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INSTALLATION
3.2.3 Installation for measurement of reflected global radiation
In the inverted position the pyranometer measures reflected global radiation.
According to the WMO the height should be 1-2 m above a surface covered
by short cut grass.
The mounting device should not interfere too much with the field of view of the
instrument. A construction as in figure 11 is suitable.
The upper screen prevents excessive heating of the pyranometer body by the
solar radiation and, if large enough, it keeps the lower screen free of
precipitation. The lower screen prevents direct illumination of the domes by
the sun at sunrise and sunset.
Offset signals generated in the pyranometer by thermal effects (see
paragraph 1.2.1) are a factor of 5 more significant in the measurement of the
reflected radiation due to the lower irradiance level.
The mast in the construction of figure 11 intercepts a fraction D/2 π sr. of the
radiation coming from the ground.
In the most unfavourable situation (sun at zenith) the pyranometer shadow
2
2
decreases the signal by a factor R
/H
.
A rule of thumb is:
A black shadow with radius
= 0.1 H on the field below
decreases the signal 1%.
Secondly 99% of the signal
comes from an area with
radius 10 H.
Figure 11
Arrangement
to
measure
reflected global radiation
26
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