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An RTD is a temperature sensing device with resistance that increases with temperature. An RTD is usually constructed with wire coil or deposited film of pure metal. RTDs can be made of different metals and have different nominal resistances, but the most popular RTD is platinum and has a nominal resistance of 100 Ω at 0 °C.
Signal conditioning is generally required to measure temperature using an RTD. Because an RTD is a resistive device, you must pass a current through the device to produce a measurable voltage. Providing current to take a resistive measurement is a form of signal conditioning called current excitation. In addition to producing current excitation for the RTD, signal conditioning amplifies the output voltage signal, and filters the signal to remove unwanted noise. You also can use signal conditioning to electrically isolate the RTD and the monitored system from the DAQ system and the host computer. Refer to Signal Conditioning Requirements for Thermistors and RTDs for more information.
Numerous types of RTDs exist, and they are typically defined by their material, their nominal resistance, and their temperature coefficient of resistance (TCR). The TCR of an RTD is the average temperature coefficient of resistance of the RTD from 0 to 100 °C and is the most common method of specifying the behavior of an RTD. The TCR for platinum RTDs is determined by the Callendar-Van Dusen equation.