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Some predictions of a validated physical model of Pt–Rh thermocouple drift above 1200 C
Metrologia ( IF 2.4 ) Pub Date : 2021-04-29 , DOI: 10.1088/1681-7575/abeb80
Jonathan V Pearce

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.



中文翻译:

1200°C 以上 Pt-Rh 热电偶漂移的验证物理模型的一些预测

最近提出了一个简单的模型,该模型将 Pt-Rh 热电偶的电动势 (emf) 漂移率与 Pt 和 Rh 氧化物的蒸气压联系起来。该模型假设这些氧化物的蒸发会导致 Pt 和 Rh 的浓度沿导线长度以不同的速率不断变化,这会导致塞贝克系数发生变化。该模型通过与可比环境下的高精度测量进行比较来测试。通过考虑不同成分的各种热电偶,证明计算的漂移率与测量的漂移率成正比,这代表了模型的验证。在当前的研究中,该模型用于对 1200 C 以上的一组最佳“零漂移”热电偶线成分进行一些预测。结果表明,对于超过几个 %Rh 的 Pt-Rh 导线,有相应的导线可以制作热电漂移几乎为零的热电偶,并且这几乎与温度无关。值得注意的是,发现这种最佳关系与基于经验技术的先前优化非常吻合。一个有趣的发现是,当测量结点在 1285 C 左右时,无论导线成分如何,漂移率都非常低;其原因由模型解释。这对在接近 1285 C 的温度下进行的热电偶漂移测试有影响,如果如模型所建议的那样,无论两个热电偶的成分如何,漂移本质上都很低,那么这可能是不可靠的。Co-C 的熔点,1324 C,常用于热电偶漂移评估,

更新日期:2021-04-29
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