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An RTD is a resistance temperature detector. The principle of RTD function is similar to a thermistor. Unlike thermocouples, RTDs are not self-powered. A current must be passed through the RTD, the same as with thermistors, and the change of voltage with temperature is measured. Materials for RTDs can be gold, silver, copper or platinum. Platinum, however, has become the most-used metal for RTDs. A thin film of platinum or a thin platinum wire is deposited on a flat ceramic material and sealed.
Platinum has a nearly linear temperature versus resistance relationship. The Callendar-Van Dusen equation approximates the RTD curve:
The lowest practically used resistance of RTDs is 100 Ohm at 0°C. The operating temperature range is from –220°C to 850°C. RTDs have a self-heating error that depends on the electrical energy input. The formula for the self-heating error is:
dt = P/EK
You may compare thermocouples, thermistors and RTDs by clicking here.
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