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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 response 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 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 with the formula 2:
+
A
=
S
+
a
B
= Sensitivity of the standard pyranometer at 20 °C.
S
b
A
= Output of pyranometer at position 1
A'
= Output of pyranometer at position 2
B
= Output of standard pyranometer at position 2
B'
= Output of standard pyranometer at position 1
= Sensitivity of the 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 outer glass dome and also, indirectly, the inner one. When
the pyranometers are shaded, there still remains a small signal up to + 20 µV
due to long wave infrared radiation from the inner dome to the sensor. This
zero offset is decreasing with a time constant (1/e) of several minutes.
40
A
'
⋅
s
(formula 2)
b
B
'
CALIBRATION
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