Chapter 3________________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 21
An extra photodiode measures the light scattered backwards from the
lens, other objects or contaminants. This signal is also monitored by
the CPU.
Light Receiver
The Light Receiver consists of a PIN photodiode, preamplifier,
voltage to frequency converter, backscatter measurement light source
LED, and some control and timing electronics.
The receiving PIN photodiode senses the transmitted light pulses
scattered from the aerosol particles. The signal voltage is filtered and
detected by a phase sensitive lock-in amplifier synchronized with the
transmitter.
An ambient light level as high as 30 kcd/m2 does not influence the
detection of the photo diode, neither does it saturate the preamplifier.
The AMBL signal (proportional to the ambient light) is lead to the
CPU for monitoring.
Additional Measurements
PWD22 includes RAINCAP
®
for estimating the water content of
precipitation, and the TS Temperature Sensor for measuring the sensor
crossarm temperature (TS). Both additional sensors are measured and
interfaced by the PWC22 board. The RAINCAP
®
temperature TDRD
is measured for the RAINCAP
®
heating (drying) control, and the box
(PWC22) temperature TB is measured for the box heating monitoring.
Vaisala RAINCAP
®
Rain Sensor
The RAINCAP
®
signal is proportional to the water amount on the
sensing surfaces. Water on RAINCAP
®
changes the capacitance of the
sensor elements. The capacitance of the elements controls the output
frequency of an oscillator. The maximum amplitude is normalized to
1.0 using the DRY and WET commands as a setup.
The frequency signal is measured once a second. The frequency is
handled in the PWD22 internal units. The DRY value is about 800,
which is also shown in the Status (STA) message.
