Kipp & Zonen CM 11, Instruction Manual

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CALIBRATION
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Reference (maintained at Davos, Switzerland) embodied by several absolute
pyrheliometers (black body cavity type).
The comparisons are in-doors or at one of the regional Radiation Centres, see
Appendix III.
These institutes sometimes offer calibration facilities.
There are several procedures for transferring calibration from a narrow field of
view instrument (pyrheliometer) to a wide field of
view instrument (pyranometer). E.g. 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 unknown pyranometer 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 radiance 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
obtain a working standard pyranometer. The latter is extensively described in
International standard ISO 9846.
Transfer from the working standard pyranometer to other pyranometers can
be done in sunlight. 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.
Transfer from another pyranometer in the laboratory is only possible when
both pyranometers are of the same type and have the same glass domes and
optical coatings. Kipp & Zonen can recalibrate pyranometers according to this
method for a charge.