33
Figure 4.2. Ventilator with the motor located
beneath the instrument. Note the extra
ventilation holes near the top of the housing
used to reduce snow accumulation (Davos,
Switzerland).
The two recommended styles of ventilated housing are:
(1) Where the ventilator fan is situated beside the instrument and the pyranometer is completely
enclosed so that the air flows evenly around the dome. Figure 4.1 illustrates this type of blower
as used by the Deutscher Wetterdienst. The advantage of this design is the ease in which
a fan can be replaced without tampering with the pyranom eter. Conversely, the design is
more complex because the air is entering from one side of the pyranometer and must be
funnelled around the instrument to pass over the dome equally from all directions. This may
require the use of a larger fan than those ventilators that pass air around the instrument from
beneath. The temperature of the instrument in the encapsulated ventilator and, to a lesser
extent the instrument dome, will rise slightly above the ambient temperature due to the heating
of the air by the blower motor.
(2) Where the housing encloses the pyranometer and the ventilator fan is located beneath the
instrument and blows air from beneath the housing, around the instrument and over the dome
(Figure 4.2). This is the more common of the two recommended ventilation systems. The
power dissipation heats the incoming air by approximately 1°C, which in turn heats the body
and dome of the enclosed instrument. Unlike (1), the instrument must be removed from the
housing before a fan can be replaced. The area beneath the instrument must also be kept
free from obstructions to maintain airflow.
The heating effect on the dome due to the fan motors is negated when wind speeds are moderate
to high.
Heating resistors can be added to both ventilators if required during cold weather operations. Care
must be taken, however, in that these may also alter the overall response of the instrument.
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