NREL Comparison of Absolute Cavity Pyrgeometers, InfraRed Integrating Sphere, and Pyrgeometers Traceable to World Infrared Standard Group: September 25-October 6, 2023

The comparison of the absolute cavity pyrgeometers (ACPs) with the InfraRed Integrating Sphere (IRIS), Eppley Precision Infrared Radiometer (PIR) pyrgeometers, and Kipp & Zonen (KZ) pyrgeometers traceable to the World Infrared Standard Group (WISG) was held during NREL ACP and IRIS Comparisons (NAIC) from September 25 to October 6, 2023. Data from all instruments was collected during nighttime clear sky conditions only. The irradiance measured by the ACPs is collected in 30 seconds intervals during the measurement period of two hours, and 10 seconds intervals during the calibration period of 6 minutes. During the comparison, the average (av) irradiance difference measured by ACPs and IRIS9 varied from -0.75 W/m2 to 0.76 W/m2, standard deviation (sd) from 0.78 W/m2 to 1.04 W/m2, and uncertainty U95 from 1.96 W/m2 to 2.07 W/m2. The average irradiance difference measured by ACP95F3 minus the irradiance measured by all pyrgeometers varied from 1.64 to 3.96 W/m2, sd from 1.70 W/m2 to 1.86 W/m2, and uncertainty U95 from 3.78 W/m2 to 5.42W/m2. Note that from September 25th at 18:31 to September 29th at 5:30 ACP96F3 irradiance is calculated using Bruce, et al 2023 method.