Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 44)

32
Figure 4.1. Ventilator with m otor located beside the instrum ent as used by Deutscher
W etterdienst.
Spring loaded bolting devices for m ounting the instrum ent are also an excellent m eans of
guaranteeing the instrum ent will rem ain fixed while providing the added ability of levelling
the instrum ent without requiring the bolts being loosened.
(3) The instrum ent should be levelled using the supplied three levelling feet. By first adjusting
the foot closest to the bubble, the instrum ent should be adjusted until the bubble is centred
within the inner circle of the supplied bubble. W hen com pletely centred, and radiom etrically
levelled, the bubble level indicates that the therm opile is horizontal to within ±0.1° causing
an azim uthal variation of ±1% at a solar elevation of 10°.
(4) Carefully tighten the retaining screws so that the instrum ent is im m ovable. 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 therm opile surface.
4.2.2.1 Ventilated housing
The recom m ended procedures for the m easurem ent of global radiation require the use of a ventilated
housing to im prove the overall stability of pyranom eter m easurem ent by dam ping changes in the
pyranom eter body tem perature due to solar loading and potentially reducing the therm al offset. In
som e clim ates, the use of a ventilator also im proves the am ount of recoverable data by elim inating
dew and reducing the num ber of occurrences of frost and snow on the instrum ent dom es. Measurem ents
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 com plexity to the installation
and m aintenance of a station a thorough analysis of its requirem ent should be m ade before installation.
Locations where a ventilated housing are recom m ended 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
m ay reduce therm al-offset,
(4) where the hum idity is high during portions of the year a ventilator will reduce the possibility
of water dam age and reduce the frequency of desiccant changes.