Calibration of pyranometers

Methods currently used for calibrating pyranometers and some probable errors which may arise are discussed. Some important precautions that must be taken which could improve these methods are indicated.Two new calibration methods using a sub-standard pyranometer as reference are described. The first of these methods consists of direct comparison of the outputs of the sub-standard and working pyranometer on a X-Y recorder. The limitations of this method are discussed.The second method is independent of clear sky conditions. The pyranometer to be calibrated is compared with a sub-standard instrument placed in the same aspect and nearby. The output from the two instruments, or more if need be, is fed directly to a computer data acquisition system. The output from the computer, including calibration constant, real solar time, and ambient temperature in the form of BCD information on paper tape is later fed to a larger computer for statistical analysis. Correlations of calibration coefficient against time (position of the sun) and ambient temperature have been made.Results of calibrations carried out by this method for various designs of pyranometers are given for various climatic conditions. The probable limit of accuracy of this method is discussed with special reference to its advantages over more conventional methods. Other meteorological parameters that could be usefully included in the analysis are considered.