Long-term changes in electromotive force and microstructure of Rh-Pt thermocouples

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.

“…Alloys with lower concentrations of Rh tended to only form rhodium oxide at the grain boundaries. These results are in good agreement with other studies on rhodium oxidation [15,26,27].…”

Section: Gradient Furnace Aging Between 170 °C and 950 °Csupporting

confidence: 93%

Section: Variations In Emf Due To Minor Changes In Rh Concentrationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Webster

2020

“…The vapour pressures of pure Pt and Rh [3] are several orders of magnitude lower and can be ignored when oxygen is present. The evaporation of Pt and Rh oxides causes local changes in the bulk wire composition, generally expressed in terms of the Rh mass fraction as a percentage, mainly due to the evaporation of PtO 2 and RhO 2 [1,[4][5][6][7][8]. Below about 1200 • C, PtO 2 has a higher vapour pressure than RhO 2 , causing depletion of the fraction of Pt in the wire.…”

Section: Introductionmentioning

confidence: 99%

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

Pearce

2021

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.

“…The vapour pressures of pure Pt and Rh [9] are several orders of magnitude lower and can be ignored when oxygen is present. The evaporation of Pt and Rh oxides causes local changes in the bulk wire composition, generally expressed in terms of the Rh mass fraction as a percentage, mainly due to the evaporation of PtO 2 and RhO 2 [7,[10][11][12][13]. Below about 1200 • C, PtO 2 has a higher vapour pressure than RhO 2 , causing depletion of Pt in the wire.…”

Section: Introductionmentioning

confidence: 99%

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

Pearce

2020

Pt and Rh oxide vapour transport is thought to dominate the thermoelectric drift of Pt-Rh thermocouples above about 1200 °C. A simple model is 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 has been tested by comparison with high precision measurements of the thermoelectric drift of a Pt-Rh thermocouple assembly consisting of five thermoelements, repeatedly exposed to the melting of a Co-C high temperature fixed point (1324 °C) for a total duration of 500 h. By considering various thermocouples of different compositions, it is demonstrated that the calculated drift rate is proportional to the measured drift rate, which represents a validation of the model.