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Taking the heat: thermocouples that can compensate for their own drift makes their use at high temperatures more reliable.

Thermocouples are widely used sensors for measuring temperatures in industry, with applications across the range of process industries and in other places such as power plants and jet engines.

They are robust sensors often put into very tough situations and their performance can be affected by a lot of different factors: the atmosphere they work in, the materials they're in contact with, and by the temperatures they are there to measure.

It's particularly at high temperatures that thermocouples show the effects of these factors. Now, through a research project which brought together the National Physical Laboratory (NPL), which is the UK's national measurement institute, and the European Space Agency (ESA), an investigation into whether the thermocouples themselves might be modified to compensate or correct for the loss of accuracy that is inherent in the system has been completed.

The concept that the project aimed to prove was self-validation: the introduction of fixed reference points into the thermocouple that enable the user to know by how much the reading has drifted away from its calibration and therefore allow an adjustment to its output to compensate for that drift.

At high temperatures, the thermocouple drifting problem is a significant one. For the nickel alloy based (Type K and Type N) sensors that are commonly used up to 1200[degrees]C, it is known that readings can fall outside the manufacturers' stated tolerance limits within 15 hours of operation in an oxidising atmosphere.

For the much higher temperatures, up to 2300[degrees]C, that some processes demand, only a tungsten-rhenium thermocouple (known as a Types C, D or G) can be used. However above 1900[degrees]C, researchers have reported "severe drift" in excess of 10[degrees]C after as few as 10 hours. Since many of these sensors operate in continuous processes or inaccessible places, and since high temperature cycling makes them brittle, the practicalities of removal for recalibration make it impossible. These factors mean that most, if not all, high temperature processes, controlled by tungsten-rhenium thermocouples are always running suboptimally, with impact on energy use and product quality.

What the NPL researchers looked to in order to overcome these difficulties is an adaptation of a standard process for calibrating thermocouples in the laboratory. Calibration is performed by putting the thermocouple measurement junction into a "temperature fixed point cell" that consists of a pure metal ingot whose melting or freezing point is known and ensures complete immersion into the ingot. The output of the thermocouple (in volts) can then be plotted against the known temperature of the fixed-point cell as it is melting (or freezing), which gives a reference output against which the thermocouple readings can be calibrated.


Until recently for contact temperature measurements, the maximum defined fixed-point temperature was that at the freezing point of copper (at 1084.62[degrees]C). This gave a severe limitation on achievable calibration uncertainties. However, recent developments have seen the use of metal-carbon eutectic alloys as high-temperature fixed points (HTFPs), the melting temperatures of which are known to be extremely repeatable: to better than 0.05[degrees]C.

There are a wide range of these HTFPs with the "top of the range", rhenium-carbon, melting at 2474[degrees]C. NPL has pioneered the calibration of thermocouples using these new HTFPs and was the first national measurement institute in the world to offer 1S017025 accredited calibrations at the metal-carbon eutectic fixed points of Co-C (1324[degrees]C) and Pd-C (1492[degrees]C). Typical uncertainties for the calibration of platinum-based Type R or S thermocouples are 0.5[degrees]C at the Co-C point rising to 0.7[degrees]C at the Pd-C point (at a 95% confidence level, k = 2).

The principle of self-validation takes these laboratory based methods and seeks to apply them in real world situations. What the research team, led by Graham Machin at NPL, has done is to demonstrate the feasibility of permanently attaching a small HTFP ingot around the thermocouple measurement junction. Each time the thermocouple temperature passes through the melting or freezing point of the HTFP ingot, the thermocouple output undergoes a pause as the HTFP melts (or freezes). If the thermocouple output is different from what it should have been at the known temperature of the HTFP, subsequent readings from the thermocouple can be adjusted to correct for this drift.

This makes the thermocouple essentially "self-validating" and, though the uncertainty is higher than for a full-sized HTFP cell in the laboratory, any deviation is far smaller in practice than that of a standard thermocouple where no adjustment is possible.

The concept was proved in a series of tests that used a range of HTFP materials attached to Type C thermocouples specially made by the sensor manufacturer Omega Engineering. Placed in a graphite furnace and raised to temperatures above the melting point of the HTFP and then down past the freezing point, the self-validating thermocouple's reading showed the characteristic pause each time the fixed-point temperature was passed. The user can then, if required, apply a suitable correction to take the thermocouple readings back into line with the real furnace temperature.

The proof of the concept does not mean, though, the end of the work. The decline in performance of a thermocouple is not a constant factor and it may not be practical to attach an HTFP to a thermocouple in some applications. Currently, the research team believe this self-validating device is usable immediately in some inert or vacuum environments, and with further refinement of design, could be used in other situations too. With these innovations it is clear that thermocouple thermometry has just become more reliable.
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Title Annotation:Temperature
Publication:Environmental Engineering
Geographic Code:4EUUK
Date:Dec 1, 2012
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