Calibrating a Dial Thermometer
Some thermometers, especially dial thermometers, can be significantly out of calibration. Fermentation problems encountered form seemingly normal mashes can be a result of using a poorly calibrated thermometer during the mash.
It’s a good idea to check the accuracy of thermometers using during the mashing and fermentation process. This can easily be done by measuring the temperature of melting ice and boiling water. Melting ice should be 32 degrees F (0 degrees C) and boiling water should be 212 degrees F (100 degrees C).
To calibrate a dial thermometer, fill a glass with ice cubes and then fill the glass with water. In a few minutes the water temperature will drop to 32 degrees F. If your thermometer is off, rotate the adjustment screw on the back of the face so that it reads 32 degrees F (0 degrees C).
Floating thermometers cannot be adjusted. They are typically more reliable than dial thermometers because they are calibrated when manufactured and don’t change over time.
The crystal strips which stick to the bucket or carboy are intended to monitor fermentation temperature. They may not be calibrated and are not accurate enough to use for measuring mash temperature or the internal temperature of a wort or must for yeast pitching.