so that the temperature reads 212 degrees F, which is the boiling point of water. If you have a digital thermometer, press the reset button to adjust the temperature.
When using the boiling water method, be sure to consider your geographic location; the boiling point of water is 1 degree F lower for every 550 feet rise above sea level. For example, at 550 feet above sea level, the boiling point of water is 211 degrees F. If you live in a mountainous area, you can either look up the local boiling point or make this calculation yourself based on the area's sea level.