Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 143
The RAINCAP
®
sensing surface is heated by the heating element built
into the surface. The heating power varies by software control.
PWD32 is equipped with a double-plate RAINCAP
®
to improve drizzle
detection in certain conditions.
When heating of the sensor elements is switched off, they become very
sensitive to ambient moisture. If the relative humidity of the air is more
than approximately 70%, the surfaces produce a measurable signal even
with no precipitation. In PWD32 this is used as a Relative Humidity
estimate. The estimated humidity is used to separate between dust and
mist.
As PWD32 detects rain both in the optical signal and the RAINCAP
signal, these can be cross-checked against each other at above zero
temperatures. This way false detections of, for example, mosquitoes as
rain can be eliminated.
Interface Unit FSI102
The interface unit consists of the AC (mains) power supply (
trans switch), the communication controller board (FSC202),
and the optional battery backup (FSB101).
The power required for the measurement unit and the power for the
heating of the optics heads' hoods is supplied by the interface unit. There
is also an interface connection for optional Background Luminance
Sensor LM21 and optional Obstruction Light Sensor FS11OBS.
The external power input and the communication interfaces of the FS11P
sensor are located in the interface unit. All external interfaces are
equipped with surge protection circuitry.
Communication Controller Board FSC202
Communication Controller Board FSC202 contains all external
communication interfaces. The FSC202 board polls the Measurement
unit FSM102, Precipitation Identifier PWD32 and Background
Luminance Sensor LM21 through the internal RS-485 sensor bus and
combines and forms the FS11P data message from their response
messages. The available data line options are the serial lines RS-232 and
(external) RS-485. FSC202 also includes a separate RS-232 maintenance
line.
Summary of Contents for FS11P
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