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Air temperature sensors: dependence of radiative errors on sensor diameter in precision metrology and meteorology

de Podesta, M; Bell, S; Underwood, R (2018) Air temperature sensors: dependence of radiative errors on sensor diameter in precision metrology and meteorology. Metrologia, 55 (2). pp. 229-244.

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In both meteorological and metrological applications, it is well-known that air temperature sensors are susceptible to radiative errors, both from direct irradiation from the Sun and lighting sources, and from thermal sources such as the walls of a room, an enclosure or a human body.
The key insight reported in this paper is that the radiative error measured by an air temperature sensor in flowing air depends upon sensor diameter, with smaller sensors reporting values closer to true air temperature. This is not a dynamic effect caused by low sensor mass, but a geometrical fluid-dynamical effect arising from sensor size. Although this result has been published [1] and is in meteorology text books [2], it does not appear to be widely known, and as a consequence air temperature can be - and probably is being - widely mis-estimated.
In metrological applications, air temperature is required in the estimation of refractive index for dimensional measurements and in the estimation of air density for buoyancy corrections in precision weighing. In these applications, errors on the order of 0.1 °C are significant, representing limiting uncertainties in their respective fields. In this paper we show that errors of this magnitude can easily be present even inside well-controlled metrological laboratories. We then describe how to detect such errors, and how to minimise and correct for them.
In meteorological applications, air temperature is the most commonly reported measurand and it is known to be subject to radiative errors both from direct insolation and thermal radiation even within a thermometer `screen'. The significance of the error is also known to be correlated with low wind speeds. However the diameter dependence of the correction is not widely appreciated. Understanding of the diameter dependence allows direct detection and assessment of the magnitude of the radiative error and, with proper design, it also enables minimisation of the error.

Item Type: Article
Keywords: Air Temperature, Metrology, Meteorology
Subjects: Engineering Measurements > Thermal
Divisions: Engineering, Materials & Electrical Science
Identification number/DOI: 10.1088/1681-7575/aaaa52
Last Modified: 27 Apr 2018 14:03
URI: http://eprintspublications.npl.co.uk/id/eprint/7891

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