One of the most used temperature measuring and sensing tool is the thermocouple wire. Thermocouple usage is almost everywhere in a temperature measurement process.
It is mostly integrated on Temperature Controllers and Indicators as a sensor to detect temperature like on ovens, incubators, water heaters, thermo-hygrometers and other processes that require temperature monitoring.
It is also used as a probe on some thermometers used in meat or food thermometers and as a surface probe used in hot plates or hot surface. In this topic, I will present to you the different ways on how to calibrate or verify the accuracy of a thermocouple.
What is a thermocouple?
A thermocouple is a pair of two dissimilar metal wires connected together (welded or just twisted) at one end. The connection between these two dissimilar metals creates a reaction in which an electromotive force ( emf) is generated, a voltage source.
The generation of voltage happens when the connected side or end is exposed to a different temperature. Since thermocouple wires create a voltage change every time it senses or detects the temperature difference, it is also called a thermocouple sensor.
Types of thermocouple wires
So many types of thermocouple wires are being manufactured mainly because each has its unique qualities, capability, and ranges depending on the suitability of the environment where they are used and exposed to. Below are some examples of commonly used thermocouple types with their ranges.
Why Calibrate Thermocouple Wires?
To verify accuracy because of:
- Aging or decay
- Contamination by the working environment- make them brittle and shortens the life
- Oxidation in the environment when unprotected.
- Mechanical stress or breakage
3 Ways to Calibrate a Thermocouple Wire
Since a thermocouple wire can be considered as a probe and a sensor connected to an indicator, there are more ways to have it calibrated or verified for accuracy. Just take note that a thermocouple wiring should be noted, it has a positive and a negative polarity for it to function properly.
There is a plus and minus sign on its connector so this won’t be a problem if you check before you connect the wirings, also, it is color-coded, so make sure to check it before perform wiring connections. Below are the different ways to verify the accuracy of a thermocouple wire.
1. Through actual temperature verification using a controlled temperature from a metrology well and an indicator (Fluke 754 or Fluke 1524)
This is a set up using a Metrology Well ( fluke 9173) and an indicator ( Fluke 1524). The Unit Under Calibration (UUC) is the thermocouple. This is an actual verification of temperature where the thermocouple wire (probe) is soaked in the well and heat generated is read through the Fluke 1524 indicator.
This thermocouple calibration procedure or set up is used when the thermocouple wire has a welded end. This set up takes more time because you are using an actual temperature that requires stabilization on every temperature set point.
2. Through a simulated temperature which is an electrical signal generated Using a Fluke 5520A calibrator and an indicator ( Fluke 754 or Fluke 1524)
Another set up is through the used of a calibrator which is the Fluke 5522a. This is a procedure where a simulation is used to create the desired temperature.
The desired temperature can be simulated from the Fluke 5522A calibrator, you can select various types of thermocouples then the required temperature. By using an indicator ( Fluke 754 or equivalent indicator), we can display the temperature generated by the calibrator.
This set up is applicable to those thermocouple wires that has an open end connected to a male thermocouple connector or adaptor. This set up is more simple and takes less time to do compared to a thermocouple with welded end or junction
3. Through a simulated output or input of a millivolt electrical signal Using a multimeter (Fluke 8846), 5522a calibrator or fluke 754
Now, we will use a multimeter as a display instead of a temperature or thermocouple indicator. The simulated signal will be generated also by Fluke 754 or Fluke 5522a calibrator. But instead of displaying a temperature readout, it will now be a voltage output (mV).
Just be sure to have the necessary thermocouple connectors or adaptors for it to be connected to the Multimeter (Fluke 8846 or any multimeter capable to display a millivolt read out to at least 3 resolutions) and take note again the polarity.
Since thermocouples can detect or produces an EMF during temperature differences, this emf or voltage generated can be measured in millivolts. Likewise, you can also generate a millivolt input and read it as a temperature output.
This is also applicable only to those thermocouples with open end junction with adapter ( unless you will cut a welded junction and perform this procedure and welded it back again once done).
Depending on the type of thermocouple used, each generated temperature has an equivalent voltage in millivolt. By using a table that has been designed or computed, a temperature equivalent can be taken. Below is a sample table for a Type K thermocouple.
All of these Thermocouple Wire calibration Set Up can be carried out in any combination depending on your needs and availability of instruments or standards. Also, this set up is applicable when calibrating a temperature indicator or a controller in which the thermocouple is used as a probe or sensor.
For a procedure on how a thermocouple wire is calibrated, please visit this thermocouple wire calibration procedure-type k thermocouple.
For a list of thermocouple wires, you may click this link thermocouple wires.
Hope this thermocouple setup provides you an additional knowledge, please feel free to comment for any clarification or feedback.
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