Kipp&Zonen CM 4 Pyranometer - Manual (Page 33)

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Accurate calibrations can also be done outdoors under clear
conditions by comparison to a reference pyrheliometer. Many
National Weather Services have calibration facilities. Their standard
pyrheliometer is compared with the World Radiometric Reference
(maintained at Davos, Switzerland) embodied by several absolute
pyrheliometers (black body cavity type).
The comparisons are performed indoors or outdoors at one of the
regional Radiation Centres, see Appendix III. These institutes
sometimes offer calibration services.
There are several procedures for transferring calibration from a
narrow field of view instrument (pyrheliometer) to a wide field of view
instrument (pyranometer). For example, the direct component of the
solar radiation is eliminated temporarily from the pyranometer by
shading the whole outer dome of the instrument with a disk. There is
however no thermal equilibrium with this method and some
pyranometer models show zero-offset drift.
There is another procedure, during which the pyranometer to be
calibrated remains in its normal operating condition. This
'component' method involves measuring the direct component with a
pyrheliometer and the diffuse component with a disk shaded
pyranometer. As, during a clear day, the diffuse irradiance is only
about 10% of the global radiation, the sensitivity of the second
pyranometer does not need to be known very accurately. Both
procedures are suitable to recalibrate a pyranometer. The latter is
extensively described in International standard ISO 9846. A
summary of calibration methods is also found in the WMO guide of
1996.
Another procedure to recalibrate pyranometers is described in the
International Standard ISO 9847. Here the pyranometer to be
calibrated is compared to a reference pyranometer under clear sky
conditions. The pyranometers must be mounted side by side so that
each views the same sky dome. It is desirable to integrate, or
average, the outputs over a period of time and then compute the
calibration constants on the basis of these averages. This reduces
the errors due to changing parameters during the day.