Bimetallic Thermometers

About bimetallic thermometers

The bimetallic principle is probably more commonly known in connection with its use in thermostats. It is based on the fact that if two strips of different metals are bonded together, any temperature change will cause the strips to bend, as this is the only way in which the differing rates of change of length of each metal in the bonded strip can be accommodated. In the bimetallic thermostat, this is used as a switch in control applications.

If the magnitude of bending is measured, the bimetallic device becomes a thermometer. For such purposes, the strip is often arranged in a spiral configuration, as this gives a relatively large displacement of the free end for any given temperature change. Strips in a helical shape are an alternative for this purpose. The measurement sensitivity is increased further by choosing the pair of materials carefully such that the degree of bending is maximized, with Invar (a nickel-steel alloy) and brass being commonly used.

The system used to measure the displacement of the strip must be carefully designed. Very little resistance must be offered to the end of the strip, otherwise the spiral or helix will distort and cause a false reading in the measurement of the displacement. For visual indication purposes, the end of the strip can be made to turn a pointer mounted in low-friction bearings which moves against a calibrated scale. If an electrical output is required, the linear variable differential transformer (LVDT) is a suitable form of translational displacement transducer. Alternatively, a fiber optic device known as the shutter sensor can be used.

Bimetallic thermometers are used to measure temperatures between -75C and + 1500C. The accuracy of the best instruments can be as good as 0.5% but such devices are quite expensive. Many instrument applications do not require this degree of accuracy in temperature measurements, and in such cases much cheaper bimetallic thermometers with substantially inferior accuracy are used.

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