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.

与温度和电动势相关的贱金属热电偶的参考函数和表格由热电偶类型指定，而不是由合金成分指定。常见的热电偶类型——E、J、K、N 和 T——需要满足规定公差带内的规格；被称为误差限制（ASTM，美国）或等级（IEC，英国和欧洲）。在此要求中隐藏了一个基本问题：公差是制造规范。尽管如此，容差通常被用作预期行为的代理。然而，这些热电偶类型在短时间暴露于适度温度后会发生快速且不可逆的变化。因此，容差声明不能很好地预测使用行为。本文首先回顾了贱金属热电偶的发展，然后确定了许多导致偏离可接受公差带的冶金过程。重点介绍了几种 K 型和 N 型热电偶中发生的过程，这些过程体现了其他贱金属热电偶中常见的变化。从这些结果可以看出，使用传统的校准技术可能是徒劳的。然后在收到的热电偶和经过热处理的热电偶之间进行比较，这些热电偶旨在允许进行有意义的校准，同时还抑制漂移。这些结果表明，热处理和校准是一种可行的方法，可以最大限度地减少典型使用期间的漂移​​，同时提供小的不确定性，完全在制造公差范围内。最后，建议制造商提供更多有关用于建立与表格和参考功能一致性的测试的信息，并遵循标准化的测试程序。