Calibrating a Bimetal Thermometer

There are a couple of ways to calibrate a bimetal thermometer. The best way is simply to use an accurate thermometer, like a mercury filled lab thermometer. Simply put both thermometers in water, stir the water, and compare temperatures. Since I do not have a lab thermometer, the way I do it is using boiling and freezing water.

First, boiling water: You can put the thermometer in boiling water and read the temperature. At sea level, it will be 212° F. Of course, I do not live at sea level. Since, the boiling point of water will vary with barometric pressure, and barometric pressure changes with altitude, the boiling point of water is different where I live. In fact, because I live at about 2,600 feet above sea level, subtracting 0.9° F for every 500 feet puts the boiling point of water at about 207° F, where I live.

Freezing water is a different matter. The freezing point of water does not vary with with altitude. You can calibrate a thermometer by sticking it in a glass filled ice and water. I use crushed ice and water, making a slushy solution. The liquid in the glass will be very close to freezing. It should read about 32° F.

I try and calibrate at both positions, freezing and boiling, if possible. The reason is that a bimetal thermometer can be off differently at high and low positions. For instance, I have a thermometer that is dead on at freezing, but is off by a degree at boiling. Of course, it is not always possible to do so. For instance, my Blichmann BrewMometer goes from 60° F to 220° F, so I cannot calibrate at freezing.

My thermometers have a screw on the back that will adjust the temperature reading. By turning the screw, the needle will change position. Some bimetal thermometers have a nut that, when loosened, will allow you to turn the housing with all the marks and numbers, to a new position. Once at the new position, it can be tightened down again. Other bimetal thermometers are not adjustable; you simply have to add or subtract from the reading to be accurate.

That is about it when it comes to calibrating a bimetal thermometer. Good luck.

This is my BrewMometer. It has a screw on the back that adjusts the temperature.

This is my Fermentap thermometer. It has a small hex key on the side to adjust temperature.

This another hand-held bimetal thermometer with a long stem. It cannot be adjusted.