Drift as a Function of Temperature in Platinum–Rhodium-Alloyed Thermoelements

Webster, E. S.; Edler, Frank

doi:10.1007/s10765-016-2166-6

Cited by 11 publications

(20 citation statements)

References 14 publications

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“…To limit the chemical changes caused by oxidation during key measurements, large wire diameters were used (∼3 mm Ø), but some oxidation effects will be implicit in the current set of reference functions. Unknown at that time were the instabilities caused by reversible crystallographic changes, which are now known to cause hysteretic effects in most thermocouple alloys [4,5]. Crystallographic changes usually involve a reversible ordering/disordering transformation of the alloying components (absent in pure thermoelements).…”

Section: Historical Backgroundmentioning

confidence: 99%

“…The type K was calibrated between 200 • C and 800 • C, whereas the type was calibrated between 50 • C and 1100 • C. Both were given a 500 • C 1 h preconditioning anneals before starting the calibration, then again at the end of calibration. Figure 5 shows the calibration data for these two thermocouples when calibrated by comparison with a type S thermocouple in a six-zone isothermal furnace [5]. The associated uncertainty budget is given in table 1, with the hysteretic component set to zero to reflect a fixed installation (with fixed immersion) and for which the process being measured operates at a fixed temperature.…”

Section: Calibrationmentioning

confidence: 99%

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Base-metal thermocouple tolerances and their utility in real-world measurements

Webster¹

2021

Meas. Sci. Technol.

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The reference functions and tables for base-metal thermocouples, which relate temperature and electro-motive-force, are specified by thermocouple type, rather than by alloy composition. The common thermocouple types—E, J, K, N, and T—are required to meet the specifications within defined tolerance bands; referred to as either limits-of-error (ASTM, US) or class (IEC, UK and Europe). In this requirement hides a fundamental problem: the tolerances are a manufacturing specification. Despite this, the tolerances are often used as a proxy for expected behaviour. However, these thermocouple types are subject to rapid and irreversible changes after short exposure times to modest temperatures. Thus, tolerance statements are poor predictors of in-use behaviour. This paper initially reviews the development of the base-metal thermocouples before identifying many of the metallurgical processes that lead to departures from the accepted tolerance bands. Focus is specifically given to the processes that occur in several type K and N thermocouples, which exemplify the changes common in the other base-metal thermocouples. From these results, it is shown that the use of traditional calibration techniques can be a futile exercise. Comparisons are then made between as-received thermocouples and those given thermal treatments, that have been designed to allow meaningful calibration while also inhibiting drift. These results show that thermal treatments and calibration are a practicable means to minimise drift over typical periods of use while offering small uncertainties, well within the span of manufacturing tolerances. Lastly it is suggested manufacturers give more information on the testing used to establish conformance with tables and reference functions and that they follow standardised testing procedures.

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Section: Historical Backgroundmentioning

confidence: 99%

Section: Calibrationmentioning

confidence: 99%

Base-metal thermocouple tolerances and their utility in real-world measurements

Webster¹

2021

Meas. Sci. Technol.

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“…Oxidation was facilitated by heat treatment in air at 900 °C and at 1400 °C, for durations between 10 h and 80 h. It was found that external oxidation occurred at both temperatures after heat treatment. On the surface of the 900 °C samples, rhodium oxide formed (rhodium oxide is known to dissociate above about 1140 °C [29][30][31][32][33][34][35][36]); neither rhodium oxide nor zirconium oxide formed a dense protective layer on the surface.…”

Section: Grain Growthmentioning

confidence: 99%

Stability assessment of a new high strength, high temperature Type R and Type S thermocouple

Pearce

Tucker

Rawal

et al. 2020

Meas. Sci. Technol.

