Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 15)

CMP/CMA series manual
Page 14
3. Principle components of pyranometers
The detector of the Kipp & Zonen CMP/CMA series pyranometer is based on a passive thermal
sensing element called a thermopile. Although the detector construction differs from model to model,
the fundamental working principle is applicable to all CMP/CMA series radiometers.
The thermopile responds to the total power absorbed by the black surface coating, which is a non-
spectrally selective paint, and warm up. The heat generated flows through a thermal resistance to the
heat-sink (the pyranometer body). The temperature difference across the thermal resistance of the
detector is converted into a voltage as a linear function of the absorbed solar irradiance.
The rise of temperature is easily affected by wind, rain and thermal radiation losses to the environment
('cold' sky). Therefore the detector is shielded by two domes (the entry-level CMP 3 has only one dome
to reduce size and cost). These domes allow equal transmittance of the direct solar component for
every position of the sun on the celestial sphere. A drying cartridge (dessicator) in the radiometer
housing is filled with silica gel and prevents dew on the inner sides of the domes, which can cool down
considerably on clear windless nights.
Figure 8 Construction details of a pyranometer
3.1. Dome
The dome material of the radiometer defines the spectral measurement range of the instrument. In
general about 97 – 98% of the solar radiation spectrum will be transmitted through the domes and will
be absorbed by the detector. The solar irradiance can come from any direction within the hemisphere
above the radiometer and therefore the domes are designed to minimize errors in measurement at all
incident angles.
CMP/CMA series radiometers, except the CMP/CM 3, have two high optical grade hemispherical
domes, one inner dome and one outer dome. In the chapter ‘pyranometer physical properties’ the
physical relation between dome and pyranometer performance will be explained.
For each particular model the specific dome material and spectral range is shown in the chapter
containing the instrument specifications.