The base-metal mineral-insulated (MI) thermocouple is the most widely used temperature sensor for industrial temperature measurements and has been for over 80 years. The reasons why this has been and will continue to be the case is due to the fundamentals of the thermocouple itself; that is, it is a sensor that is simple, robust and low-cost with an easily measured output.
The base-metal thermocouple (initially as type K, J, E or T sensors) has been used in the temperature range of -200 to 1000°C (-328 to 1800°F) in all aspects of industry from food production to metals processing. Initially in the form of insulated-wire constructions, by the 1940s it took the form of mineral-insulated (MI) thermocouple constructions, which had the effect of providing a more robust device as well as increasing the temperature range of the base-metal thermocouple (Fig. 1).
This was particularly the case for the type K combination for use above 1000°C (1800°F). In the 1960s, the development of the type N thermocouple combination pushed the operating temperature range of the base-metal thermocouple even higher, as it gradually became recognized as the base-metal thermocouple combination of choice for work in the region up to 1250°C (2380°F).
Thermocouples in the form of a type K or type N have remained the sensor of choice for industry in the temperature range up to 1250°C due to their overall general performance. This has allowed them to become the most cost-effective sensors in this temperature range and the most widely used by industry.
Due to this long-term experience in industry, the limitations of the base-metal thermocouple are also widely understood. The major issue is that of drift or change in output when exposed to high temperature. The higher the temperature the greater the drift and, therefore, the greater the error when using these sensors for temperature measurements.
Industry has countered this limitation of type K and type N thermocouples by having specifications detailing in-process checking procedures and limiting the operational life of base-metal sensors. Specifications such as the heat-treatment specification published by SAE, AMS 2750, detail the maximum temperature and number of uses after exposure to these temperatures. This is how and why type K and N thermocouples have remained the most cost-effective temperature sensors for industrial applications.
Today, we can inform you of a major development in base-metal thermocouple technology that will further enhance and strengthen the position of type K and N thermocouples in industry while also increasing the possible temperature range of reliable operation for these industrial sensors. Due to groundbreaking fundamental scientific work conducted by researchers at the University of Cambridge, a new low-drift, high-temperature type K and type N mineral-insulated thermocouple sensor has been designed.
The researchers have spent the last six years studying the reasons and mechanisms of drift in these sensors. Once the fundamentals of the actual reasons for the drift had been understood, they turned their attention to applying their extensive expertise to working out how to reduce or stop the main causes of the drift process. As with all the best developments, the concept they arrived at was a very simple one.
Once the concept of this new low-drift type K and type N thermocouples had been developed, the next step was to conduct tests to see how the theory and practice actually worked. Extensive testing was conducted by the laboratories at the University of Cambridge and further independent testing was conducted in parallel by an industrial IEC 17025-accredited calibration laboratory based in the U.K. The results of the testing confirmed beyond doubt how the newly designed type K or type N mineral-insulated thermocouples significantly reduced drift to a level that had not been seen before.
How does the new design work?
Technical publications and white papers written by the researcher responsible for this development, Dr. Michele Scervini, and the technical director of CCPI Europe Ltd., Trevor Ford, have been published. They provide details of the new low-drift MI cable design and the results of extensive testing that confirm the superior low-drift performance of these sensors to any base-metal thermocouple counterpart (Fig. 2).
Why is this new design unique?
Most developments in science, temperature measurement being no exception, are usually at the fundamentals of measurement, such as the development of the new pure-element thermocouple combinations (Palladium v Platinum or Gold v Platinum) that have applications for increasing accuracies in the high-temperature range. As tends to be the case, however, this development will be mainly for operation in the laboratory and not for direct use for industrial applications. This low-drift development in MI type K and N sensor technology not only gives a major leap forward in low-drift temperature-measurement performance but also provides a sensor that can increase the confidence directly in those everyday industrial temperature measurements.
This is one of the first times there has been a major increase in operation performance of an industrial contact temperature sensor that does not require the change of measurement equipment, wiring or procedural use by industrial users to get the dramatic increase. To the user both in appearance and in operation, the new low-drift MI type K and N thermocouple sensors will outwardly appear the same. In fact, the only differences the user will see is the remarkable reduction in drift and, therefore, increase in accuracy measurement performance.
How can this new technology be applied in industry?
As discussed, there are many specifications that have been developed to make sure when existing designs of type K and N MI thermocouples are used for accurate or critical temperature measurements. Drift limitations are minimized by stipulating operational times and temperatures. Unfortunately, these same specifications will result in the slow adoption of this new thermocouple design. Since the new low-drift thermocouple is still designated type K or type N, the specification limitations will still apply. This will result in the user not being able to take advantage of the lower-drift, higher-temperature, higher-stability properties of the sensor.
How do we make sure 20 years does not pass, as happened with type N thermocouple development, before industry can take advantage of this step forward in temperature-measurement technology? The best way would be for users to write to the relevant standards committee asking them to review current standards limitations on base-metal thermocouples with a view to adding this new design to the standard.