Calibrating Your Thermometer

Ice Point Method:

• Fill a plastic or metal container (a large Styrofoam cup, for example) with chipped or crushed ice; then add clean freshwater to a depth of at least 10 cm (4 inches) - 50% ice, 50% water.
• Stir the ice and water then wait a minimum of 2 min to be certain the water is completely cooled and good mixing has occurred.
• Suspend the stem of the dial thermometer or the probe of the electronic thermometer in the ice slurry.
• Wait 2 minutes until the indicator stops changing.
• While thermometer is in the ice water adjust the thermometer to 0ºC (32ºF), if necessary, by following the manufacturer's directions. These thermometers are generally adjusted with a zeroing screw.>

Note: Depending on the material that the container is made of, the temperature of the container could be higher or lower than the temperature of the water so one should avoid touching the bottom or sides of the container.

Boiling Point Method:

This is much more dangerous than calibrating with ice so be extra careful. You must first calculate the boiling point of freshwater for your working altitude. See the section on physical constants for the factors to use to calculate the adjusted boiling point below.

Because of the complexity involved in calibrating a thermometer in boiling water, this method should only be used to confirm that a thermometer calibration at 0ºC (32ºF) is measuring accurately at higher temperatures.

• Heat a pot of freshwater until a rolling boil is achieved.
• Immerse the stem of the dial thermometer or the probe of the electronic thermometer in the boiling water.
• After at least 1 minute, read the temperatures on the thermometer without removing it from the boiling water.
• While thermometer is in the boiling water adjust the thermometer to 100ºC (212ºF), if necessary, by following the manufacturer's directions. These thermometers are generally adjusted with a zeroing screw.

Note: The boiling point of water is influenced by air pressure (weather and altitude) and solutes in the water. Always use distilled water (it can be purchased in grocery or hardware stores) and check with Bureau of Meteorology for the normal boiling point of water in your area. The boiling point lowers about 0.6ºC (1ºF) for each 168 meters (550 feet) above sea level.

Cannot adjust a thermometer?

If you have a thermometer that cannot be adjusted, record the amount that the temperature is off by and adjust your future readings by this amount.

For example, if your thermometer reads 2.5 degrees Celsius instead of 0.0 degrees Celsius in the ice mixture, it is 2.5 degrees higher than it should be and you should subtract 2.5 degrees from your observed measurement. If, on the other hand, your thermometer reads -3.0 degrees Celsius in the ice mixture, it is 3.0 degrees lower than it should be and you should add 3.0 degrees to your observed measurement.

If possible, apply a note with clear tape on the thermometer itself about what correction is needed following a measurement. This is not an exact solution but it should serve your purposes.