246
Chapter 25
The Sun-and-shadow method is commonly used for calibration of the
pyranometers. According to our experience, better results can be derived
using special tube (Fig. 4), geometrical proportions of which are in
accordance with these of the reference pyrheliometer, i.e.,
where is the diameter of the aperture diaphragm, D the diameter of the
field diaphragm and
the distance between them of the reference
pyrheliometer, is the diameter of the receiving surface of the pyranometer,
the diameter of the field diaphragm of the tube and
the distance from
the sensitive surface of the pyranometer to the field diaphragm of the tube.
When both the pyrheliometer and the tube are directed towards the Sun, the
output of the pyranometer enclosed into the tube is proportional to the direct
solar radiation measured by the reference pyrheliometer. To avoid
overheating of the pyranometer in the bottom of the tube around
pyranometer the openings must be made for air circulation. The temperature
of the pyranometer has to be measured as the sensitivity (calibration factor)
of all types more or less depends from the temperature.