
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.
Some of the most common types of transmitters and transducers can be found on pressure instruments. Examples of these include: drop-in well pressure devices and devices that measure the pressure in a pipe or gas chamber. Transmitters and transducers can also be found on flow and speed measurement devices.
What is the difference between transmitters and transducers? The main difference is the way they send data to the monitoring device, like a PLC. In terms of measurement, the transmitter will reference a measuring unit by sending a milliamp signal (such as 4-20 mA). A transducer will reference a measuring unit by sending a voltage signal, typically in volts or millivolts.
Why have the two different types of transmission methods? The method will depend on the application. If the electrical connection in the application travels a short distance, a pressure transducer is the better choice. Pressure transducers are smaller with fewer active electronic components that can be upset by electromagnetic interference. Also, the voltage signals transmitted over longer distances are subject to voltage drop and signal corruption. A transmitter is better for transmitting signals over a long distance.