ChapTer | 21 Temperature Measurement 285 21.3.5 solid expansion Thermal expansion of solids, usually metals, forms the basis of a wide range of inexpensive indicating and control devices. These devices are not particularly accurate; typi- cally errors of as much as !5° or more may be expected, but due to their low cost they find wide application, especially in consumer equipment. As indicated earlier in this section, this technique is also used to provide temperature compen- sation in many instruments. The temperature-sensitive elements using solid expansion fall into two groups: rodsensing probes and bimetal strips. There are so many applications that only one or two examples are given here to illustrate the techniques. thermostats, thought not particularly accurate and having a switching differential of several degrees Celsius, provide a very rugged and reliable control system for a noncritical application such as domestic hot water control. Figure 21.13 shows another rod application. in this case, to achieve greater sensitivity the expanding component is coiled. 21.3.5.2 Bimetal Strip Thermometer Bimetal strips are fabricated from two strips of different metals with different coefficients of thermal expansion bonded together to form, in the simplest case, a cantilever. Typical metals are brass and invar. Figure 21.14 illustrates this principle. As the temperature rises, the brass side of the strip expands more than the invar side, resulting in the strip curling, in this case upward. in this "straight" form a bimetal strip can form part of a micro-switch mechanism, thus forming a temperature- sensitive switch or thermostat. To construct a thermometer, the bimetal element is coiled into a spiral or helix. Figure 21.15 shows a typical coiled thermometer element. A long bimetal strip, consisting of an invar strip welded to a higher expansion nickel-molybdenum alloy wound around 21.3.5.1 Rodsensing Probes The widest application of this technique is for immersion thermostats for use in hot water temperature control. Figure 21.12 shows diagrammatically the operation of an immer- sion thermostat. The microswitch is operated by the thermal expansion of the brass tube. The reference length is provided by a rod of low thermal expansion, such as invar. These