Understanding how bimetallic strips work

Article By : John Dunn

Bimetallic strips can be useful for making thermometers and thermostats, or for providing temperature compensation for other devices.

According to Wikipedia, the basic invention of bimetallic strips comes from 18th-century clockmaker John Harris (1693-1776). Two strips of dissimilar metals are mechanically joined either by rivets or by being fused together along their length to form a structure that responds to changes of temperature by bending in a preferred direction.

this diagram shows the operation principles of bimetallic stripsFigure 1 This diagram shows the operating principle of a bimetallic strip.

Steel typically has a thermal coefficient of expansion of 12 parts per million per degree Celsius or 12 ppm/°C, whereas copper exhibits a larger coefficient of 16.6 ppm/°C and brass exhibits a still larger coefficient of 18.7 ppm/°C. When a steel strip is combined with either a copper or a brass strip, the combination responds to heating by bending in the direction of the lesser coefficient of thermal expansion. With applied heating, the structure bends toward the steel.

Bimetallic strips can be made quite physically long versus their width. Formed into a spiral, we see the following very useful trait.

diagram of spiral-shaped bimetallic strips reacting to temperatureFigure 2 Spiral-shaped bimetallic strips react to cold and hot temperatures.

A pointer attached as shown in Figure 2 can serve for temperature measurement. If care is taken to avoid the elastic limits of the spring, this makes a very nearly linear thermometer, as shown in the following example.

photo of a meat thermometer displayFigure 3 A meat thermometer provides an application example.

Yeah, I cheated here. I took away the thermometer’s pointer in the above photograph to make the linear temperature scale easier to see. This thermometer then went right back into its kitchen drawer.

Getting back to electronics though, bimetallic strips can be useful for making thermometers or temperature control thermostats or for providing temperature compensation for other devices or for making temperature alarms, etc.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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