Prepare a mixture of fresh water and crushed ice with more ice than water in your container.
Put the thermometer into the ice-water bath. The bulb of the thermometer must be in the water. NOTE: This will not work for some digital thermometers that should not be immersed in water. Only immerse the probe part of a digital thermometer, if it has one.
Allow the ice-water bath and thermometer to sit for 10 to 15 minutes.
Gently move the thermometer around in the ice-water bath so that it will be thoroughly cooled.
Read the thermometer. If it reads between -0.5˚ C and +0.5˚ C, the thermometer is fine.
If the thermometer reads greater than +0.5˚ C, check to make sure that there is more ice than water in your ice-water bath.
If the thermometer reads less than -0.5˚ C, check to make sure that there is no salt in your ice-water bath.
If the thermometer still does not read between -0.5˚ C and +0.5˚ C, replace the thermometer.