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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.

Within the work package one of the ongoing EMPIR project „Developing Traceable Measurement Capabilities for Monitoring Thermocouple Performance” (MetForTC), the novel concept of dual-type temperature probes was developed. By using dual-type temperature probes, users would be able to continuously monitor the thermocouple drift in-situ, including environments with varying temperatures. The concept is developed mainly for use in industry, where the drift of thermocouples can lead to various problems, such as increased energy consumption, loss of raw material, or decrease in quality of products. To mitigate the problems related to thermocouple drift, industrial users tend to replace thermocouples regularly, in some cases even after only a few days of usage. Using this approach, thermocouples can be occasionally replaced too early as well as too late. Dual-type temperature probe consists of a monitored thermocouple sensor and an additional temperature sensor, accommodated within the same protective sheath. The use of an additional (reference) sensor, which experiences a lower drift than the monitored thermocouple (e.g. higher grade thermocouple wires, platinum resistance sensors, or fiber-optic sensors), enables monitoring of thermocouple drift by continuous comparison of the differences in their temperature readings. While the drift of the monitored thermocouple can be tracked continuously, the concept also enables occasional tracking of drift of the reference sensor.

Launched in 2011 within the European Metrology Research Programme (EMRP) of EURAMET, the joint research project ‘MeteoMet’—Metrology for Meteorology—is the largest EMRP consortium; national metrology institutes, universities, meteorological and climate agencies, research institutes, collaborators and manufacturers are working together, developing new metrological techniques, as well as improving existing ones, for use in meteorological observations and climate records. The project focuses on humidity in the upper and surface atmosphere, air temperature, surface and deep-sea temperatures, soil moisture, salinity, permafrost temperature, precipitation, and the snow albedo effect on air temperature. All tasks are performed using a rigorous metrological approach and include the design and study of new sensors, new calibration facilities, the investigation of sensor characteristics, improved techniques for measurements of essential climate variables with uncertainty evaluation, traceability, laboratory proficiency and the inclusion of field influencing parameters, long-lasting measurements, and campaigns in remote and extreme areas. The vision for MeteoMet is to take a step further towards establishing full data comparability, coherency, consistency, and long-term continuity, through a comprehensive evaluation of the measurement uncertainties for the quantities involved in the global climate observing systems and the derived observations. The improvement in quality of essential climate variables records, through the inclusion of measurement uncertainty budgets, will also highlight possible strategies for the reduction of the uncertainty. This contribution presents selected highlights of the MeteoMet project and reviews the main ongoing activities, tasks and deliverables, with a view to its possible future evolution and extended impact.