Vaisala FD12P - Manual (Page 21)

Chapter 2 ___________________________________________________________Product Overview
VAISALA ________________________________________________________________________ 21
Operating Principle
The FD12P Weather Sensor is a microprocessor controlled, intelligent
sensor combining optical forward scatter measurement, capacitive
precipitation sensing, and temperature measurement. The main units
of the FD12P are shown in Figure 3 below.
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Figure 3 FD12P Block Diagram
The FD12P evaluates Meteorological Optical Range (MOR) by
measuring the intensity of infrared light scattered at an angle of 33
°.
The scatter measurement is converted to the visibility value (MOR)
after a careful analysis of the signal properties. Special processing is
used in case of precipitation.
The FD12P software detects precipitation droplets from rapid changes
in the scatter signal. The droplet data is used to estimate optical
precipitation intensity and amount. In addition to the optical signal,
the analog output of the DRD12 Rain Detector is used to estimate the
precipitation intensity and type.