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the lamp optics etc. Therefore the pyranometers are interchanged
and the whole procedure is repeated. We get another pair of values:
A' and B'.
6.3.3 Calculation
The sensitivity of the unknown pyranometer is calculated using
formula 2:
=
S
a
= Sensitivity of the reference pyranometer at 20 °C.
S
b
A
= Output of test pyranometer at position 1
A'
= Output of test pyranometer at position 2
B
= Output of reference pyranometer at position 2
B'
= Output of reference pyranometer at position 1
= Sensitivity of the test pyranometer at 20 °C.
S
a
Output = mean value at 100% response minus zero offset signal
6.3.4 Zero offset
The lamp housing and diaphragms are emitting long wave infrared
radiation, which heats up the glass dome. When the pyranometers
are shaded, there still remains a small signal up to + 20 µV due to
longwave infrared radiation from the dome to the sensor. This zero
offset is decreasing with a time constant (1/e) of several minutes.
A zero offset is also embodied in the response due to illumination. To
correct for this unwanted response, the zero offset read after 70 s
shading is subtracted.
34
+
A
A
'
⋅
s
+
b
B
B
'
(Formula 2)
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