Vaisala WS425FG - Manual (Page 62)

User's Guide ______________________________________________________________________
60 ______________________________________________________________________________
0505-216
Measuring the six transmit times allows Vw to be computed for each of
the three ultrasonic paths, which are offset to each other by 120°. The
computed wind speeds are independent of altitude, temperature, and
humidity even though the velocity of sound affects individual transit
times because the algebra that produces the formula above cancels the
effect of the static speed of sound.
Bad readings, which can happen when a large raindrop or ice pellet hits
a transducer, are eliminated by a proprietary signal processing
technique. The one Vw that is most affected by turbulence error is
eliminated so wind speed and wind direction are calculated from the
best two vectors.
0506-018 Figure 15 Ultrasonic Wind Sensor Block Diagram
where
Vw =
Wind velocity
L = Distance between two transducers
tf = Transit time in the positive direction
tr = Transit time in the negative direction
V
w
0.5
L
1
t
f
1
t
r
e – e u u =