Calibrating a dial thermometer

26 02 2010

How accurate is your kitchen thermometer? And if it’s accurate at one temperature, does that mean that it’s accurate over its entire range?

What brings me to this question is, of course, coffee. I’ve found that some kinds of coffee are very sensitive to the brewing temperature. Just a couple degrees in either direction can make a good coffee bitter or sour, and if the temperature is just right, it can bring out undertones of cocoa, delicate berry notes, and perhaps even a lush, winelike acidity. OK, I admit that I just copied that last bit from fancy coffee websites. My coffee palate may not be particularly refined, but I do know from experience that sourness and bitterness depend on temperature.

The adjustment nut

The kind of thermometer most commonly found in the kitchen is a dial thermometer, also called an instant-read thermometer. (But what thermometers aren’t read in an instant?) They are typically calibrated in one of two ways. The first way is to put it in boiling water, check the reading, and if it’s not at 212 degrees (or 100, if you live in one of those other countries), use a wrench to turn the adjustment nut on the underside of the dial until it is. A couple of things to be careful of when doing this: First, it’s hot! Second, unless you live at sea level, the boiling temperature of water isn’t exactly 212 degrees. I live in Chicago, which is at about 600 feet, and the boiling temperature here is just a tiny bit lower, at 211 degrees. (The boiling point of water decreases by about 1 degree F for every 500 feet of elevation.)

The second way to calibrate a thermometer is to put it in ice water, and adjust the nut so that it reads 32 degrees. When it comes out of the freezer or in from the outdoors, ice is usually colder than 32 degrees, and water from the tap is generally much warmer. When you put the two together, it takes some time for the ice to warm up to 32 degrees and for water to cool to 32 degrees, so before adjusting the thermometer, you should wait a while for the water and ice to stabilize at the freezing point.

This brings us to the next question: If you calibrate your thermometer at the freezing or boiling point, does that mean that it will be accurate over its entire range?

To find out, I checked the calibration of my thermometer, first on the cold end, and then on the hot end. I don’t have an ice maker and I didn’t have any ice made in an ice tray, but fortunately, there’s a readily available supply of snow at this time of year.

This chunk of snow was conveniently located on a plate in my kitchen.

Thermometer in ice water, reading 32 degrees.

Thermometer in boiling water, reading 203 degrees.

The thermometer was right on at freezing, but 8 degrees low at boiling.This means that I could be brewing coffee with water that’s 8 degrees hotter than I think it is! Note to those for whom I’ve made bad coffee: don’t blame me, blame the thermometer.

Unfortunately, there’s no way to calibrate this thermometer for both ends of the spectrum, so I have to choose whether I want it to be accurate at high temperatures or low temperatures, or somewhere in-between. Since I use the thermometer primarily for hot stuff, I decided to calibrate it for high temperatures. I set it to read a few degrees lower than 211 at boiling (when the temperature actually is 211), so that it would be most accurate around 170 degrees.

A cautionary note: don’t let your thermometer fall into the water. Water can seep in at the base where the dial meets the probe. Mine dropped briefly into the boiling water, and later on it got kind of stuck at around 90 degrees. Things turned out all right, though, after I baked it for an hour at 200 degrees. I think that drove out any water that had gotten inside.

Bonus picture: The infrared thermometer says that the ice water is just about 32 degrees.

OK, the snowball didn't magically appear on a plate. I took it from the bathtub.

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