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6
CALIBRATION
6.1
INITIAL CALIBRATION
The ideal pyranometer should always have a constant ratio of voltage output
to irradiance level (outside the instrument in the plane of the sensing
element). This ratio is called sensitivity (S
The sensitivity figure of a particular pyranometer is unique. It is determined in
the manufacturer's laboratory by comparison against a standard pyranometer.
The standard pyranometer is calibrated outdoors regularly at the World
Radiation Centre (Davos, Switzerland). The spectral content of the laboratory
lamp differs from the outdoors solar spectrum at the Radiation Centre of
course. However, this has no consequences for the transfer of calibration,
because standard and unknown pyranometer have the same black coating
and quartz glass domes.
The supplied sensitivity figure is valid for the following conditions:
An ambient temperature of 20 °C.
For a horizontal pyranometer as well as for a tilted pyranometer.
Normal incident radiation of 500 W/m
Spectral content as clear sky solar radiation.
6.2
RECALIBRATION
Pyranometer sensitivity changes with time and with exposure to radiation.
Periodic calibration (at least every two years) is advised. Accurate calibrations
can be done outdoors under clear conditions by reference to a standard
pyrheliometer. Many National Weather Services have calibration facilities.
Their standard pyrheliometer is compared with the World Radiometric
CALIBRATION
) or responsivity.
ensitivity
2
.
37
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