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How to Calibrate a Thermometer

I got a new quick read probe thermometer, but I'm not sure it is reading accurately.  Is there an easy way to check it?
-- Dan

There are two temperatures that are very easy to check a thermometer for in the home kitchen -- the freezing point of water, and its boiling point.

Water freezes at 32°F (0°C).  If your thermometer will read that low, then fill a glass half full of cold tap water and then add as many ice cubes as will fit.  Allow to sit for a few minutes for the temperature to even out and then take a reading, locating the tip of the probe about in the center of the glass.  You may need to watch and wait to see if the temperature is still dropping, but as long as there is still a fair amount of ice left, it will stay at the freezing point.

Checking the boiling point is a little harder.  Water boils at 212°F  or 100°C, at sea level under normal conditions.  If you are at higher altitude, however, the boiling point drops by about 2°F (1°C) for every 1,000 feet (300 meters) of elevation.  This is the reason why some recipes need to be adjusted for higher elevations (see High Altitude Cooking).  So if you live in Saskatoon, as I do, the elevation is 1,579.7 ft (481.5 m).  That means that water here boils at around 209°F  or 98.5°C.  Depending on the day, there may also be some atmospheric effects due to air pressure, but these are more difficult to calculate.  If you don't know the elevation where you live, you may be able to Google it, or find out be calling the reference department at your library.

To test for the boiling point, again assuming your thermometer is designed to read in that temperature range, place a pan about three quarters full of water over medium heat and bring it to a rolling boil.  Once it is boiling, carefully read the temperature, locating the tip of the probe well under the surface but away from the sides and bottom of the pan.  If the probe touches the bottom of the pan, you will be reading the temperature of the metal, not the water.  The temperature should read the boiling point of water corrected for your elevation, if necessary.

Water hardness shouldn't have a significant effect on these reading, but if you are compulsive you can use distilled water in both tests just to be sure.

Once you have determined how accurate your thermometer is, you may be able to adjust it.  For mechanical probe or bi-metal thermometers you should either find an adjusting screw or hex nut on the back.  While holding the body of the thermometer you can turn the screw or nut to move the dial the number of degrees that need to be corrected.  If the thermometer is off by the same amount in the same direction for both boiling and freezing, then adjust by that amount and re-check.  If the amounts vary then you can split the difference or adjust to the temperature closer to the environment in which you will use the thermometer -- ie. if it is for checking the temperature of roasts, then set it so the boiling point is more accurate.  If it is a fridge thermometer, set it so that the freezing point is right.

Electronic thermometers are not usually adjustable at home.  If yours is, follow the manufacturer's directions.

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