To test your thermometer, bring a pot of water to a vigorous boil. Hold the thermometer stem or probe in the water, making sure not to touch the sides or bottom of the pot, and take your reading.

Remember that there are several factors that affect the boiling point of water:

• As atmospheric pressure decreases, the boiling point decreases. Atmospheric pressure will vary depending on your altitude and local weather conditions.
• Hard water boils at a temperature 1-2°F higher than soft water, due to dissolved mineral salts.
• Using a tall, narrow pot will result in a boiling point about 1°F higher than a short, wide pot.

If you live at high altitude, you'll need to take that into account when testing your thermometer. The table below lists the approximate boiling point for a number of different altitudes. As a general rule, the boiling point decreases approximately 1.8°F for every 1000-foot increase in altitude. Note that the actual boiling point may be higher or lower depending upon atmospheric pressure in your area on any given day.

Another way to determine your boiling point is to use a Boiling Point Calculator. By entering your current barometric pressure and your elevation, you can get a good estimate of your boiling point.

Ice Bath Test

If your thermometer measures temperatures of 32°F or below, you can test it using an ice bath. Fill a Styrofoam cup with crushed ice and then add cold water. Insert the thermometer stem or probe into the ice bath and stir continuously. An accurate thermometer will read 32°F.

One advantage of using this method is that you don't have to take atmospheric pressure into account. An accurate thermometer will read 32°F in an ice bath at any altitude or atmospheric pressure.

What To Do If Your Thermometer Is Inaccurate

When testing a thermometer for accuracy, all you're trying to do is make sure that your unit is not grossly out of whack. You should be worried about being off by +/-20°F, not +/-2°F. If your testing shows that your thermometer is off by only a few degrees, don't do anything--just take those few degrees into account when reading your thermometer.

It should be noted that even high-quality, industrial-grade thermometers from manufacturers like Trend are only accurate to +/-1% of their scale. This means that at a "normal" 212°F boiling point, these thermometers may read up to 2°F above or below the actual temperature and still be within manufacturing specifications.

Bottom line: Don't sweat your thermometer being off by a few degrees!

If your thermometer is off substantially, you may want to consider replacing it with a higher quality model. See Measuring Temperature In The WSM to learn more about available options.

A very few thermometers can be recalibrated. Make sure to consult the instructions that came with the thermometer before attempting to recalibrate it. The process usually consists of adjusting a nut on the back of the dial, as shown in Picture 1.