There are two methods that you could use to calibrate thermometers -
Ice Point Method - Fill a container with crushed ice and water. Ensure that the container have enough crushed ice to provide an environment of 32 degrees F, so you may need to add more ice into the container during the process. It will takes about 4 to 5 minutes for the mixture of water to stabilized. Then insert the thermometer which needs to be calibrated into the appropriate immersion depth. Hold the thermometer away from the bottom and sides of the container to avoid error. If your thermometer is not accurate at 32 degrees F, adjust the thermometer accordingly or provide for error correction.
Boiling Point Method - Fill a container with distilled water and heat it up. After the temperature in the water has reached the boiling point, insert the thermometer . Be sure to leave at least a two inch gap between the thermometer sensing element and the bottom and sides of the container to avoid error. If your thermometer is not accurate at 212 degrees F, adjust the thermometer accordingly.