A temperature transmitter is essentially a form of amplifier, filtering, conditioning and boosting the signal for better reception at the place where the temperature is read. Thermocouple or RTD wires are connected directly to the transmitter which scales the input to a 4 to 20 mA output signal, a voltage such as 0 to 10 Vdc or a digital output such as RS232, RS485, and Modbus®. This goes out over twisted pair wiring to wherever the temperature needs displaying or recording.

Temperature transmitters are available in many forms and with varying degrees of sophistication, from compact DIN railmountable units to programmable and “smart” devices with diagnostic capabilities.

Benefits of Using Temperature Transmitters

Historically, direct wiring of temperature sensors to display or data logging equipment was the norm. Increasingly though, engineers are recognizing the benefits of using temperature transmitters. These include:

Improved signal integrity, especially over long distances. Using shielded twisted pair wiring provides even higher EMI resistance.

Standardized output. Unlike the output from direct wiring, the 4 to 20 mA signal is compatible with most data acquisition, recording and display systems, allowing standardization of hardware.

Improved accuracy. Clipping the milliamp output to part of the sensor’s range can improve resolution and yield greater accuracy.Additionally, some transmitters can detect thermocouple drift and provide an alert before problems arise.

Less expensive wiring. Thermocouple extension wires are usually made from the same material as the device itself, so are more expensive and more fragile than standard twisted pair wiring. Twisted pair is more easily “pulled” and as it costs less per foot results in substantial savings over the long run.

Simplified maintenance. Twisted pair wiring stands up better in hostile environments so cable breaks and short circuits are less common and easily detected if they do occur. “Smart” transmitters can send diagnostic information so problems are understood before a technician starts tracing wiring and looking for faults.

Retrofit and upgrade flexibility. Once temperature transmitters are installed, should the process change and a different thermocouple be needed, only the sensor itself has to be changed. Conversely, a transmitter can be installed using existing direct wire connections (although EMI immunity will be lower).