NREL Comparison Between Absolute Cavity Pyrgeometers and Pyrgeometers Traceable to World Infrared Standard Group and the InfraRed Integrating Sphere

The comparison of the Absolute Cavity Pyrgeometers (ACPs) Comparison with a PIR Pyrgeometer Traceable to the World Infrared Standard Group (WISG) and a Kipp & Zonen (KZ) pyrgeometer traceable to the InfraRed Integrating Sphere Radiometer (IRIS) was held during the NREL Pyrheliometer Comparison (NPC-2018) from September 24 to October 5, 2018. Data from all instruments was collected during night time clear sky conditions only. The irradiance measured by the ACPs and the PIR pyrgeometer traceable to the WISG were collected every 10 seconds during the ACPs self-calibration and every 30 seconds during the measurement runs. The measurement runs were for two hours while the calibration runs were for 6 minutes. The irradiance measured by the Kipp & Zonen pyrgeometer was collected every 30 seconds. During the comparison the average (av) irradiance difference measured by three ACPs varied from 0.2 W/m2 to -0.5 W/m2, with a standard deviation (sd) from 0.5 W/m2 to 0.8 W/m2. The average irradiance difference measured by the three ACPs minus the irradiance measured by the PIR is 4.3 W/m2 and sd 4.2 W/m2, and that of measured irradiance by the KZ is 2.3 W/m2 and sd 3.5 W/m2.