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To measure air temperature precisely using contact sensors requires that the sensor is (in equilibrium/in adiabatic conditions) with the surrounding air. This is difficult to achieve because heat exchange with the air can only be accomplished through the surface of the thermometer to the nearby air, while radiation may transmit energy to or from faraway objects. To make matters harder, heat exchange across the surface is also affected by the state of the air, such as density, water content and wind speed. To a calibration laboratory this represents a dilemma: should the sensor be calibrated in a liquid bath to obtain the best possible calibration uncertainty, or should it be calibrated in air to more closely resemble the actual use conditions at the cost of a higher calibration uncertainty? As part of a EURAMET project (1459) an interlaboratory comparison (ILC) was launched in 2019 with the aim of establishing a set of best practices for calibration procedures of contact thermometers. 8 different probe models, from 6 different manufacturers, were shipped around Europe to 26 NMIs or DIs, which reported data at air temperatures ranging from -80 °C to +60 °C. In total the ILC provided more than 1600 independent observation points. The sensors were thoroughly characterised prior to and after the circulation. We present two important observations from the aggregate results of this ILC. On the one hand, there is a substantial scatter in the reported reference uncertainty, pointing to a strong variation in the measurement setups and performance. Secondly, the scatter of results show a temperature dependency which is not seen in the reported uncertainties. The standard deviation of this scatter ranges from around 40 mK at 20 °C up to more than 200 mK at -40 °C. This variation is larger than the typical reported uncertainty. We discuss possible implications of this observation.
Topic : Theme 1: Atmospheric Chemistry and Physics.
Reference : T1-A31
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