34
Figure 4.3. An one-axis tracker used in shading a
pyranom eter. Note the use of two fine wires to m aintain
the stability of the shading disk. (Developed by
Deutscher Wetterdienst).
4.2.3 Mechanical installation of shaded sensors (pyranometers and pyrgeometers)
The general installation of shaded sensors follows the guidelines set out in 4.2.2, but includes the added
complexity of aligning the shading device with the instrument. Within the BSRN, shade rings (diffusographs)
are not accepted as a means of shading an instrument because of the field of view the ring subtends.
Two types of devices are commonly employed within the BSRN.
Figures 4.3 and 4.4 illustrate synchronous motor shade devices as designed by the Germans and
the Swiss. The German device uses a single axis to carry the pyranometer about which the shade-disk
rotates. This design is significantly more complex than the Swiss design, but is more efficient in using
space. The Swiss design fixes the pyranometer on a stable, level stand mounted separately from the
equatorial motor. The position of the shade disk must be moved along the shading arm with the changing
solar declination. The size of the arm holding the disk may reduce the amount of diffuse irradiance
detected by the sensor. To reduce this effect, the German design has reduced the overall dimensions
of the shade-arm by adding guy-wires to the design. Consideration must be given to the location of
the instrumentation when determining how robust the design and dimensions of the shade-arm need
be. Locations that experience high winds, driving precipitation or significant snowfalls will require a
more robust design than less hostile environments.
While inexpensive to fabricate, these devices require more daily maintenance than shade systems
that operate on two-axes trackers. Problems associated with synchronous motor shade arms are similar
to those of solar trackers used for the measurement of direct beam irradiance (Section 4.4).
The general installation of devices sim ilar to those of the Germ an and Swiss that use a single-axis
synchronous motor system for shading are equatorial mount are similar.
(1) The pyranometer be maintained in a stable, horizontal position.
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