A basic physical property of a metal is that its electrical resisivity changes with temperature. All RTDs are based on this principle. The heart of the RTD is the resistance element. Several varieties of semi-supported wire-wound fully supported bifilar wound glass, and thin film type elements are shown here. Some metals have a very predictable change of resistance for a given change of temperature; these are the metals that are most commonly chosen for fabricating an RTD.
A precision resistor is made from one of these metals to a nominal ohmic value at a specified temperature. By measuring its resistance at some unknown temperature and comparing this value to the resistor’s nominal value, the change in resistance is determined. Becasue the temperature vs resistance characterstics are also known, the change in temperature from the point initally specified can be calculated. We now have a practical temperature sensor, which in its bare form is commonly referred to as a resistance element.
Through years of experience, the characteristics of various metals and their alloys have been learned, and their temperature vs. resistance relationships are available in look-up tables. For some types of RTD’s, there are also equations that give you the temperature from a given resistance. This information has made it possible for instrument manufacturers to provide standard readout and control devices that are compatible with some of the more widely accepted types of RTD’s.