Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 163
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 transmitter backscatter is measured with an analog circuit using the
transmitter LED as a light source. The transmitter backscatter value is
higher with higher backscatter signals.
The receiver backscatter is measured with the main receiver circuitry
using an additional LED as a transmitter. The signal is higher when more
light is scattered back.
A BACKSCATTER HIGH alarm is generated when the backscatter
signal of either the transmitter or receiver has increased considerably
from the reference signal level.
The window contamination measurement circuitry uses a separate LED
and a detector located on the opposite side of the window edges to
measure window contamination. For the measurement principle, see
Figure 40 on page 137. The change of the contamination signal from the
clean value is proportional to the contamination on the window. The
window contamination value of the status message indicates the
reduction in transmittance of the window. For example, a contamination
value of 10% means that the window transmittance has decreased 10%
and it is now only 90% of the original value.
The total transmission of the optical path is calculated from the
transmitter and receiver window transmittances. A window
contamination warning WINDOW CONTAMINATED is given when the
transmission has decreased to 85% but still allows reliable scattering
light measurements.
Contamination Compensation
The total transmission value is also used to calculate the required
compensation for the visibility values. The contamination compensation
is only performed with moderate contamination when reliable visibility
measurements can still be made. If the calculated total transmission value
has decreased but still allows reliable scattering light measurements, a
window contamination warning is generated to indicate a proactive
maintenance need. Notice that the visibility values are still reliable and
delivered normally.
If the contamination compensation is disabled with the command SET
CONTAMINATION_COMPENSATION VISIBILITY_SENSOR
OFF, no contamination compensation is made. However, in this case the
window contamination warning is still generated if the total transmission
of the windows has decreased.
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