Temperature Transmitter ExplainedLearn about temperature transmitters, sensors used with them, how they are connected in a control loop, and how they are calibrated.
In this article, we’re going to introduce you to a very common device used in process control today, the Temperature Transmitter.
We’ll discuss the sensors used with them, and show you how they are connected in a typical control loop. We’ll also give you an insight into how they are calibrated.
Different shapes and sizes
Temperature Transmitters are available in all different shapes and sizes depending on the application and environment they are expected to operate.
To name a few, there are:
Before we look at a Temperature Transmitter in detail, let’s look at where it fits in a typical process control system.
What is a temperature transmitter?
A Temperature Transmitter is a device that converts the signal produced by a temperature sensor into a standard instrumentation signal representing a process variable temperature being measured and controlled.
The most common transmitter instrumentation output signal is 4 to 20 mA.
The signal from the Temperature Transmitter is sent to a Controller that determines what action is required and generates an appropriate output signal. Controllers are either a PLC or a DCS in process control today.
The Heating/Cooling source exerts a direct influence on the process as directed by the controller.
2-Wire Temperature Transmitter
A Temperature Transmitter requires a power supply to operate. Most Temperature Transmitters are 2-wire types having only two wires. These two wires provide power for the transmitter and are also the signal lines!
4-Wire Temperature Transmitter
A 4-wire transmitter has two wires connected to a power supply and two signal wires connected to the PLC. The power supply can be AC or DC depending on the vendor and model.
Most common temperature sensors
We haven’t said much about the temperature sensors, so let’s do that now. The two most common temperature sensors used in process control are the thermocouple and the Resistance Temperature Detector (RTD).
The thermocouple produces a change in voltage with a temperature change. Thermocouples have only two wires.
An RTD produces a change in resistance with a temperature change. Unlike a thermocouple that has only two leads, an RTD can have two, three, or four leads.
It’s common to see Temperature Transmitters that are capable of working with either an RTD or a thermocouple.
HART temperature transmitter
You may have heard the term Digital Temperature Transmitter. This term has been around for a while now and its meaning is evolving. Historically, a Digital Temperature Transmitter was a Smart Transmitter.
The SMART transmitter is an intelligent transmitter that has an analog output of 4 to 20 mA and also provides digital communication signal information such as Instrument Tag Names, Calibration Data, and Sensor Diagnostics.
Temperature transmitter calibration
As with any instrumentation transmitter, a Temperature Transmitter needs to be adjusted or calibrated so that the current output of 4 to 20 mA represents the temperature range of the process variable being measured.
For example, a process variable temperature range of 0°C to 100°C will produce a current range of 4 to 20 mA.
In every calibration scenario, the sensor is replaced by a device that can simulate the output of the sensor over the entire process variable temperature range.
For a thermocouple, the sensor is replaced by a device that produces a range of millivolts. As you can imagine, it’s not easy to create these small voltages.
For an RTD, the sensor is replaced by a device that produces a range of resistances. Historically the device that produced a range of resistances was a decade box.
Today we’ve got Signal Reference Calibrators that can output a variety of electrical signals that can simulate RTDs and Thermocouples making it much easier to calibrate Temperature Transmitters. How are the actual adjustments performed?
Old style temperature transmitter calibration
Older style Temperature Transmitters have multi-turn potentiometer adjustments called Zero and Span.
The Zero pot is adjusted to produce 4 mA when the simulated temperature is 0%, and the Span pot is adjusted to produce 20 mA when the simulated temperature is 100%.
As an example, let’s calibrate a temperature transmitter connected to a PT100 RTD assuming a temperature range of 32°F to 100°F.
We can get the resistance values at our calibration temperatures from a PT100 table. We will start with the Zero calibration. As you can see in the calibration table, a PT100 RTD has a resistance of 100 ohms at 32°F, so we’ll set 100 ohms on the decade box.
The multimeter current should be adjusted to 4 mA by rotating the Zero adjustment screw.
For Span calibration, we’ll find the high range temperature on the table and set the corresponding resistance on the decade box. The multimeter current should be adjusted to 20 mA by rotating the Span adjustment screw.
New style temperature transmitter calibration
Newer style Temperature Transmitters do not have external Zero or span adjustments but are programmed or calibrated using the software. The Transmitter is connected to a computer via a serial USB connection.
Wireless temperature transmitter
This article wouldn’t be complete if we didn’t talk about one of the newest trends in Transmitters; the Wireless Transmitter.
As the name implies, the transmitter does not send its signal to a receiver via wires. The signal is transmitted to a receiver via a Wi-Fi signal.
Once the signal is received, it can be sent to a computer so readings can be saved and later printed or exported to a spreadsheet. Or, the received signal can be retransmitted via wires as a 4 to 20 mA signal to a PLC.
Let’s review what we’ve discussed…
– Temperature Transmitters are available in several different shapes and sizes.
– A Temperature Transmitter is a device that converts the signal produced by a temperature sensor into a standard instrumentation signal representing a process variable temperature being measured and controlled.
– The most common transmitter instrumentation output signal is 4 to 20 mA.
– Most Temperature Transmitters are 2-wire types having only two wires.
– The two most common temperature sensors used in process control are the thermocouple and the Resistance Temperature Detector (RTD).
– Many Temperature Transmitters are capable of working with either an RTD or a thermocouple.
– Signal Reference Calibrators can output a variety of electrical signals that can simulate RTDs and Thermocouples making it much easier to perform calibration.
– Older style Temperature Transmitters have multi-turn potentiometer adjustments called Zero and Span used for calibration.
– Newer style Temperature Transmitters are programmed or calibrated using the software.
– A wireless Temperature Transmitter sends signals to a receiver via a Wi-Fi signal.
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