Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 23
Additional Measurements
PWD22/52 includes RAINCAP for estimating the water content of
precipitation, and a temperature sensor in the sensor cross arm or in an
external radiation shield (TS). Both additional sensors are measured and
interfaced by the PWC22/52 board. The RAINCAP temperature TDRD
is measured for the RAINCAP heating (drying) control, and the box
(PWC22/52) temperature TB is measured for the monitoring box heating.
Vaisala RAINCAP
®
Rain Sensor
The RAINCAP signal is proportional to the water amount on the sensing
surfaces. Water on RAINCAP
®
changes the capacitance of the sensor
elements. The capacitance of the elements controls the output frequency
of an oscillator.
The frequency signal is measured once a second. The frequency is
handled in the PWD22/52 internal units. The DRY value is about 800,
which is also shown in the Status (STA) message.
0008-012
Figure 4 RAINCAP Block Diagram
The heating element built into the surface heats the RAINCAP. The
heating power is controlled by software.
PWD22/52 is equipped with a double-plate RAINCAP to improve drizzle
detection in certain conditions.
When the sensor element heating is switched off, they become highly
sensitive to ambient moisture. If the relative humidity of the air is more
than about 70 %, the surfaces produce a measurable signal even with no
precipitation. In PWD22/52 this is used as an estimated humidity
measurement. The estimated humidity is used to differentiate between
dust and mist.
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