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An absolute cavity pyrgeometer to measure the absolute outdoor longwave irradiance with traceability to International System of Units, SI



Ibrahim Reda, Jinan Zeng, Jonathan Scheuch, Leonard Hanssen, Daryl Myers, Boris Wilthan, Tom Stoffel


This article describes a method of measuring the absolute outdoor longwave irradiance using an absolute cavity pyrgeometer (ACP). The ACP consists ofa domeless thermopile pyrgeometer, gold- plated concentrator, temperature controller, and data acquisition. The dome was removed from the pyrgeometer to remove errors associated with dome transmittance and the dome correction factor. To avoid thermal convection and wind effect errors resulting from using a domeless thermopile, the gold-plated concentrator was placed above the thermopile. The concentrator is a dual compound parabolic concentrator (CPC) with 180° view angle to measure the outdoor incoming longwave irradiance from the atmosphere. The incoming irradiance is reflected from the specular gold surface of the CPC and concentrated on the 11mm diameter of the pyrgeometer's blackened thermopile. The CPC's interior surface design and the resulting cavitation result in a throughput value that was measured by the National Institute of Standards and Technology. The ACP's pyrgeometer was installed horizontally outdoor on an aluminum plate connected to the circulating bath to control the pyrgeometer's case temperature. The responsivity of the pyrgeometer's thermopile detector was determined by lowering the case temperature and calculating the rate of change of the thermopile output voltage versus the changing irradiance. The responsivity is then used to calculate the absolute atmospheric longwave irradiance with an uncertainty estimate (U95) of less than ±1.23% with traceability to the International System of Units, SI. The measured irradiance was compared with the irradiance measured by two pyrgeometers calibrated by the World Radiation Center with traceability to the Interim World Infrared Standard Group, WISG. A total of 408 readings were collected over three different nights. The calculated irradiance measured by the ACP was 1.5 W/m2 lower than that measured by the two pyrgeometers that are traceable to the WISG.
Journal of Atmospheric and Solar-Terrestrial Physics


ACP, ATmospheric-IR, Measurement-Equation, WISG


Reda, I. , Zeng, J. , Scheuch, J. , Hanssen, L. , Myers, D. , Wilthan, B. and Stoffel, T. (2012), An absolute cavity pyrgeometer to measure the absolute outdoor longwave irradiance with traceability to International System of Units, SI, Journal of Atmospheric and Solar-Terrestrial Physics (Accessed April 20, 2024)
Created February 29, 2012, Updated October 12, 2021