We know that the resistance of metallic conductors increases with temperature. Thermal sensors using this property of metallic conductors are called Resistance Temperature Detector(RTD).
Construction of RTD
The resistance temperature detector is constructed by wounding the resistance wire on a mica base. The wire is wound like a helical coil on the support to reduce the inductance effect. The terminals(Leads) are brought out of the pipe. The coil is protected by a stainless steel case. The structural view of a wire wound RTD is shown in the figure.
Copper, Nickel and Platinum are the most used RTD materials. These metals have positive temperature co-efficient and possess poor thermal sensitivity. Also, the resistance-temperature characteristics of these materials are approximately linear.
Another type of RTD is thin-film RTD that is constructed by depositing a thin layer of resistive material onto a ceramic substance.
Working Principle of RTD
Resistance Temperature Detectors (RTD) operates on the principle that the resistance of a metal changes with changes in temperature.
The variation of resistance R with temperature t can be represented by the equation,$$R_t=R_0(1+\alpha \: \Delta t)$$
Where α is the temperature co-efficient at t0 and R0 is the resistance at t0.
- Can be operated in a wide temperature range.
- Good stability at high temperature.
- High accuracy.
- Low sensitivity.
- More Expensive.
- Affected by shock or vibration.