Bimetallic strip thermometers are mechanical thermometers. They are widely used in industry for temperature control because of their robustness, temperature range and simplicity.

It consists of two strips made of dissimilar metals and bonded together with one end fixed and the other free. The principle is that as the temperature changes one strip expands more than the other, causing the pair to bend at the free end.

Most bimetallic strips use a high thermal expansion alloy, such as steel or stainless steel, coupled with a low thermal expansion alloy such as Invar.

		Two constructions are available: -a spiral strip: the bimetallic strip is coiled into a spiral attached to a dial that indicates temperature
		-a cantilever strip: the bimetallic strips are bonded together in a cantilever. The deflection is used to indicate temperature.

Bimetallic strips are used in thermostats for measuring and controlling temperature. The strip is connected to a switch and as the temperature changes the strip flexes and opens or closes a contact. They are also used in ovens for measuring temperature. The strip is coupled to a dial which is calibrated for temperature indication.

Bimetallic strip advantages and disadvantages

• Power source not required
• Robust, easy to use and cheap but not very accurate. Can be used to 500 °C
• Limited to applications where manual reading is acceptable, e.g. a household thermometer
• Not suitable for very low temperatures because the expansion of metals tend to be too similar, so the device becomes a rather insensitive thermometer

References

1. T D McGee (editor), Principles and methods of temperature measurement, John Wiley & Sons, ISBN 0 471-62767-4
2. J Jouanneau (editor), Pyromètres à bilames, Technique de l'Ingénieur R 2 540
3. P R N Childs, J R Greenwood, C A Long, Review of temperature measurement, Review of scientific instruments, vol 71, n°8 2000