Comparisons of Two Calibration Methods for the Net Pyrradiometer

China doesn't conduct operational observations of net radiation until the 1990s, when the remote detection for radiation is applied. Temporary reference standard for the net pyrradiometers is based on 3 CN-11 type net pyrradiometers which are imported from Japan in 1980. The structures of CN-11 net pyrradiometers and homemade pyrradiometers are of the same. The inductive parts consist of the thermopile and two black inductive surfaces. The black inductive surface can receive the whole wave-band radiation with the wavelength 0.3-100 μm. The upper inductive surface receives the whole radiation from the sky and the lower surface receives the radiation from the earth surface. The net pyrradiometer is more complex than other radiometers because of two inductive surfaces and the broad range of received radiation. To stop wind and protect the inductive surface, a polyvinyl film cover is installed. But the measuring uncertainty is increased by it because the film cover is difficult to clean and keep sealed, while it gets distorted and aging easily. According to the suggestions of WMO, two pyrgeometers (CG4) and two pyranometers (PSP) are used together as the new standard group. Under different environment conditions, six working net pyrradiometers are calibrated using four components method as well as the CN-11 type pyrradiometer standard group and the results are analyzed. Experiments are conducted to provide scientific basis for the updating of standard group, through which the relationship between the new standard group and the old standard group in the calibration environment are found out. The random error of the results using the new standard group is calculated. Analysis suggests the measuring accuracy can be improved 9.2% than before in daytime and 5.2% at night after using the new standard group. The data of the net pyrradiometers calibrated by the new standard group prove to be closer to the true value. In September 2005, two CG4 type pyrgeometers are traced to the infrared radiation standard of World Radiation Center (WRC). The new net pyrradiometer calibration regulation is established in 2006. China Meteorological Administration has taken the combination of the two pyrgeometers (CG4) and two pyranometers (PSP) as the formal standard group of net pyrradiometers since 1 January 2007. It is proved that using the new standard group could ensure the transfer of the radiation value reliable and raise the measuring accuracy for net radiation in China.