Resistance Temperature Detectors or resistive thermal devices (RTDs), are temperature sensors that exploit the predictable change in electrical resistance some materials with changing temperature. As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability.
An RTD is basically a temperature sensitive resistor. It is a positive temperature coefficient device, which means that the resistance increases with temperature. The resistance of the metal increases with the temperature. The resistive property of the metal is called resistivity. The resistive property defines length and cross sectional area required to fabricate an RTD of a given value. The resistance is proportional to length and iversely proportional to the cross sectional area.
The criterion for selecting a material to make an RTD is:
This is a measurement of drift over time. This is often referred to as long term stability. Most manufacturers specicfy stability at less than 0.05 C per year. Stability is affected by the sensor design. A well designed, high quality sensor will have less drift. Stability is also affected by the sensor's service environment and thermal shock will affect stability.
Interchangeability is the measure of variability of base tolerance and temperature coefficient from sensor to sensor. This is important when replacing a sensor, to insure that the system retains its related performance characteristics without re-calibration. Systems designers should know what sensor tolerance will support their performance expectations when specifying the sensor.
Response time is the sensors ability to react to temperature changes in the process. The ability to track process changes depends on the sensors thermal mass and proximity to the process. Direct immersion sensors generally are the fastest responding, but in many applications a thermowell is used so that the process does not have to be drained to change a sensor.