A validated physical model of the thermoelectric drift of Pt-Rh thermocouples above 1200 °C

Pearce, J. V.

doi:10.1088/1681-7575/ab71b3

Cited by 5 publications

(5 citation statements)

References 31 publications

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“…Note that according to (1) and (2) the cross-over temperature Measured drift rate versus calculated drift rate [11] for the thermocouples where data is available [12], showing a consistent linear relationship. Each point represents a different thermocouple composition, denoted by the composition of the two wires (e.g.…”

Section: The Modelmentioning

confidence: 74%

“…A simple model of Pt and Rh oxide vapour transport was recently presented [11], which predicts the influence of oxide transport to different regions of the thermocouple wire on the resulting thermoelectric stability. In this paper the model has been employed to make some predictions about the relative thermoelectric stability of different Pt-Rh alloys.…”

Section: Discussionmentioning

confidence: 99%

“…In [11] a model was described which relates the thermoelectric drift rate of Pt-Rh thermoelements to the vapour pressure of Pt and Rh oxides. The model assumes that the evaporation of these oxides gives rise to a continuously changing concentration of Pt and Rh, at different rates (which depend on the local temperature) along the length of the wires, which causes a local change in the Seebeck coefficient.…”

Section: Introductionmentioning

confidence: 99%

“…In this paper the model is used to make some predictions concerning the set of thermocouple wire compositions which give rise to minimal thermoelectric drift, and assess the temperature dependence of this. In section 2 the model is summarised (further details can be found in [11]). In section 3 the model is used to make some predictions, which are compared with experimental findings.…”

Section: Introductionmentioning

confidence: 99%

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Some predictions of a validated physical model of Pt–Rh thermocouple drift above 1200 °C

Pearce

2021

Metrologia

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A simple model was recently presented which relates the electromotive force (emf) drift rate of Pt–Rh thermoelements to the vapour pressure of Pt and Rh oxides. The model assumes that the evaporation of these oxides gives rise to a continuously changing concentration of Pt and Rh, at different rates along the length of the wires, which causes a change in the Seebeck coefficient. The model was tested by comparison with high precision measurements under comparable circumstances. By considering various thermocouples of different compositions, it was demonstrated that the calculated drift rate is proportional to the measured drift rate, which represented a validation of the model. In the current study, the model is used to make some predictions concerning the set of optimum ‘zero-drift’ thermocouple wire compositions above 1200 °C. It is shown that for a wire of Pt–Rh with more than a few %Rh, there is a corresponding wire to make a thermocouple which has nearly zero thermoelectric drift, and that this is almost independent of temperature. Remarkably, this optimum relation is found to agree very well with a previous optimisation that was based on an empirical technique. An intriguing finding is that when the measurement junction is at around 1285 °C, the drift rate is very low, regardless of wire composition; the reason for this is explained by the model. This has implications for thermocouple drift testing at temperatures close to 1285 °C, which may be unreliable if the drift is inherently low regardless of the composition of the two thermoelements, as suggested by the model. The melting point of Co–C, 1324 °C, commonly used for thermocouple drift assessment, is far enough away from 1285 °C for this effect not to be a problem.

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Section: The Modelmentioning

confidence: 74%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Some predictions of a validated physical model of Pt–Rh thermocouple drift above 1200 °C

Pearce

2021

Metrologia

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“…In EMPRESS, a systematic investigation of a number of different Pt-Rh thermocouples showed that the most stable thermocouple consistent with readily available Pt-Rh wire compositions, at least in the temperature range 1324 °C to 1492 °C, is the Pt-40%Rh versus Pt-6%Rh thermocouple [19][20][21]. This is due to the combined effects of thermoelectric drift due to vaporisation of Pt and Rh oxides [34,35,39] A preliminary reference function was also drafted (Figure 2) [19]. In EMPRESS2, to facilitate uptake of the new thermocouple type, NMIs will systematically determine its reference function.…”

Section: Wp2: Low-drift Thermocouplesmentioning

confidence: 99%

Enhancing process efficiency through improved temperature measurement: the EMPRESS projects

Pearce

Edler

Fateev

et al. 2023

J. Phys.: Conf. Ser.

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EMPRESS 2 is a European project to enhance the efficiency of high value manufacturing processes by improving temperature measurement and control capability. This project seeks to address four contemporary thermometry challenges in this sector, and new developments from this and its predecessor project, EMPRESS, will be described: • Below 1000°C many industrial processes require reliable surface thermometry e.g. welding, coating, forging and forming. Conventional non-contact surface thermometry techniques e.g. thermal imaging are prone to large errors (tens of degrees) due to reflected thermal radiation and unknown emissivity. Contact thermometry approaches are prone to similarly large errors. Traceable imaging phosphor thermometry is being developed to overcome these difficulties, and is being combined with quantitative thermography to determine emissivity for thermometry over wide fields of view. • Above 1300°C sensor drift is a significant unaddressed issue for casting, forging and heat treatment, causing large errors. There is a need for more stable sensors and standardisation of at least one new thermocouple type to fill the gap from 1300°C to 1800°C. This is being addressed through improved Pt-Rh thermocouples and optimisation of double-walled mineral insulated, metal sheathed thermocouples by mitigating insulation breakdown and drift effects. • Combustion temperature measurement is very challenging and traceability is almost non-existent; for example, thermocouple measurements of flame temperatures can be in error by hundreds of degrees. A ‘standard flame’ that can be transported to users’ sites has been developed, and is being deployed in several high value manufacturing and industrial applications to a) demonstrate the possibility of reducing flame temperature uncertainties by at least an order of magnitude and b) for the first time to demonstrate in-situ traceability to the International Temperature Scale of 1990 (ITS-90). • Many processes are not amenable to any conventional thermometry techniques due to inaccessibility, ionising radiation, electromagnetic interference, and contamination; here methods based on optical fibres are ideal but there are no traceable calibration techniques for such sensors currently available. A suite of different fibre-optic thermometers and calibration techniques is being developed to address this. In some cases (ionising radiation) darkening of the fibre is a problem, and this is being overcome by the development of novel thermometry approaches based on practical ‘hollow core’ fibres.

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Research on the methodology and instrument of traceable measurement of surface temperature based on an “ideal plane” model

2022

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Because the emissivity of a measured sample is difficult to determine in an in situ measurement and the emissivity of a pyrometer is different in the in situ measurement and calibration, the measurement results of the pyrometer are not traceable to a standard—The International Temperature Scale of 1990. To solve this problem, an ideal plane is proposed in this paper. The measured sample and the thermocouple are in a vacuum blackbody, and a gold-plated reflector is in contact with the measured sample. The surface can emit blackbody radiation during the measurement. The emissivity of the measured surface is always the same as that during calibration, so the uncertainty of emissivity is eliminated. In addition, the temperature of the measured sample is the same as that of the thermocouple in the vacuum blackbody; in doing so, the temperature differences between them are eliminated, and the uncertainty of the traceable measurement of the measured surface is greatly reduced. The instrument has been developed and tested. The results show that the average difference of traceable measurements in the range of 600–900 °C is 2.29 °C, while the uncertainty is 0.52 °C.

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A validated physical model of the thermoelectric drift of Pt-Rh thermocouples above 1200 °C

Cited by 5 publications

References 31 publications

Some predictions of a validated physical model of Pt–Rh thermocouple drift above 1200 °C

Some predictions of a validated physical model of Pt–Rh thermocouple drift above 1200 °C

Enhancing process efficiency through improved temperature measurement: the EMPRESS projects

Research on the methodology and instrument of traceable measurement of surface temperature based on an “ideal plane” model

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