Vaisala FD12P - Manual (Page 100)

User's Guide _______________________________________________________________________
100 _____________________________________________________ M210296en-A
9807-031
Figure 24 DRI21 Block Diagram in the FD12P Application
DRD12 Rain Detector
The DRD12 analog signal is proportional to the water amount on the
sensing surfaces. Water on the DRD12 changes the capacitance of the
sensor elements. The capacitance of the elements controls the output
frequency of an oscillator. This frequency is amplified and also
converted into a voltage signal for direct analog measurement. With
dry surfaces, the DRD12 outputs about 3 V and with totally wet
surfaces 1 V. Refer to Figure 29 on page 104, section DRD12 Signal
Processing on page 104.
A droplet detector monitors the voltage signal. When a new droplet
hits the DRD12 sensing surface, the voltage changes rapidly and the
detector circuit reacts. The detector triggers a delay circuit, which
controls the precipitation ON/OFF output. When new droplets are
detected often enough, the delay circuit output will stay constantly on.
The voltage signal is measured once a second by an analog channel of
the DRI21 interface board. In addition, the precipitation (ONN/OFF
signal) is read with a digital input.