Calibrate a thermometer by filling a container with finely crushed ice and water, stirring the mixture and immersing the stem of the thermometer at least 2 inches into the water without touching the sides or bottom of the container. If you are calibrating a food thermometer, adjust the nut under the dial so the unit reads 32 degrees Fahrenheit without removing the probe from the ice water. These methods help to verify the accuracy of food and other thermometers.
A second method of calibration requires using boiling water. For this method, use distilled water and take into consideration the atmospheric pressure of the location where you are working. Minerals dissolved in tap water and changes in atmospheric pressure can vary the boiling point of water by as much as 5 degrees Fahrenheit. At one atmosphere of pressure, water boils at 212 degrees Fahrenheit.
Some thermometers do not have the adjusting nut. With these thermometers, just check the accuracy of the unit. For minor inaccuracies, add or subtract the difference from the reading from the calibration and thermometer reading. If the thermometer reads 214 degrees Fahrenheit in the boiling water bath, subtract 2 degrees from the reading for a more accurate measure of the temperature. If the thermometer is grossly inaccurate, replace it.
In certain industries, particularly in the food processing sector, calibrating a thermometer is essential to ensure accurate temperature readings. The inexpensive and simplest methods of utilizing ice water and boiling water are widely used in adjusting a thermometer.