13/10/2017
The pyranometer is based on a thermopile sensor. The thermopile
sensitive surface is coated with a black matt paint, which allows the pyranometer not
to be selective at different wave lengths. The pyranometer spectral range is
determined by the transmittance of the two glass domes type K5.
Radiant energy is absorbed by the thermopile black surface, creating a difference of
temperature between the center of the thermopile (hot junction) and the pyranometer
body (cold junction). Thanks to the Seebeck effect, the difference of temperature
between hot and cold junction is converted into a Difference of Potential.
In order to grant the thermopile a proper thermal insulation from the wind and to
reduce the sensitivity to thermal irradiance, the pyranometer is equipped with two
concentric domes having a diameter of 50mm and 30mm, respectively. The domes
protect the thermopile from the dust, which, laying down on the black surface, might
change spectral sensitivity.
The shadow ring prevents the direct solar radiation to reach the sensor the whole day
long, so that only the diffuse solar radiation will be measured. Because the elevation
of the sun changes day by day, it is necessary to change the height of the shadow ring
every two days.
