Vaisala PWD22/52 - Light Receiver; Additional Measurements; Vaisala RAINCAP Rain Sensor

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USER'S GUIDE____________________________________________________________________

22 __________________________________________________________________ M210543EN-E

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/m

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

PWD22/52 includes RAINCAP

for estimating the water content of

precipitation, and the TS Temperature Sensor for measuring the sensor

cross arm temperature (TS). Both additional sensors are measured and

interfaced by the PWC22/52 board. The RAINCAP

temperature TDRD

is measured for the RAINCAP

heating (drying) control, and the box

(PWC22/52) 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 frequency signal is measured once a second. The frequency is

handled in the PWD22/52 internal units. The DRY value is about 800,

which is also shown in the Status (STA) message.