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![]() Eppley Precision Spectral Pyranometer (PSP) ![]() ![]() ![]() Specifications ![]()
![]() Accuracy ![]() The absolute accuracy of calibration is ±3-4%. The relative accuracy of calibration is about ±2%. Deviation from a true cosine response is the reason for the large absolute uncertainty in calibration. ![]() Field performance ![]() Yearly calibrations are useful because the responsivity of PSPs has been observed to deteriorate over time. The most accurate way to calibrate a pyranometer is to measure the direct normal beam irradiation with an Absolute Cavity Radiometer and the diffuse irradiance with a black and white pyranometer shaded by a disk. The direct normal data are projected onto the horizontal surface and the diffuse values are added to obtain the best estimate of the total (global) irradiance on the horizontal surface. ![]() Cosine response of a PSP ![]() ![]() For a PSP, the calibration factor varies with zenith angle. The cosine response of a pyranometer that has been in the field for 20 years is shown in the above figure. Each data point is the average calibration factor of data within a 10° bin, centered at the zenith angle of the data point, on a clear day in August or September, 1999. Because of the change in calibration factor with zenith angle, the National Renewable Energy Laboratory defines the calibration factor of a PSP as the average of calibration numbers obtained when the zenith angle is between 45° and 55°. The UO SRML currently uses calibration numbers obtained with the NREL definition. For the pyranometer in shown in the above figure the calibration factor is 7.00 µV/Wm-2, although the calibration number varies depending on the zenith angle. Notice that for any given zenith angle the standard deviation is much smaller than the overall standard deviation. ![]() ![]() ![]() © 2023, UO Solar Radiation Monitoring Laboratory. ![]() ![]()
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