Vaisala WXT536, Руководство пользователя

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User’s Guide ______________________________________________________________________
30 __________________________________________________________________ M211840EN-C
The unit calculates wind speed from the measured transit times using
the following formula:
0505-216
Measuring the six transit times allows V
w
to be computed for each of
the three ultrasonic paths. The computed wind speeds are independent
of altitude, temperature, and humidity, which are cancelled out when
the transit times are measured in both directions, although the individual
transit times depend on these parameters.
The V
w
values of two array paths are enough to compute wind speed
and wind direction. A signal processing technique ensures that wind
speed and wind direction are calculated from the two array paths with
the best quality.
The wind speed is represented as a scalar speed in selected units (m/s,
kt, mph, km/h). The wind direction from which the wind comes is
expressed in degrees (°). North is represented as 0°, east as 90°, south
as 180°, and west as 270°.
The wind direction is not calculated when the wind speed drops below
0.05 m/s. In this case, the last calculated direction output remains until
the wind speed increases to the level of 0.05 m/s.
The average values of wind speed and direction are calculated as a
scalar average of all samples over the selected averaging time
(1 ... 3600 s) with a selectable updating interval. The sample count
depends on the selected sampling rate: 4 Hz (default), 2 Hz, or 1 Hz.
The minimum and maximum values of wind speed and direction
represent the corresponding extremes during the selected averaging
time. See
Appendix D Wind Measurement Averaging Method, on page
201 .
where
V
w
= Wind speed
L = Distance between the two transducers
t
f
= Transit time in forward direction
t
r
= Transit time in reverse direction
V
w
0.5 L 1 t
f
1 t
r
e
–
e u u
=