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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
responsivity.
The calibration (sensitivity) factor of a particular pyranometer is
unique. It is determined in the manufacturer's laboratory by
comparison against a reference pyranometer.
The reference pyranometer is regularly calibrated outdoors at the
World Radiation Centre in Davos, Switzerland. Of course the
spectral content of the laboratory lamp differs from the outdoor solar
spectrum at the Radiation Centre. However, this has no
consequences for the transfer of calibration, because the reference
pyranometer and the pyranometer under test have the same black
coating and glass dome.
The supplied calibration factor is determined under 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 the same as clear sky solar radiation.
6.2
RECALIBRATION
The pyranometer sensitivity changes with time and with exposure to
radiation, this deviation is also known as the non-stability.
Periodically a radiometer calibration is advised, at least every two
years. Recalibration can be done at Kipp & Zonen. When sending
back a pyranometer to Kipp & Zonen for recalibration it is
recommended to use the recalibration form in the back of this
manual, Appendix IV.
6. CALIBRATION
) or
ensitivity
2
.
31
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