All metals will change in resistance when subjected to a temperature change. This relationship can be predicted by use of a constant alpha (the temperature coefficient of resistance). No two different metals have the same alpha and if the properties of the metal are known, a resistance vs. temperature curve may be established which can be accurately duplicated.
A metal's alpha may be changed by alloying it with another material or by mechanically stressing it. Thus, only a few materials are used for resistance temperature detectors (RTD). Other factors which limit the use of some metals and alloys as RTDs include: low electrical resistance, difficulty in forming coils, availability in the pure state, linearity of resistance vs. temperature curve, long-term drift and stability characteristics, resistance to contamination and chemical attack, etc. When all of these factors are considered, the most commonly used materials for RTDs are platinum and nickel. On special applications tungsten and copper are also used.
The sensing element of a RTD may consist of a coil, foil or thin film deposited material. This sensing element may be wound or deposited onto a suitable insulating core and then encapsulated with an insulating material.
RTD extension leads may be of any convenient material (typically copper wire with insulation requirements to suit the environment of use) provided all leads are of the same material. The number of extension wires can consist of two, three or four wires. Two extension leads are used where the lead wire resistance is not very important. Higher accuracy is achieved if three wires are used - the third wire compensating for the lead wire resistance. Four leads are typically used only with dual element RTDs, or when extreme accuracy, or long lead lengths are required. An RTD system consists of the sensor, lead wire, and an instrument/readout device.