To correctly understand the thermocouple calibration procedure, firstly we must understand the difference between a thermocouple and a thermometer. Whilst both measure temperature, ultimately the difference is that a thermocouple is much more accurate – in a situation where even a small spike in temperature requires recording, a measurement device like a thermocouple is the instrument of choice.
In this article, we will cover why it is necessary to calibrate a thermocouple, how often a thermocouple should be calibrated, and the most common methods a calibration house would typically use during the calibration procedure of a thermocouple.
Why is it necessary to carry out thermocouple calibration?
A thermocouple is essentially a sensor that can accurately detect a change in temperature – almost like a high-range thermometer – but like any measurement device in continual use, its efficiency can depreciate over time. In a setting where a thermocouple is required, accuracy, continuity and reliability is paramount. Therefore, thermocouple calibration is essential to ensure this often expensive piece of equipment is working correctly.
How does thermocouple calibration work?
There are 2 differing approaches for thermocouple calibration. A comparative calibration & a fixed-point calibration.
- A comparative calibration procedure uses a known-accurate ‘reference’ thermocouple. These are of a higher quality than the instrument to be tested and exhibit an accuracy of three or four times greater.
- A fixed-point calibration uses temperatures set by the International Temperature Scale of 1990 (ITS-90). These fixed temperature points are the triple, freezing and melting points at which the physical state of a given substance will alter.
There are several varying approaches known in the calibration of temperature transmitters that all require specialist equipment. Many customers have a personal preference as to which of the thermocouple calibration procedure they would favour, but sometimes the equipment itself determines the process due to the demands of the accuracy required.
- Dry block (comparative method)
The probes of the temperature device are inserted into a metal dry block. The metal is then heated or cooled using the reference thermocouple to the desired temperature, and the tested thermocouple readings are measured against it. - Stirred bath or furnace (comparative method)
A water bath or furnace will be brought to the required temperature using the reference thermocouple, and the test thermocouple measured alongside it. A variance in readings between the tested and the reference thermocouple will reveal any discrepancy that requires attention.
Both the stirred bath and furnace calibration systems are limited in their range and can take longer than other approaches.
- Thermodynamic (fixed-point method)
Using the ITS-90 determined reference points, a thermocouple calibrated using this method is measured against an International Standard. A number of fixed points will be used and based on the test results an output curve (measured in Ohms, mV etc.) will be established.
Used for the reference thermocouples in other calibration methods, highly stable Platinum Resistance Thermometers (PRTs), are tested using this technique as it is the most accurate way to service a temperature sensor.
A simulated process, some customers prefer to test at ranges that are more commonly used in their own setting, as opposed to a range determined by a governing body.
After a completed calibration, the thermocouple will be returned with a traceable calibration certificate detailing the test points and results, along with any advisories such as condition notes.
How often should I carry out thermocouple calibration?
To ensure they maintain reliable and accurate temperature measurements, it is recommended for thermocouples to be calibrated once every 12 months based on light to heavy usage – as a minimum. If a thermocouple is used more often, or perhaps is due for replacement, more frequent calibration will be recommended.
Where can I get my thermocouple calibrated?
GNW Instrumentation are expert in thermocouple calibration and all types of temperature equipment, at our test laboratory in Southport, Merseyside.
We can arrange for the collection of your thermocouple typically the next working day or attend the site where possible to minimise the downtime of any equipment reliant on its use.
With prices starting from just £30 + VAT it is more cost-effective than you may have initially thought. If a replacement is deemed necessary, we can manufacture a suitable instrument as soon as possible to ensure your workflow is uninterrupted.
Conclusion
In conclusion, the calibration of a thermocouple is essential to guarantee the reliability in the measurements it is recording.
From this article, we now understand:
- The difference between a thermocouple and a thermometer.
- Using a reference thermocouple is a ‘comparative’ method.
- Simulating temperatures against the ITS-90 National Standard is a ‘fixed point’ method.
- Calibration of thermocouples should be performed regularly, no less than once a year.
- GNW can handle your requirements professionally and efficiently with minimal disruption from as little as £30 + VAT.
If you find yourself needing thermocouple calibration, here at GNW we can support your needs, simply call one of our friendly experts on 01704 536010 or email us at sales@gnw.co.uk and we will get back to you.