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3.2
MECHANICAL INSTALLATION
The mechanical installation of the pyranometer depends upon the measuring
purpose. Different measuring methods will be explained in the next
paragraphs.
3.2.1 Installation for measurement of global radiation
The following steps must be carefully taken for an optimal performance of the
instrument:
1.
Location
Ideally the site for the pyranometer should be free from any obstructions
above the plane of the sensing element, and at the same time the
pyranometer should be readily accessible to clean the outer dome and inspect
the dessicator. If this is not possible, the site should be chosen in such a way
that any obstruction over the azimuth range between earliest sunrise and
latest sunset should have an elevation not exceeding 5° (The apparent sun
diameter is 0.5°). This is important for an accurate measurement of the direct
solar radiation. The diffuse (solar) radiation is less influenced by obstructions
near the horizon. For instance, an obstruction with an elevation of 5° over the
whole azimuth range of 360° decreases the downward diffuse solar radiation
by only 0.8%.
It is evident that the pyranometer should be located in such a way that a
shadow will not be cast on it at any time (for example, by masts or exhaust
pipes). Note that hot (over 100 degrees centigrade) exhausted gas will
produce radiation in the spectral range of the CM 11 Pyranometer.
The pyranometer should be distant from light-coloured walls or other objects
likely to reflect sunlight onto it.
In principle no special orientation of the instrument is required.
22
INSTALLATION
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