Vaisala WXT520 - Manual (Page 28)

User’s Guide ______________________________________________________________________
26 __________________________________________________________________ M210906EN-C
The wind speed is calculated 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.
Using V
w
values of two array paths is enough to compute wind speed
and wind direction. A signal processing technique is used so that wind
speed and wind direction are calculated from the two array paths of best
quality.
The wind speed is represented as a scalar speed in selected units (m/s,
kt, mph, km/h). The wind direction is expressed in degrees (°). The
wind direction reported by WXT520 indicates the direction that the
wind comes from. 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 again 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 also
Appendix D, Wind Measurement Averaging Method, on page 163 .
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
=