In order to identify limitations on the use of Pt/Pd thermocouples as high-accuracy thermometers, we have studied the change in thermocouple emf at the Al and Ag freezing points following a series of heat treatments at various temperatures for several lots of Pd wire. Reversible changes in the thermocouple emf at the Ag point (962°C) occur after heat treatments in the approximate temperature range 550°C to 1000°C, and slower, irreversible changes in emf occur after exposure to higher temperatures. Surprisingly, the sign of the reversible change was different for thermocouples fabricated from two lots of Pd of different purity. The magnitude of the changes in emf can be as high as the equivalent of 0.7 K per 200 h of heat treatment for the irreversible changes and 0.07 K per 200 h for the reversible changes. The consequences of these effects for metrology applications of Pt/Pd thermocouples are discussed.
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机译：为了确定使用Pt / Pd热电偶作为高精度温度计的局限性，我们研究了在不同温度下对几批Pd线进行一系列热处理后，Al和Ag凝固点的热电偶电动势的变化。在大约550°C至1000°C的温度范围内进行热处理后，Ag点（962°C）处的热电偶电动势会发生可逆变化，而暴露于更高的温度后，电动势会发生较慢的不可逆变化。出乎意料的是，由两批不同纯度的Pd制成的热电偶的可逆变化的符号是不同的。电动势的变化幅度可高达不可逆变化的每200 h热处理0.7 K和可逆变化的每200 h 0.07K。讨论了这些效应对Pt / Pd热电偶计量应用的影响。
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