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2.4.3. Measurements during a sunny day
The CGR 4 di ers from all other pyrgeometers in that it allows accurate daytime measurements on
sunny days without the need for a shading device. Due to the unique construction of the CGR 4, solar
radiation of up to 1000 W/m² induces window heating of less than 4 W/m² in the overall calculated
downward radiation.
Formula 1 can be applied without any problems with the following exception; one must take note of the
amount of Infrared radiation in the solar spectrum. The amount of solar infrared radiation depends on
many parameters; for example the water vapour content in the atmosphere (humidity), location of the
CGR 4 at a certain altitude and the sun's declination angle. Figure 7 indicates the possible infrared
radiation in the solar spectrum in the case of low water content in the atmosphere. The amount of solar
infrared detected at the CGR 4 (and the CGR 3) is expected to be very low (0 to 3 W/m²) because of the
filter cut-on at 4.5 µm. Other types of pyrgeometers can exhibit 0 – 10 W/m².
Figure 7: Direct solar irradiance in Davos, Switzerland at solar noon in mid-September
2.5. Measurement uncertainty
When a pyrgeometer is in operation, the performance of it is correlated to a number of parameters, such
as temperature, level of irradiance, angle of incidence, etc. Normally, the supplied sensitivity figure is
used to calculate the irradiances. If the conditions di er significantly from calibration conditions,
uncertainty in the calculated irradiances must be expected.
Kipp & Zonen expects maximum uncertainty below 3% for daily totals for the CGR 4 pyrgeometer. This
remaining uncertainty can be reduced further if the sensitivity of the pyrgeometer under the prevailing
conditions is used, with corrections calculated from the e ects of parameters such as temperature and
non-linearity. This is especially convenient with a programmable data acquisition system.
CGR 4 Manual
Page 13
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