Kipp&Zonen CFR Calibration Facility - Manual (Page 14)

CFR manual
13
3 Calibration operation
The indoor calibration procedure, according to ISO 9847 Appendix III, is based on a side-by-side
comparison with a reference radiometer under a stable artificial sun. Kipp & Zonen uses a 150 W Metal-
Halide high-pressure gas discharge lamp with voltage stabilisation. Behind the lamp is a reflector with a
diameter of 16.2 cm. The reflector is 115 cm above the radiometers producing a vertical beam. The
irradiance at the radiometers is approximately 500 W/m².
To minimise stray light from the walls and the operator, the light is restricted to a small cone around the two
radiometers. The unknown test radiometer 'T' and the reference radiometer 'R' are placed side-by-side on a
small table. The table can rotate to interchange the positions (1 and 2) of the radiometers. The lamp is
centred on the rotating axis of this table. Actually there is no normal incidence of the radiation, but the
angle of incidence is the same for both radiometers (3°) so this cannot give rise to errors. The two
radiometers are not levelled with the adjustable feet, but placed on their bases. The effect of the small
beam tilt is negligible (compare cos. 3° = 0.9986 and cos. 4° = 0.9976).
Once the Calibration Facility has been installed calibration is quick and simple, but the procedure must be
carried out carefully and thoroughly. Section 3.1 describes the procedure and 3.2 gives step-by-step
instructions to follow. The philosophy behind the calibration is given in 3.3. In 3.4 an example of a
calibration form is given and the form is shown in Annex I.
Wherever output voltages are indicated, R refers to the reference and T refers to the test pyranometer.
When dark signals or offsets are indicated, the "E" symbol is used. Measurements in position 1 are
indicated by "1" (see paragraph on turntable), measurements in position 2 are indicated by "2". The symbol
"S" refers to sensitivity.
3.1 Description of the procedure
The reference pyranometer and the test pyranometer are placed side-by-side on the turntable in position 1.
The table can turn in order to exchange positions. The lamp illuminates both pyranometers with an intensity
of approximately 500 Watts per square meter.
After illuminating for one minute, the output voltages of both pyranometers are measured shortly after each
other with the voltmeter. The obtained values are called R and T. Next, a shield covers both pyranometers,
so that they do not receive any light. After one minute, the output voltages of both pyranometers are again
measured. The values obtained, called ‘zero offsets’, are RE and TE respectively. The order of magnitude
of this offset for a Kipp & Zonen pyranometer is a few W/m
2
. For pyranometers from other manufacturers, it
could be larger. These zero offsets must be subtracted from the values R and T. The resulting values are
R1 (equals R-RE) and T1 (equals T-TE).