Kipp & Zonen CGR 4 Instruction Manual

Kipp & Zonen CGR 4 Instruction Manual Manual pdf 16 page image

3. Principle components of pyrgeometersThe detector of the Kipp & Zonen CGR series pyrgeometer is based on a passive thermal sensing elementcalled a thermopile. Although the detector construction differs from model to model, the fundamentalworking principle is applicable to all CGR series radiometers.The thermopile responds to the total power absorbed by the black surface coating, which is a nonspectrallyselective 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 isconverted 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 3has a flat silicon window to reduce cost). A drying cartridge (desiccator) in the radiometer housing isfilled with silica gel and prevents dew on the inner sides of the domes, which can cool down considerablyon clear windless nights.Figure 10: Construction details of CGR 4 pyrgeometer3.1. WindowThe inner surface of the silicon dome / window has an interference filter deposited on it for transmittingthe long-wave radiation and blocking the short-wave solar radiation from reaching the detector. Thesilicon window material and the deposited ‘solar blind’ filter defines the spectral measurement rangeof the pyrgeometer.The silicon window allows transmittance of the atmospheric long-wave radiation up to approximately42 μm and about 50 - 60% of the radiation spectrum will be transmitted through to the detector. Theouter surface of the CGR 4 dome has a hard-carbon, diamond-like layer, deposited as additionalprotection against environmental influences in harsh environments and to smooth out the windowtransmission beyond 30 μm.Page 16 CGR 4 Manual