32
Figure 4.1. Ventilator with motor located beside the instrument as used by Deutscher
Wetterdienst.
Spring loaded bolting devices for mounting the instrument are also an excellent means of
guaranteeing the instrument will remain fixed while providing the added ability of levelling
the instrument without requiring the bolts being loosened.
(3) The instrument should be levelled using the supplied three levelling feet. By first adjusting
the foot closest to the bubble, the instrument should be adjusted until the bubble is centred
within the inner circle of the supplied bubble. When completely centred, and radiometrically
levelled, the bubble level indicates that the thermopile is horizontal to within ±0.1° causing
an azimuthal variation of ±1% at a solar elevation of 10°.
(4) Carefully tighten the retaining screws so that the instrument is immovable. To do so, gently
tighten the bolts alternately until secure. Be careful not to over-tighten.
(5) Place and adjust the radiation shield or ventilated housing cover so that it is parallel to, and
level with or below the thermopile surface.
4.2.2.1 Ventilated housing
The recommended procedures for the measurement of global radiation require the use of a ventilated
housing to improve the overall stability of pyranometer measurement by damping changes in the
pyranom eter body tem perature due to solar loading and potentially reducing the thermal offset. In
some climates, the use of a ventilator also improves the amount of recoverable data by eliminating
dew and reducing the number of occurrences of frost and snow on the instrument domes. Measurements
in other regions, however, have not shown a significant increase in accuracy or percent data recovered
with the use of ventilated housings. As each ventilator adds extra cost and complexity to the installation
and maintenance of a station a thorough analysis of its requirement should be made before installation.
Locations where a ventilated housing are recommended are:
(1) where dew, frost or snow is prevalent,
(2) where natural ventilation is infrequent or variable,
(3) where there is significant radiative cooling during portions of the year, a ventilated housing
may reduce thermal-offset,
(4) where the humidity is high during portions of the year a ventilator will reduce the possibility
of water damage and reduce the frequency of desiccant changes.
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