Kipp&Zonen CM 4 Pyranometer - Manual (Page 34)

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6.3
CALIBRATION PROCEDURE AT KIPP & ZONEN
6.3.1 The facility
The calibration facility at Kipp & Zonen consists of a good quality film
sun (Osram) fed by an AC voltage stabiliser. This is used as an
artificial sun. It embodies a 150 W Metal Halide lamp with compact
filament.
To minimise stray light from the walls and the operator, the light is
limited to a small cone around the two pyranometers. The unknown
pyranometer 'a' and the standard pyranometer 'b' are placed side by
side on a small table. The table can rotate to interchange the
positions (1 and 2) of the pyranometers. 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
pyranometers (3°) so this cannot give rise to errors. The two
pyranometers are not levelled with the screws, but placed on their
bases. The effect of a small tilt is almost zero (Compare cos. 3° =
0.9986 and cos. 4° = 0.9976). The irradiance of the pyranometers is
approx. 500 W/m
2
. The colour temperature of the light is 3300 K.
6.3.2 Procedure
After illuminating for 70 s, the output voltages of both pyranometers
are integrated over 20 s with a solar integrator. Next, a blackened
‘hat’ covers both pyranometers. After 70 s the zero offset signal of
both pyranometers is integrated again.
The problem of the zero offset is described below. This zero offset
has to be subtracted to obtain the response due to illumination. So
we get responses A and B respectively.
The irradiance at position 1 (pyranometer 'a') may be slightly
different from that at position 2 (pyranometer 'b') due to asymmetry in
6. CALIBRATION