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Chapter 3 ______________________________________________________ Functional Description
Optical Measurements
VAISALA _______________________________________________________________________ 21
Optical Arrangement
Transmitter
Figure 3
PWD22/52 Optical System
PWD22/52 measures light scattered at an angle of 45°. This angle
produces a stable response in various types of natural fog. Precipitation
droplets scatter light in a different manner as fog and their contribution to
visibility must be analyzed separately. PWD22/52 can detect and
measure precipitation droplets from the optical signal and use this
information in processing the scatter measurement results.
PWD22/52 has a small sample volume of about 0.1 liters (see Figure 3
above). This allows for independent particles to be measured even at
relatively heavy precipitation intensities. The signal levels from even the
smallest precipitation droplets can also be detected.
Transmitter Unit PWT11
The transmitter unit consists of an infrared LED, control and triggering
circuits, LED intensity monitor, and backscatter receiver.
The transmitter unit electronics pulses the IR-LED at a frequency of
2 kHz. A PIN photodiode monitors the transmitted light intensity. The
transmitter intensity level measurement is used to keep automatically the
LED's intensity at a preset value. The "LEDI" feedback voltage is
monitored by the CPU to get information of the aging of the LED and
possible defects.
The feedback loop compensates for temperature and aging effects of the
light emitting diode. On the other hand, the active compensation slightly
accelerates the LED aging. Thus, the initial LED current is set to a value,
which ensures several years of operation without maintenance.
3 cm
Sample volume
Receiver
4 cm
0403-103
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Troubleshooting
