Kipp&Zonen CGR 4 Pyrgeometers - Manual (Page 16)

3. Principle components of pyrgeometers
The detector of the Kipp & Zonen CGR series pyrgeometer 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 CGR series radiometers.
The thermopile responds to the total power absorbed by the black surface coating, which is a nonspectrally
selective paint, and warm up. The heat generated flows through a thermal resistance to the heat-sink
(the pyrgeometer 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 CGR 4 detector is shielded by a silicon meniscus dome (the entry-level CGR 3
has a flat silicon window to reduce cost). A drying cartridge (desiccator) 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 10: Construction details of CGR 4 pyrgeometer
3.1. Window
The inner surface of the silicon dome / window has an interference filter deposited on it for transmitting
the long-wave radiation and blocking the short-wave solar radiation from reaching the detector. The
silicon window material and the deposited ‘solar blind’ filter defines the spectral measurement range
of the pyrgeometer.
The silicon window allows transmittance of the atmospheric long-wave radiation up to approximately
42 µm and about 50 - 60% of the radiation spectrum will be transmitted through to the detector. The
outer surface of the CGR 4 dome has a hard-carbon, diamond-like layer, deposited as additional
protection against environmental influences in harsh environments and to smooth out the window
transmission beyond 30 µm.
Page 16
CGR 4 Manual