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New Absolute Cavity Pyrgeometer equation by application of Kirchhoff's law and adding a convection term
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Journal Article
New Absolute Cavity Pyrgeometer equation by application of Kirchhoff's law and adding a convection term
2023
Overview
An equation for the Absolute Cavity Pyrgeometer (ACP) is
derived from application of Kirchhoff's law and the addition of a convection
term to account for the thermopile being open to the environment, unlike a domed radiometer. The equation is then used to investigate four methods to
characterise key instrumental parameters using laboratory and field
measurements. The first uses solar irradiance to estimate the thermopile
responsivity, the second uses a minimisation method that solves for the thermopile responsivity and transmission of the cavity, and the third and
fourth revisit the Reda et al. (2012) linear least squares calibration
technique. Data were collected between January and November 2020, when the ACP96 and two IRIS radiometers monitoring terrestrial irradiances were
available. The results indicate good agreement with IRIS irradiances using
the new equation. The analysis also indicates that while the thermopile
responsivity, concentrator transmission and emissivity of an ACP can be
determined independently, as an open instrument, the impact of the
convection term is minor in steady-state conditions but significant when the base of the instrument is being subjected to rapid artificial cooling or
heating. Using laboratory characterisation of the transmission and
emissivity, together with use of an estimated solar calibration of the thermopile, generated mean differences of less than 1.5 Wm−2 to the two IRIS radiometers. A minimisation method using each IRIS radiometer as the reference also provided similar results, and the derived thermopile
responsivity was within 0.3 µV W−1 m2 of the solar-calibration-derived infrared responsivity estimate of 10.5 µV W−1 m2 estimated
using a nominal solar calibration and provide irradiances within ±2 %
of the terrestrial irradiance measured by the reference pyrgeometers
traceable to the International System of Units (SI). The calibration method using linear least squares regression introduced by Reda et al. (2012) that relies on rapid cooling of
the ACP base but utilising the new equation was found to produce consistent results but was dependent on the assumed temperature of the air above the thermopile. This study demonstrates the potential of the ACP as another
independent reference radiometer for terrestrial irradiance once the
magnitude of the convection coefficient and any potential variations in it have been resolved.
Publisher
Copernicus GmbH,Copernicus Publications, EGU,Copernicus Publications
