Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 147
Calculations
The FS11P software calculates the extinction coefficient separately for
droplets and other scattering media, and these are summed using weights.
MOR is calculated from the summed extinction coefficient with the
following known formula of the 5% contrast ratio:
MOR = - ln 0.05 / σ = 3 / σ
Where σ is the extinction coefficient.
Detecting Precipitation
Detecting the precipitation is based on measuring the optical signal peaks
caused by precipitation droplets. The peak amplitudes that are detected
during 10 minutes are summed. When the sum exceeds a threshold value,
the sensor will indicate precipitation. The threshold value is a
Precipitation limit parameter, and it can be changed by the operator.
In addition to the optical detection, the RAINCAP
®
signal is used in
lowest precipitation intensities. RAINCAP
®
is also used for some cross-
checking of optical detection.
Precipitation cessation is detected when the summed optical signal
amplitudes decrease below another, lower threshold value. The summing
interval depends on the detected precipitation intensity. PWD32 detects
precipitation cessation within three minutes.
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