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.
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.
The most common types of inputs for 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.
RTD inputs are designed based off of their resistance value. The resistance is known by the material used when creating the wire-wound. Platinum is a commonly used material and has a temperature coefficient of 0.00392 //℃. From this, it is possible to make a wide variety of resistance values. They can range from 10 to 10,000, but the most common is 100 known as a PT100.
Once you know the resistance value of your RTD you're halfway done specifying the input for the transmitter. The next step is to decide on the number of wires for the RTD. RTD are made in 3 different wiring configurations: 2-wire, 3-wire and 4-wire. Each topology requires slightly different circuits. The 3-wire configuration tends to be the most economical and provides sufficient accuracy and precision. Although, you will be able to find transmitters for all types of wiring.
Thermistors are used when the temperature range is small, but the response needs to be high. If you need a transmitter for a thermistor they can be sourced similarly to a thermocouple.
Temperature transmitters act as the medium of the sensor and the receiver, thus they should output a value that is measurable by the receiver instrumentation. Across the industry, the standard output of a temperature transmitter is 4-20mA, 0-5VDC or 0-10VDC.
