Most industrial and scientific thermometers can be calibrated to make them as accurate as possible. The thermometer should be adjusted to ensure accuracy whenever it is dropped, just prior to its maiden use or when the device is used to measure conditions at opposite temperature extremes.
Locate the calibration control for the thermometer. Usually this is either a thumbscrew or nut somewhere along the side or back of the thermometer. When you turn it, either the needle or dial behind it should move slightly.
Prepare a condition where you have a known temperature to which you can calibrate your dial thermometer. An ice water bath is the easiest and safest possible way to do this.
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Take a medium-size container and fill it half of the way up with ice. Now pour cold water over the ice to fill the container the rest of the way. Wait 10 to 15 minutes for the temperature to stabilize. There should be still both water and ice in the container.
Place the dial thermometer's stem in the ice bath and wait for the needle to settle on a temperature. If this temperature is zero degrees Celsius / 32 degrees Fahrenheit, the thermometer is correct. If it is not reading this temperature, use the calibration control to correct it. Turn the adjustment in the appropriate direction until it gives you a reading of the freezing temperature of water.
If you have another thermometer available, you can test conditions at various temperatures and double-check your calibration.