Field recalibration of radiometers by using the shading technique

Use of radiometers to measure radiant energy components in energy budgets is widespread in water-resources studies. Previous experience has shown that readings of radiometers may contain substantial errors. These errors may be due to deterioration of the instrument during use and (or) inaccurate initial calibration. Both of these errors may be detected and corrected by proper field recalibration. After 10 months in the field, two Beckman-Whitley total hemispherical radiometers and one Eppley pyranometer were recalibrated by using the shading technique. The working standard was an Eppley pyrheliometer. Errors as great as 38 percent were discovered. New calibration constants were determined for all three field instruments. The shading technique is a rapid, accurate method of recalibrating under field conditions. It is important that the working standard be directly traceable to a primary standard which has participated in the International Comparisons of Pyrheliometers. With, an accurate working standard and careful technique, very accurate field recalibration is possible.