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In-situ determination of thermocouple drift using dual-type thermometers

This paper describes the novel concept of dual-type temperature sensors, that enables in-situ monitoring of thermocouple drift. The concept aims to provide users, who are performing temperature measurements by thermocouples, a relatively cheap and simple solution for monitoring its drift under conditions of typical use, including environments with varying temperatures. For industrial users, thermocouple drift causes many problems such as higher energy costs and lost time through process disruption due to the replacement or recalibration of failed thermocouples. This usually requires processes to be shut down when the measurement errors become too large. Such process disruptions can further lead to loss of raw material or the production of semi-finished products. For these reasons, users requiring high accuracy temperature measurements tend to replace thermocouples regularly as a precautionary measure, sometimes even if they have been used for only a few days. The main idea of the dual-type thermometers is that a drift of a thermocouple can be determined by monitoring the difference in its temperature readings and the readings of an additional (reference) thermometer if the reference sensor experiences a lower drift than the thermocouple under test. In many cases, the monitored thermocouple sensor cannot be simply replaced with a thermometer of lower drift, since it is connected to the electronic unit that is, in general, not suitable for measurement with a different type of temperature sensor. Even if changing the thermocouple type would be feasible, the replacement thermocouple would still experience drift that would be impossible to track in-situ. The concept of dual-type temperature sensors can solve this problem as a drift of a reference sensor can be estimated after every replacement of the monitored thermocouple. Also, in many industrial scenarios, thermocouples are not calibrated before being put in operation and users rely on their stated tolerance classes. In such cases, the methods described in this paper can be used for checking if a monitored thermocouple has departed outside the specified tolerance limits in time. Together with the description of the concept, this paper provides schematic diagrams of related measurement setups, drawings of dual-type temperature sensors (utilizing noble metal thermocouples and platinum resistance thermometers as the reference sensors) as well as an algorithm for drift determination. This work is performed within the framework of the European Metrology Programme for Innovation and Research (EMPIR) project 18RPT03 MetForTC. This project (18RPT03 MetForTC) has received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme.

temperature measurement, thermocouple drift, dual-type thermometers

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