A temperature transmitter is used to transmit the measured temperature as an analogue signal to a receiver.
A temperature transmitter transmits a measured temperature as an analogue 0/4–20 mA signal to a receiver. The receiver will typically be an electronic regulator or PLC (Programmable Logic Controller).
A PLC is a small computer that is not normally able to directly measure electrical signals from a sensor. The temperature transmitter is fitted with a resistor with either PT or NTC resistance.
PT & NTC
The terms PT and NTC are abbreviations for positive and negative temperature, and describe whether resistance in the resistor is increasing or falling as the temperature rises. If a resistance sensor is connected directly to the receiver, the resistance in the wire between the two components is also measured and will give a false reading. The bias is dependent on the length of the wire and the ambient temperature.
Temperature transmitters are used to avoid voltage loss, or when a regulator or PLC cannot directly measure the signal from a resistance sensor.