Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 50)

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Figure 4.6. Canadian com puter-controlled, friction-drive tracker used
for m easuring direct beam , diffuse and infrared radiation using a
shaded pyrgeom eter. The pyrheliom eter m ounting block is capable
of holding three instrum ents, including an active cavity radiom eter. A
second m ounting place is m ounted on the opposite side of the
tracker.
4.2.4 Installation of downfacing sensors (pyranom eters and pyrgeom eters)
Downfacing sensors should only be installed when the sensor can be located a m inim um of 30 m above
the surface to increase the representativeness of the field of view. The tower from which the instrum ent
is to be deployed should be as com pact as possible while recognizing the need for individuals to clim b
the tower to service the instrum ent. Open towers provide less interference of the radiation flux than
solid towers of the sam e dim ension. The further the instrum ents are m ounted away from the tower
on boom s, the less the tower influences the radiation field. In the worse case scenario of a solid tower
of diam eter D with a boom of length L m easured from the centre of the tower, the fraction of radiation
intercepted is D/2BL.
In all cases the sensors should be installed with the tower poleward of the instrum ents. This will elim inate
self-shading except in high latitude locations where the solar disk does not drop below the horizon
during the hem isphere sum m er and in equatorial locations that are affected by the passage of the
sun between the Tropics.
As in the case of the up-facing sensors, the sensors m ust be horizontal. To accom plish this, however,
is m ore difficult because the bubble levels attached to the instrum ents no longer function properly.
Two m ethods are suggested to help overcom e the levelling problem :
(1) By assum ing that the rotation of the instrum ent about its horizontal axes is true,the instrum ent
can be levelled in the up-facing position with its own bubble level and then rotated 180°.