GHM Delta OHM HD52.3D Series, Operating Manual

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HD52.3D…
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V2.2
4 MEASURING PRINCIPLE OF WIND SPEED AND DIRECTION
Wind speed and direction are determined by measuring the time taken by ultrasonic pulses to
cover the distance from the transducer that generates the pulse to the receiving transducer.
The instrument uses 2 pairs of transducers oriented along two orthogonal axes. Detecting the
wind speed along two axes allows determining not only the intensity but also the wind direc-
tion.
The instrument measures the travel time of the ultrasonic pulse between the two transducers
of the same pair in both directions. The travel times in the two opposed directions are defined
as t
A
(forward direction time) and t
R
(reverse direction time).
If wind speed is zero, t
A
and t
R
values are the same. In the presence of wind, one of the two
time values is greater than the other and the comparison between the two time values allows
to determine the direction and the intensity of the wind.
Measuring the travel time in both directions allows to cancel the dependence of the transmis-
sion speed of ultrasounds in the air from the environmental conditions of temperature, humidi-
ty and barometric pressure.
Measurement along first axis Measurement along second axis
The travel times of the ultrasonic pulses are given by:
W
A
V C
D
t


W
R
V C
D
t


Where:
D = Distance between the two transducers of the same pair
C = Sound speed
V
W
= Component of wind speed along the measurement axis
Measuring the two travel times allows determining the wind speed component:






  
R A
W
t t
D
V
1 1
2
Axis U
Transducer
Transducer
Transducer
Transducer
Axis V