Campbell IRGASON, Руководство по эксплуатации продукта

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11. Theory of operation
11.1 IRGASON sonic anemometer
The CSAT3 measures wind speeds by determining the time of flight of sound between pairs of
transducers. However, unlike many other commercial anemometers, it does not use simple
threshold detection to determine the ultrasonic times of flight. Instead, it uses advanced digital
signal processing techniques to determine the arrival of the transmitted ultrasonic signal. In
comparison to other systems, the digital signal processing techniques result in more accurate,
lower noise measurements.
11.1.1 Wind speed
Each axis of the sonic anemometer pulses two ultrasonic signals in opposite directions. The time
of flight of the first signal (out) is given by:
Eq. 1
and the time of flight of the second signal (back) is given by:
Eq. 2
where:
t
o
= time of flight out along the transducer axis,
t
b
= time of flight back, in the opposite direction,
u
a
= wind speed along the transducer axis,
d = distance between the transducers, and
c = speed of sound.
The wind speed, u
a
, along any axis can be found by inverting the above relationships, subtracting
Eq. 2 (p. 54) from Eq. 1 (p. 54), and solving for u
a
.
Eq. 3
The wind speed is measured on all three non-orthogonal axes to give u
a
, u
b
, and u
c
, where the
subscripts a, b, and c refer to the non-orthogonal sonic axes.
IRGASON® Integrated CO
2
/H
2
O Open-Path Gas Analyzer and 3D Sonic Anemometer 54