Temperature Transmitters & Transducers
Temperature transducers are devices that convert a thermal quantity into an electrical quantity for the measurement of temperature. Temperature transducers contain a sensing element. As the temperature changes, a corresponding change occurs in certain properties of the sensing element. Three types of temperature transducers are: thermocouples, resistance temperature detectors (RTDs) and thermistors.
Thermocouples consist of two electrically bonded wires of different metals. The varying voltage between the two dissimilar metals reflects proportional changes in temperature. The accuracy of thermocouples is low, however, they operate across the widest temperature range of -200 degrees Celsius to 1550 degrees Celsius.
RTDs consist of a high-purity conducting metal wire wrapped around a glass or ceramic core. The metal’s electrical resistance changes with a change in temperature. Platinum RTDs have a highly accurate linear output across a temperature range of -200 degrees Celsius to 600 degrees Celsius.
Thermistors consist of metal oxides that alter their resistance characteristics with temperature. There are two types of thermistors. Positive temperature coefficient (PTC) thermistors exhibit linear characteristics (increase in temperature causes resistance to increase) and negative temperature coefficient (NTC) thermistors exhibit non-linear characteristics (increase in temperature causes resistance to decrease). Thermistors are sensitive to small changes in temperature and have an effective operating range between -50 degrees Celsius to 150 degrees Celsius.
Temperature transmitters are signal conditioners that isolate, amplify and filter an input signal from a temperature transducer, and then rapidly transmit a standardized output signal to the control device.
