Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 45)

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Figure 4.2. Ventilator with the m otor located
beneath the instrum ent. Note the extra
ventilation holes near the top of the housing
used to reduce snow accum ulation (Davos,
Switzerland).
The two recom m ended styles of ventilated housing are:
(1) W here the ventilator fan is situated beside the instrum ent and the pyranom eter is com pletely
enclosed so that the air flows evenly around the dom e. Figure 4.1 illustrates this type of blower
as used by the Deutscher W etterdienst. The advantage of this design is the ease in which
a fan can be replaced without tam pering with the pyranom eter. Conversely, the design is
m ore com plex because the air is entering from one side of the pyranom eter and m ust be
funnelled around the instrum ent to pass over the dom e equally from all directions. This m ay
require the use of a larger fan than those ventilators that pass air around the instrum ent from
beneath. The tem perature of the instrum ent in the encapsulated ventilator and, to a lesser
extent the instrum ent dom e, will rise slightly above the am bient tem perature due to the heating
of the air by the blower m otor.
(2) W here the housing encloses the pyranom eter and the ventilator fan is located beneath the
instrum ent and blows air from beneath the housing, around the instrum ent and over the dom e
(Figure 4.2). This is the m ore com m on of the two recom m ended ventilation system s. The
power dissipation heats the incom ing air by approxim ately 1°C, which in turn heats the body
and dom e of the enclosed instrum ent. Unlike (1), the instrum ent m ust be rem oved from the
housing before a fan can be replaced. The area beneath the instrum ent m ust also be kept
free from obstructions to m aintain airflow.
The heating effect on the dom e due to the fan m otors is negated when wind speeds are m oderate
to high.
Heating resistors can be added to both ventilators if required during cold weather ope rations. Care
m ust be taken, however, in that these m ay also alter the overall response of the instrum ent.