Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 46)

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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 W etterdienst).
4.2.3 Mechanical installation of shaded sensors (pyranom eters and pyrgeom eters)
The general installation of shaded sensors follows the guidelines set out in 4.2.2, but includes the added
com plexity of aligning the shading device with the instrum ent. Within the BSRN, shade rings (diffusographs)
are not accepted as a m eans of shading an instrum ent because of the field of view the ring subtends.
Two types of devices are com m only em ployed within the BSRN.
Figures 4.3 and 4.4 illustrate synchronous m otor shade devices as designed by the Germ ans and
the Swiss. The Germ an device uses a single axis to carry the pyranom eter about which the shade-disk
rotates. This design is significantly m ore com plex than the Swiss design, but is m ore efficient in using
space. The Swiss design fixes the pyranom eter on a stable, level stand m ounted separately from the
equatorial m otor. The position of the shade disk m ust be m oved along the shading arm with the changing
solar declination. The size of the arm holding the disk m ay reduce the am ount of diffuse irradiance
detected by the sensor. To reduce this effect, the Germ an design has reduced the overall dim ensions
of the shade-arm by adding guy-wires to the design. Consideration m ust be given to the location of
the instrum entation when determ ining how robust the design and dim ensions of the shade-arm need
be. Locations that experience high winds, driving precipitation or significant snowfalls will require a
m ore robust design than less hostile environm ents.
W hile inexpensive to fabricate, these devices require m ore daily m aintenance than shade system s
that operate on two-axes trackers. Problem s associated with synchronous m otor shade arm s are sim ilar
to those of solar trackers used for the m easurem ent 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 m otor system for shading are equatorial m ount are sim ilar.
(1) The pyranom eter be m aintained in a stable, horizontal position.