38
Figure 4.6. Canadian computer-controlled, friction-drive tracker used
for measuring direct beam, diffuse and infrared radiation using a
shaded pyrgeometer. The pyrheliometer mounting block is capable
of holding three instruments, including an active cavity radiometer. A
second mounting place is mounted on the opposite side of the
tracker.
4.2.4 Installation of downfacing sensors (pyranometers and pyrgeometers)
Downfacing sensors should only be installed when the sensor can be located a minimum of 30 m above
the surface to increase the representativeness of the field of view. The tower from which the instrument
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 instrument. Open towers provide less interference of the radiation flux than
solid towers of the same dimension. The further the instruments are mounted away from the tower
on booms, the less the tower influences the radiation field. In the worse case scenario of a solid tower
of diameter D with a boom of length L measured 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 instruments. This will eliminate
self-shading except in high latitude locations where the solar disk does not drop below the horizon
during the hemisphere summer 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 must be horizontal. To accomplish this, however,
is more difficult because the bubble levels attached to the instruments no longer function properly.
Two methods are suggested to help overcome the levelling problem:
(1) By assuming that the rotation of the instrument about its horizontal axes is true,the instrument
can be levelled in the up-facing position with its own bubble level and then rotated 180°.
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