Campbell Scientific Kipp and Zonen CMP-Series Pyranometers - Manual (Page 23)

CMP/CMA series manual
Page 22
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 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 radiation 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.