A posteriori calibration of a mechanical pyranograph

A method is described to homogenize a time series of hourly mean readings from the records of a Robitzsch mechanical pyranograph over a period of years. The time-dependent instrumental coefficient is represented by the product of two adimensional coefficients accounting for (i) the typical monthly error of the pyranograph at different hourly intervals of the day, (ii) the mean error of the pyranograph during each month of the year and (iii) a monthly conversion factor, slowly increasing with time, accounting for the actual irregular decay of the instrumental sensitivity. Reference hourly irradiation values are given by Kipp & Zonen pyranometers over a three-year period. The long term correction is derived with the assumption that typical monthly maximum near-noon hourly irradiations represent a stationary climatic characteristic, at a given site and in a short (13-year) period, at least between the limits of accuracy of conventional global solar radiation measurements.