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Testing Thermometers For Accuracy

Originally posted: 06/01/2000
Last updated: 09/26/2014

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I've received several e-mails from people saying, "No matter what I do, I can't get my cooker over 200°F." It usually turns out that they've got a bad thermometer and were actually cooking at over 275°F!

You should check your thermometer for accuracy when you buy it and probably once a year thereafter, or any time it is has been dropped or subjected to abuse.

Here are two methods you can use to test a thermometer, followed by tips on what to do if you find your thermometer is inaccurate.

Click on any of the pictures below for a larger image.

Boiling Water Test

Boiling water test
Photo 1

The most common way to test a thermometer is to place it in boiling water. An accurate thermometer will read about 212°F in boiling water at sea level under normal atmospheric conditions.

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.

Altitude (feet) Boiling Point (F/C)
Sea Level 212.0 / 100.0
500 211.1 / 99.50
1000 210.2 / 99.01
1500 209.3 / 98.52
2000 208.5 / 98.03
2500 207.6 / 97.54
3000 206.7 / 97.05
3500 205.8 / 96.57
4000 204.9 / 96.08
4500 204.1 / 95.60
5000 203.2 / 95.12
5500 202.4 / 94.64
6000 201.5 / 94.16
6500 200.6 / 93.69
7000 199.8 / 93.21
7500 198.9 / 92.74
8000 198.1 / 92.26
8500 197.2 / 91.79
9000 196.4 / 91.32
9500 195.5 / 90.86
10000 194.7 / 90.39

Source: Boiling Point of Water vs. Altitude, K. Loomis, New Mexico State University Apache Point Observatory

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

Ice bath test
Photo 2

If your thermometer will measure temperatures of 32°F or below, you can test it using an ice bath. The advantage of 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.

Fill a glass with ice, and then add just enough water to cover the ice, but not so much water that the ice floats.

After a couple of minutes, insert the thermometer stem or probe into the middle of the ice bath and stir gently. Don't let the thermometer rest against the ice or you'll get a low reading.

An accurate thermometer will read 32°F.

What To Do If Your Thermometer Is Inaccurate

Adjusting the calibration nut on a thermometer
Photo 3

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 are only accurate to +/-1% of their scale. This means that at a standard 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 by +/-5°F or more, you may want to consider replacing it with a higher quality model. See Measuring Temperature In The Weber Bullet to learn more about available options.

A very few thermometers can be recalibrated. Make sure to consult the instructions that came with your thermometer before attempting to recalibrate it. The process usually consists of adjusting a nut on the back of the dial, as shown in Photo 3.

More Thermometer & Temperature Links On TVWB

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