Chapter 5 ___________________________________________Theory of Operation
VAISALA__________________________________________________________31
Diagnostic and Supportive Circuits
For reliable operation, the device includes diagnostic and
supportive circuits. These include control circuits for the
external hood and dew heaters, backscatter measurement,
sensor sensitivity, and window contamination.
Optical Contamination and
Blocking Monitoring
Optical contamination and blocking of the sensor are
continuously monitored. The LM21 sensor monitors optical
contamination and blocking by measuring window
contamination and backscattered signals. The FS11
CALIBRATE WINDOW_CLEAN BL_SENSOR
command (or CALIBRATE WINDOW_CLEAN when
given directly to LM21 via a maintenance terminal) is used
to set clean reference values for the contamination and
blocking signals. If the alarm limit is exceeded in the
backscatter measurement, the visibility data is set as missing
(/////) and an alarm is generated.
The backscatter is measured with a dedicated receiver diode
and circuit using an IR LED as a light source. The signal is
higher when more light is scattered back. A
BACKSCATTER HIGH alarm is generated when the
backscatter signal has increased considerably from the
reference signal level.
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