Kipp&Zonen CFR Calibration Facility - Manual (Page 17)

CFR manual
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3.3 Calibration uncertainty
The main purpose of the calibration procedure is to perform a one-to-one comparison of the reference
pyranometer and the test pyranometer. In order to achieve this, both pyranometers need to be exposed to
exactly the same irradiance, under the same circumstances.
There are a number of error sources that could affect the calibration measurement. Potential sources of
error are: Lamp instability, pyranometer zero offsets (A and B), voltmeter offset, differences in sensor
height and tilting of the pyranometers. These error sources are described below.
The method of performing the calibration, and the subsequent calculations, have been chosen such that
the effects of all these error sources have been minimised.
3.3.1 Instability of the lamp output power
Instability of the lamp output power due to mains voltage variations and changes in the light field is
fortunately only a secondary source of error. By using formula 2 one can easily assess the total lamp power
instability and calibrations where the deviation is greater than 2 % should be rejected.
3.3.2 Pyranometer offsets
Pyranometer offsets cause an output from the instrument, even when no light is present. These offsets are
corrected for during the calibration procedure by performing the dark measurements and are taken into
account in Formula 1.
ISO 9060 specifies two kinds of offsets; "zero offset a" which results from exchange of thermal radiation
between the pyranometer and its surroundings, and "zero offset b" which results from changes in the
ambient air temperature.
The offsets are basically caused by the fact that the pyranometer incorporates a thermal sensor, which is
sensitive to all heat flows. A high offset can usually be cured by waiting. It will settle down slowly.
Important note!
1 . Pyranometers should be kept out of the lamplight when exposure is not necessary.
2 . Pyranometers must be operated in a thermally stable environment during calibration.
More detailed information about zero offsets is explained in the general pyranometer manual.