NREL Comparison of Absolute Cavity Pyrgeometers and Pyrgeometers Traceable to the World Infrared Standard Group and the Infrared Integrating Sphere: September 23-October 4, 2019

The comparison of the absolute cavity pyrgeometers (ACPs) with a precision infrared radiometer (PIR) pyrgeometer traceable to the World Infrared Standard Group (WISG) and a Kipp & Zonen (KZ) pyrgeometer traceable to WISG and the InfraRed Integrating Sphere Radiometer (IRIS) was held during the National Renewable Energy Laboratory (NREL) Pyrheliometer Comparison (NPC-2019) from September 23 to October 4, 2019. Data from all instruments was collected during nighttime clear sky conditions only. The irradiance measured by the ACPs, PIR, and CG4 pyrgeometers collected 30 seconds during the measurement runs. The measurement runs lasted for two hours, while the calibration runs lasted for 6 minutes. During the comparison, the average (av) irradiance difference measured by three ACPs varied from 0.2 W/m2 to 0.8 W/m2, with a standard deviation (sd) from 0.4 W/m2 to 0.8 W/m2. The average irradiance difference measured by the three ACPs minus the irradiance measured by the PIR was 7.1 W/m2 and sd 1.0 W/m2, and the average irradiance difference measured by the KZ traceable to WISG was 8.1 W/m2 and sd 0.9 W/m2, and average irradiance difference measured by the KZ traceable to IRIS was 4.0 W/m2 and sd 0.9 W/m2.