Resistance Temperature Detectors
ByCharlie Douglous
Posted Date: Friday, February 02, 2007 | Viewed: 274
Resistance Temperature Detectors works on property possessed by all conductors and semiconductors, that their resistance varies as a function of temperature. This property is more or less pronounced, depending on the particular material. The relative change in the resistance as a function of temperature (dR/dt) is known as the temperature coefficient, the value of which is usually not constant over the range of temperature of interest, but is itself a function of temperature. The resistance temperature detector is made up of a platinum coil. The wire coil is either fused into glass or embedded in ceramic. To meet today’s requirements for more compact dimensions and higher resistance values, extremely thin platinum layers are applied to a ceramic substrate instead of wires. To protect them against mechanical damage these measuring elements are usually installed into suitable protective tubes. This also ensures easy replacement without the need to replace the complete fitting. As the sensor has to reach the temperature of the medium in which measurement is to be performed, the housing has to be adapted to the application. When using resistance thermometers for temperature measurement, the fact that the measurement result is influenced by the resistance of the selected lead wire must be taken into account. Three types of circuits are commonly used: 2-wire, 3-wire and 4-wire circuits. The most accurate measurements are obtained with the 4-wire circuit, as in this case the measurement is not affected by lead wire resistance or environment temperature of lead wires. The 3-wire circuit is normally used for eliminating the lead wire resistance (Wheatstone bridge). In the case of the 2-wire circuit the lead wire resistance is fully measured by the measuring bridge. By the use of modern control equipment the influence of the lead wire resistance at 2-wire circuit can be compensated by a line compensation resistor, which is independent of temperature.
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