Thermometers need calibrating daily, before the first use, when moving from one temperature to another or after the thermometers have been dropped to ensure correct temperatures are recorded. There are two methods to calibrate thermometers: the ice point and boiling point methods.
The ice point method is done by submerging the probe in a glass filled with crushed ice and water. The probe is left for 30 seconds without letting the probe touch the glass. When the indicator has stopped moving, it is adjusted until it points to 32 degrees Fahrenheit.
The boiling point method is done in a similar way in a pot of boiling water. After being submerged in the water for 30 seconds, the indicator is calibrated by turning it to read 212 degrees Fahrenheit.