You should have all devices in a process calibrated to confirm that they perform well enough to support the application. This is especially true for thermocouple calibration.
With some devices, a simple adjustment can get you back to the accuracy you had before. For instance, on a pressure transmitter, you’ll adjust the offset and slope. Or for a flow meter, you can change the K-factor after you calibrate the meter in the lab.
For a temperature sensor, you may need to replace the sensor, because you can’t adjust it. If temperature can have an effect on product quality, then you definitely need an accurate and calibrated sensor.
Before we go through thermocouple calibration, we need to define what that means. Calibration refers to the system that compares a device to a reference with known accuracy. During the procedure, you change the temperature to find the device’s error margin compared to the reference.
What is a thermocouple?
Before we proceed, we should do a quick recap on thermocouples. These sensors use the thermoelectric effect to measure. The sensor has two types of metal joined at one end. Heat creates a temperature difference in the metals that generates a voltage. Using the proper reference table, you can find the temperature based on the voltage.
When you talk about thermocouples, resistance temperature detectors (RTDs) often come up, along with the question of when to use which. The general consensus says to use a thermocouple above 400 degrees Celsius. Below 400 degrees Celsius, RTDs have better accuracy and life. As always, your mileage may vary. If you would like to know more, check out our RTD vs thermocouple comparison.
Lab and field calibration
Usually, calibration happens in a lab under controlled conditions, but you can do a field calibration for a quick check. In the lab, you need a calibration bath and a voltmeter or a calibrator. Basically, you change the temperature using the bath and measure the voltage with the voltmeter or the calibrator. If you want a comparison calibration, then you need a thermocouple with a known accuracy to compare your sensor to.
In the field, you’ll need a calibrated joint – basically a thermocouple, extension cable, and portable calibrator. You’ll measure the process point that the sensor measures, comparing it to a specific point in the application to see if it has the same error margin. You verify only one point in the field, unlike in the lab, where you can run through the whole measuring range.
Calibration with a voltmeter
Now let’s talk about thermocouple calibration using a voltmeter. You need to put the sensor in the bath, then connect the sensor to the voltmeter using an extension cable or compensation cable.
Keep in mind that you have to factor the error of the sensor, cable, and voltmeter, creating a sum of errors, right? So check the reference table, where you’ll find the temperature by the millivolt. If you don’t know what reference table to use or how to read it, don’t worry. We have an article posted on Visaya here that will help.
Thermocouple cold junction compensation
You may need to do a special installation in the lab called a cold junction. Basically, you have a terminal block with different materials where you connect the thermocouple to a copper cable, then to your voltmeter.
The voltage values in the reference table use the cold junction at zero degrees Celsius. You can use an ice bath to achieve that temperature, then have a voltage based on the same cold junction. If you want a comparison, then add a reference thermocouple and compare voltages from the two sensors.
We use it to avoid creating a thermal electromotive force (EMF) in the connection terminals. A temperature change in the terminals can change the readings. However, the transmitter should compensate for this difference automatically.
You have two ways to compensate for the cold junction: the electrical bridge method and the thermoelectric refrigeration method.
Calibration with a calibrator
When you use a calibrator, such as a FLUKE 744 or a Beamex MC6, you can set it up, then connect the sensor with an extension cable, and you’re practically done! Well, almost.
The calibrator will compensate appropriately and show the proper value to you. You can connect the sensor in the bath directly to the calibrator. As soon as you have a stable temperature, then register the value and change to the next point. Some baths can adjust the temperature to create a trend of calibration points for you.
Nowadays, customers want to verify their sensors in the field. For this, you use a thermocouple with an inspection tube. Here, you need a calibrated joint (reference sensor + cable + reader) to check if the sensor works properly, at least at a certain temperature point.
When you insert the reference sensor in the inspection tube, you’ll read the same temperature as the process sensor. Inspection thermocouples are usually type-N and no bigger than three millimeters. You must have the joint calibrated together because you need to know the error of the joint. And you can either check one point in the process or you can monitor the thermocouple for a while.
You have many ways to do a thermocouple calibration, to know more about thermocouple calibration, please ask our engineers!