What is a temperature transmitter?

A temperature transmitter is an electronic device used to send a temperature measurement over two wires to the processing unit. The transmitter is responsible for converting the small electrical signal from the temperature sensor into a more readable signal for the processing unit. In most cases, the signal is sent to some sort of programmatic logic controller (PLC) or receiver. 

If you are thinking about using thermocouples, resistance temperature detectors (RTDs), or thermal resistors and are wondering how to communicate their micro-signals to the industry standard 4-20mA, then you must use a transmitter.

Transmitters are commonly referred to as transducers because of their close proximity in definition and can be used interchangeably.

Transmitters are physically designed to accept and deliver many different types of inputs and outputs. They require a power supply voltage and have different physical shapes depending on your application.

How do temperature transmitters work?

Transmitters aim at amplifying and filtering the signal from the temperature sensor. How this is done varies slightly based on the sensor in use. 

For example, when using a RTD, a Wheatstone bridge is used to create a small voltage across its extremities. This signal is then amplified to produce a 4-20mA signal. Sometimes, this analog signal is converted to a digital signal (ADC) to allow for additional functions (such as calibration and scaling), then returned to an analog signal.  The conditioning circuitry can be designed for resistance values from 15 to 380ohms or something similar to accommodate the full range of RTD values. 

The electronics inside the transmitter will draw 4mA from the power supply when the temperature is at the low-end setpoint and will draw 20mA when the sensor is at the high-end temperature setpoint. For example, if your temperature range of your sensor is 0-100 ℃, then a 4mA signal would correspond to 0℃ . In the same way, 20mA would represent 100℃. Using 4mA as the low reference makes it much easier to notice when your system is malfunctioning. That being said, transmitters are designed for numerous inputs and outputs.

What are the inputs of a temperature transmitter?

The most common types of inputs for  Rosemount temperature transmitters are thermocouples and RTDs.Thermocouple Input

In the industry today, transmitters for thermocouples are normally designed for the base metal thermocouples. Those are type: K,T, J, and E. That doesn’t mean that you won’t be able to find transmitters for other types of thermocouples but they may be more expensive.

Thermocouples have two wires, so the transmitters will have two input terminals where the wires can be connected. It is important to wire the thermocouple correctly. The transmitter should be purchased with cold junction compensation. Cold junction compensation is used as a reference for the junction that is immersed in the environment. 

Thermocouple transmitters will have a Zero and Span dialog that will be used for calibration purposes. You must fine turn the device using these potentiometers, if you notice your device is reading incorrect values.

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