Temperature control makes a difference
Thermocouples have traditionally been used to measure high temperatures.
These measurements are crucial to many industrial processes in all
sectors. Unfortunately, ageing can irreversibly damage thermocouples.
Testing laboratories and research institutes from three European
countries worked together to produce a novel thermometer that can
measure high temperature and easily be recalibrated on site. They
are continuing their work with the help of industrial partners to
produce a commercial product.
An alcohol or mercury-filled thermometer may be useful to measure temperatures that we experience in our daily lives. However, many industrial processes require accurate measurements of temperatures well over 1000°C. From school day physics, you may recall being told that a thermocouple could be used in such situations. This device consists of two wires, made of different metals, joined together. Today, these metals are commonly tungsten-rhenium or platinum-rhodium alloys against pure platinum. If one junction is hotter than the other, voltage can be measured across the cold ends of the thermocouple. This can be measured and related to the temperature difference between the two junctions.
The problem with thermocouples
The thermocouple is useful for temperatures up to about 2000°C and it can easily detect changing temperatures. The problem with thermocouples, however, is that they age irreversibly. After this immersion-dependent ageing occurs, they can no longer be calibrated. This is where the resistance thermometer comes in. Electrical resistance is dependent on temperature so reading the resistance of this thermometer gives its temperature when a calibration table is available.
Platinum resistance thermometers are commercially available for temperatures up to 850°C. Research to try to increase the maximum temperature has been attempted but the use of such thermometers is limited because the platinum wire is very sensitive to stress. Industrial applications above 1000°C would be unlikely.
A new approach
To get round this, the problem had to be approached from a different angle. A team of scientists from the Netherlands, Germany and the UK decided to see what would happen if they used a different resistive material in platinum's place. They chose instead a conductive ceramic, based on silica and molybdenum, a transition metal. With this material, ageing effects can be corrected through calibration, unlike the thermocouple. "The advantage of this material also lies in the strong temperature dependence of its resistance when compared to platinum," adds the project coordinator, Martin De Groot of the Nederlands Meetinstituut. He continues, "No one has ever made a thermometer like ours before."
The group also discovered that the new thermometer could be recalibrated in a novel way. Usually, calibrating high temperature measurement devices requires a special calibration furnace, which is often off site. Thermodynamics, however, shows that the noise in a resistor, caused by thermally induced fluctuations of the electrons that carry the current, is dependent on temperature. All this means is that the thermometer's own thermal noise can be used directly to give a "true" temperature and thus the resistance thermometer can self-calibrate on site.
During the project, the partners used their different skills and knowledge to design and construct prototype thermometers and then carry out high temperature ageing tests. They also built a noise thermometer to be used for calibration. A number of measurements were made at fixed temperatures to measure this thermometer's accuracy.
The first round of tests on the resistance thermometer revealed that some changes would be needed to improve its performance. The team tested the second generation of prototypes for stability at high temperature (up to 1600°C). The thermometers suffered from an initial anneal effect but stabilising the device before use can eliminate this problem. Longer term stabilisation experiments showed that the thermometer can be stabilised to within the limits that today's thermocouple can achieve. "This resistance thermometer promises to be a good alternative to these thermocouples because it can be calibrated more easily," says Dr De Groot. What's more, sensitivity compares well to that of platinum resistance thermometers.
A patent application has been sent in for the resistance thermometer and there are plans to develop it into a relatively inexpensive, self-calibrating commercial product. "We have shown that we can make prototypes that survive laboratory conditions where the thermometers have been exposed to harsh conditions to simulate industrial use. We have yet to prove that the thermometer is viable for industrial conditions and that the results can be produced repeatedly within industrial requirements," says Drs De Groot.
The project participants worked well together and some will continue their studies to optimise the material and production parameters of the thermometer. Three of the partners have formed a new consortium, along with three other SMEs, an R&D organisation and two larger industries. Together, they have started a two-year CRAFT / BRITE EURAM project. With just a little further work, the thermometer could realistically be applied to industrial applications where the quality of the product is critically related to the process temperature.