Kipp&Zonen CHP 1 Pyrheliometer - Manual (Page 15)

15
7 CALIBRATION
7.1 Calibration principle
An ideal pyrheliometer gives voltage output that is proportional to the absolute irradiance level. This relationship
can be expressed as a constant ratio called ‘sensitivity’ (Sensitivity).
The sensitivity figure of a particular pyrheliometer is unique. It is determined in the manufacturer's laboratory by
comparison against a reference pyrheliometer of similar type. The reference pyrheliometer is calibrated outdoors
regularly at the World Radiation Centre (WRC) at Davos, Switzerland. The spectral content of the laboratory
calibration lamp differs from the outdoor solar spectrum at the World Radiation Centre. However, this has no
consequences for the transfer of calibration, because standard and test pyrheliometers have the same black coating
and windows.
The supplied sensitivity figure is valid for the following conditions:
An ambient temperature of +20 °C.
Normal incident radiation of 500 W/m
2 .
For any other condition the sensitivity figure can be used within uncertainty bands given in the specifications for
each model.
A summary of calibration methods is also found in the WMO guide of 1996.
7.2 Calibration procedure at Kipp & Zonen
At Kipp & Zonen, calibration is performed by indoor comparison with a reference instrument. The reference is not of
a higher standard classification. The comparison is made under a xenon lamp at an irradiance level of
approximately 500 W/m
2 . The reference has been calibrated at the World Radiation Centre. The accuracy of this
calibration is ± 0.5 %. ISO requests that each pyrheliometer, in order to obtain its classification, must periodically be
compared to a higher standard.
7.3 Traceability to world radiometric reference
Reference pyrheliometers, which are calibrated annually by the World Radiation Centre in Davos, are used for the
calibration of radiometers manufactured by Kipp & Zonen. The reference radiometers are fully characterized, i.e.
linearity, temperature dependence and directional response are recorded.
Kipp & Zonen keeps two reference radiometers for each radiometer model. These reference radiometers are sent
alternate years to WRC for calibration, so production and calibration in Delft can carry on without interruption.