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Platinum-rhodium Type R and S thermocouples fail in service because of two main reasons: excessive grain growth in the platinum limb at high temperatures (causing wire breakage) and temperature drift, i.e. deviation of electromotive force (emf) generated over time because of deposition of rhodium oxide on the platinum limb, out of accepted industry tolerances. Finding a solution to the problem is challenging because of the contradictory requirements of strength and narrow permissible emf range: emf is extremely sensitive to any dopants or alloying elements that can be used to improve strength at high temperatures. Johnson Matthey has developed a new thermocouple wire (platinum limb) called 'HTX ™ ' by doping platinum with a small quantity of zirconium which is oxidised during processing to electrically neutral zirconium oxide. Zirconium oxide improves the high temperature strength of platinum wire by restricting the grain growth but does not significantly affect the emf. The result is a wire which has the potential to last many times longer at high temperatures (>1200 °C) than standard platinum wire, whilst still achieving Class 1 tolerance (i.e. ±1 °C at 1000 °C). It also improves the thermoelectric drift characteristics of the thermocouple by counteracting any reduction in emf due to rhodium contamination by an increase in emf as any remaining zirconium is converted to zirconium oxide. We describe an impartial long-term assessment (up to 1500 h) of the thermoelectric stability of the HTX ™ wire by exposure to high temperature with periodic in situ calibration using a Co-C (1324 °C) high temperature fixed point. This yields stability measurements with very high precision.

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“…These thermocouples are typically either the Type S (Pt-10%Rh/Pt) or Type R (Pt-13%Rh/Pt). Both of these thermocouple types have now been shown to suffer from reversible drift-inducing processes in the Pt-Rh alloy caused by either crystallographic ordering changes below 600 °C or by rhodium oxidation above 600 °C [1,2]; the former cause an increase in the Seebeck effect and the latter causes a decrease. The changes in the emf caused by these two effects as a function of temperature, when homogeneity scanned at 100 °C, are illustrated in figure 1.…”

Section: Introductionmentioning

confidence: 99%

“…More recently several NMI's (national metrology institutes) have shown a renewed interest in Pt-x%Rh thermocouple alloys [7][8][9][10], with the hope of finding more stable pairs for prolonged use at temperatures over 1100 °C, principally for hightemperature, high-value manufacturing processes. A recent study by Webster and Edler [2] investigated the temperatureinduced drift characteristics of several Pt-x%Rh alloys at lower temperatures. This study revealed a distinct transition in both ordering and rhodium-oxidation behaviour at a Rh concentration of 20%, in good agreement with McLaren and Murdock's work.…”

Section: Introductionmentioning

confidence: 99%

Trials of Pt-20%Rh versus Pt thermocouples between 157 °C and 962 °C

Webster

2020

Metrologia

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For over a century the Type S (Pt-10%Rh) and R (Pt-13%Rh) thermocouples have been used as primary or secondary reference thermometers. However, the measurement uncertainties of both types at the silver point (~962 °C) are typically limited to about 0.5 °C, due to drift caused by crystallographic ordering between 200 °C and 500 °C and rhodium oxidation between 500 °C and 900 °C. Although both processes can be reversed using an 1100 °C anneal, regular annealing can be inconvenient or impracticable. This paper follows up a prior study indicating that an alternative noble-metal thermocouple comprised of Pt-20%Rh and Pt is relatively insensitive to both drift mechanisms. Four thermocouples, assembled using wire from four different manufacturers, were evaluated using a gradient-furnace and homogeneity scanner. Measurements were also made using a salt-bath (200 °C to 500 °C) and fixed points between the indium and silver points. The experiments indicate that individual thermocouples are stable at the silver point to within 0.18 °C for periods of up to 100 h and all four thermocouples have emf versus temperature characteristics within 0.3 °C of each other. This intrinsic stability and similarity, coupled with cost, assembly and use conditions identical to a Type R or S thermocouple make the Pt-20%Rh versus Pt thermocouple an attractive alternative. Recommended annealing procedures enabling the greatest stability are also given.

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Drift as a Function of Temperature in Platinum–Rhodium-Alloyed Thermoelements

Cited by 11 publications

References 14 publications

Base-metal thermocouple tolerances and their utility in real-world measurements

Base-metal thermocouple tolerances and their utility in real-world measurements

Stability assessment of a new high strength, high temperature Type R and Type S thermocouple

Trials of Pt-20%Rh versus Pt thermocouples between 157 °C and 962 °C

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