Resistance Temperature Detectors (RTDs)
Resistance temperature detectors, also well known as RTD sensors are used to measure the temperature. RTD have a linear increase in resistance as temperature rises. RTD come in 2-wire, 3-wire or 4-wire versions. A 2-wire RTDs signal is affected by the distance to the controller. 3-wire or 4-wire RTDs can compensate for line losses.
Principle of Working of RTD- PT-100
The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4 °C. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90).
The linearization equation is:
Rt = R0 * (1 + A* t + B*t2 + C*(t-100)* t3)
Rt is the resistance at temperature t, R0 is the resistance at 0 °C, and
A= 3.9083 E–3
B = –5.775 E–7
C = –4.183 E–12 (below 0 °C), or
C = 0 (above 0 °C)
For a PT100 sensor, a 1 °C temperature change will cause a 0.384 ohm change in resistance, so even a small error in measurement of the resistance (for example, the resistance of the wires leading to the sensor) can cause a large error in the measurement of the temperature. For precision work, sensors have four wires- two to carry the sense current, and two to measure the voltage across the sensor element. It is also possible to obtain three-wire sensors, although these operate on the (not necessarily valid) assumption that the resistance of each of the three wires is the same
RTDs can operate in a temperature range of (-200 to 650) °C [-328 to 1202] °F
All temperatures from cryogenic to ±600°C where fast response and accuracy are required