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Accurate and reliable temperature and humidity measurements are critical to industry, meteorology and research. Businesses and organisations need to have complete confidence in the performance of their measurements over a wide range of temperatures and in the harshest conditions.
Thermocouples are the most widely used temperature sensor in industry. They are vital for monitoring high temperature processes, such as aerospace heat treatment, quartz manufacturing and many others. They consist of two metal wires joined at one end and a temperature difference between the hot end and the cold end gives rise to a voltage. This is known as the Seebeck effect, and it can be used to determine the temperature.
“businesses and organisations need to have complete confidence in the performance of their measurements over a wide range of temperatures and in the harshest conditions”
The Seebeck coefficient
The Seebeck coefficient is a parameter which describes how much voltage is developed for a given temperature change along the wire. Every material has a different Seebeck coefficient, and thermocouple inhomogeneity describes the variation in Seebeck coefficient along the length of a thermocouple. As long as the Seebeck coefficient at constant temperature is the same at all points along the wire, the wire can be used in a thermocouple for temperature measurements with low uncertainties.
However, over time – especially when used regularly at high temperatures – the Seebeck coefficient of the thermocouple wires can change due to contamination, oxidation, and other effects. If different parts of the wire are affected in different ways (as is usually the case, because different parts of the thermocouple are at different temperatures – for example, the tip of the thermocouple is typically in a hot region, while the other end is at room temperature), this can affect measurement accuracy.
“manufacturers need to be able to check the wires are sufficiently ‘homogeneous’, i.e. free of variations in the Seebeck coefficient”
Ensuring correct measurement
To be sure that thermocouples are measuring correctly, manufacturers – as well as National Metrology Institutes and calibration labs that perform thermocouple calibrations – need to be able to check the wires are sufficiently ‘homogeneous’, i.e. free of variations in the Seebeck coefficient.
To identify these variations over the length of a thermocouple, many approaches have been developed by National Metrology Institutes, using furnaces, oil baths or salt baths. However, many of these systems are not without their drawbacks. They are:
• Slow (taking up to an hour to complete a single scan)
• Complex
• Of low resolution
These problems are caused by poor thermal coupling between the scanning medium and the thermocouple under test. Additionally, these are expensive calibrations for the end user, and they sometimes operate at very high temperatures (over 600°C) which can further degrade thermocouples during the test.
Homogeneity scanners
To provide a more practical solution, commercially available homogeneity scanners are available that operate at a more wire-friendly temperature of 100°C. The design was first developed by the Measurement Standards Laboratory of New Zealand (MSL).
The system uses water vapour to transfer heat directly to the thermocouple, making use of the latent heat of vaporisation that occurs when there is a change of state between liquid and vapour. This facilitates a very sharp temperature gradient along the thermocouple, which translates into very high resolution thermoelectric inhomogeneity measurements. The scanner uses two separate heat-pipes, a water heat-pipe at 100°C (boiling point of water), and a second, smaller acetone heat-pipe at 20°C (ambient air temperature). The thermocouple is passed from the acetone heat-pipe into the water heat-pipe, where they are directly exposed to the steam, thus ensuring a high rate of heat transfer.
“commercially available homogeneity scanners are available that operate at a more wire-friendly temperature of 100°C”
The ability to pass the thermocouple between two highly uniform temperature zones makes high-accuracy high-resolution scanning possible. The ‘low’ temperature (100 °C) of the hot zone ensures the thermocouple under test is not likely to be altered (metallurgically or chemically) during the scanning process. This is a big advantage over conventional homogeneity scanners operating above 200°C, which will affect the wire composition of most thermocouple types, often invalidating the homogeneity scan.
The company behind the development of this scanner needed independent verification that its product worked as claimed, in order to confidently take it to market. NPL’s M4R programme made it possible to carry out this important work, which otherwise would have been put on hold due to logistical challenges around COVID-19.
NPL scientists compared the newly developed scanner with their own pre-existing homogeneity scanning equipment. Both scanners were used to measure the same two thermocouples that had well-characterised pre-existing damage, and the results were compared.
The data showed that both scanners were effective and consistent in their characterisation of the damage, and that scanning at 100°C was sufficient to detect inhomogeneities.
The investigations provided an independent demonstration of result consistencies, which in turn provided confidence in the product. To this end, data from NPL can be used as an impartial verification that it is fit for purpose.
“the new device will ultimately offer a practical, low temperature, low-cost approach for properly characterising thermocouples”
This low temperature thermocouple calibration device does not otherwise exist commercially. The product is still new, but it is anticipated that the initial market will be National Metrology Institutes, with the study data giving them confidence that they can rely on this device, rather than developing their own.
The new device will ultimately offer National Metrology Institutes, calibration laboratories and thermocouple manufacturers a practical, low temperature, low-cost approach for properly characterising thermocouples and verifying their measurement uncertainties. This will increase options for measurement assurance and reduce calibration costs for customers.
Alongside supporting companies to deliver innovation, uncover efficiencies and bring confidence to decision making to drive growth for UK industry, the NPL Temperature and Humidity team has leading expertise and continues to research and develop novel techniques in:
• Contact thermometry
• Humidity and moisture measurement
• Primary thermometry
• Quantitative thermal imaging
• Non-contact thermometry
• Reliable harsh environment thermometry
These offer solutions to many real-world temperature and humidity measurement problems, including those in challenging or harsh environments, with the potential to address major global issues, such as mitigating the long-term impact of diabetes to improve patients’ quality of life, or providing measurements to validate more accurate climate models to enable better policy making. Almost every technological process relies in some way on the measurement and control of temperature, and the ability to make better measurements directly translates into improved efficiency and hence greenhouse gas emissions, improved product yield and reduced wastage, and the ability to delocalise processes across the globe without loss of quality.
