Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 141
Optical Arrangement
Receiver
Transmitter
Sample volume
3 cm
4 cm
0403-103
Figure 43 PWD32 Optical System
PWD32 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. PWD32 can detect and measure
precipitation droplets from the optical signal and use this information in
processing the scatter measurement results.
PWD32 has a small sample volume of about 0.1 liters (see Figure 43
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 automatically keep 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.
An extra photodiode measures the light scattered backwards from the
lens, other objects or contaminants. This signal is also monitored by the
CPU.
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