Vaisala FD12P - Manual (Page 96)

User's Guide _______________________________________________________________________
96 ______________________________________________________ M210296en-A
Optical Measurement
Optical Arrangement
0110-185
Figure 21 FD12P Optical System
The following numbers refer to Figure 21 above:
1 = FDT12 transmitter
2 = FDR12 receiver
3 = Sample volume
The FD12P measures light scattered at an angle of 33 °. This angle
produces stable response in various types of natural fog. Precipitation
droplets scatter light in a different manner than fog. Thus, their
contribution to visibility must be analyzed separately. The FD12P can
detect and measure precipitation droplets from the optical signal and
use this information in processing the scatter measurement results.
The FD12P has a small sample volume of about 0.1 liters (see Figure
21 above). This enables independent particles to be measured even at
quite heavy precipitation intensities. The signal levels from even the
smallest droplets can also be detected.
FDT12B Transmitter Unit
The transmitter unit consists of an infrared LED, control and
triggering circuits, LED intensity monitor, backscatter receiver, and
analog multiplexer.