Vaisala WXT520 - Manual (Page 29)

Chapter 3 ______________________________________________________ Functional Description
VAISALA_______________________________________________________________________ 27
Depending on user selection the wind speed extreme values can be
computed in two alternative ways; either with the traditional minimum/
maximum calculation or with the 3-second gust & lull calculation
recommended by the WMO (World Meteorological Organization). In
the latter case the highest and lowest 3-second average values (updated
once a second) replace the maximum and minimum values in reporting
of wind speed, while the wind direction variance is returned in the
traditional way.
The WXT520 constantly monitors the wind measurement signal
quality. If poor quality is detected, the wind values are marked as
invalid. If over half of the measurement values can be considered as
invalid, the last valid wind values are returned as missing data.
However, in the SDI-12 protocol the invalid values will be marked as
zeroes.
Precipitation Measurement Principle
WXT520 uses Vaisala RAINCAP
®
Sensor 2-technology in
precipitation measurement.
The precipitation sensor comprises of a steel cover and a piezoelectrical
sensor mounted on the bottom surface of the cover.
The precipitation sensor detects the impact of individual raindrops. The
signals from the impact are proportional to the volume of the drops.
Hence, the signal of each drop can be converted directly to accumulated
rainfall. Advanced noise filtering technique is used to filter out signals
originating from other sources than raindrops.
The measured parameters are accumulated rainfall, rain current and
peak intensity, and the duration of a rain event. Detection of each
individual drop enables computing of rain amount and intensity with
high resolution. Precipitation current intensity internally updated every
10 seconds represents the intensity during the one minute period before
requesting/automatic precipitation message sending (for fast reacting to
a rain event, during the first minute of the rain event the intensity is
calculated over the period rain has lasted in 10-second steps instead of
fixed one minute). Precipitation peak intensity represents the maximum
of the calculated current intensity values since last precipitation
intensity reset.