Calibrating Pyrgeometers Outdoors Independent from the Reference Value of the Atmospheric Longwave Irradiance

In this article, we describe a method for the calibration of thermopile pyrgeometers in the absence of a reference for measurement of atmospheric longwave irradiance. This is referred to as the incoming longwave irradiance in this article. The method is based on an indoor calibration using a low-temperature blackbody source to obtain the calibration coefficients that determine the pyrgeometer's radiation characteristics. From these coefficients the outgoing irradiance of the pyrgeometer can be calculated. The pyrgeometer is then installed outdoors on an aluminum plate that is connected to a circulating temperature bath. By adjusting the temperature bath to the approximate value of the effective sky temperature, the pyrgeometer's body temperature is lowered changing the pyrgeometer's thermopile output. If the incoming longwave irradiance is stable, the slope of the outgoing irradiance versus the pyrgeometer's thermopile output is the outdoor net irradiance responsivity (RS_net), independent of the absolute value of the atmospheric longwave irradiance. The indoor calibration coefficients and the outdoor RS_net are then used in the pyrgeometer equation to calculate the incoming longwave irradiance. To evaluate this method, the calculated irradiance using the derived coefficients was compared to the irradiance measured using a pyrgeometer with direct traceability to the World Infrared Standard Group (WISG). This is maintained at the Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Switzerland. Based on results from four pyrgeometers calibrations, this method suggests measurement agreement with the WISG to within ±3 W/m² for all sky conditions.