Decagon Devices DS-2 Benutzerhandbuch - Seite 8
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This is the basic equation for the anemometer. ∆n is proportional to
the phase difference between the forward and reverse sound pulses.
The sound comes from a 40 kHz ultrasonic transducer in the head of
the anemometer. It is transmitted diagonally across the anemometer,
bouncing off of a stainless steel disk in the center, and is received by
another transducer in the anemometer head that is opposite the first.
Once the sound pulse is received, the receiver becomes the transmit-
ter and the process is repeated. Two more sensors, mounted at 90
degrees from the first two give the other horizontal component of
the wind. The sound travels a total distance of about 72 mm from
transmitter to receiver, but d in the equations is just the horizontal
distance, which is 40 mm.
If we take u as the magnitude of the wind vector in the east-west di-
rection (east +) and v as the magnitude in the north-south direction
(north +), then compute wind speed with equation 6.
Where the overbar indicates an average of the values sampled every
ten seconds. Compute wind direction with equation 7.
The wind measurement requires 42 ms to complete. An additional
60 ms are required for the computations to determine phase differ-
ences. The anemometer samples every 10 seconds (or more often
if requested). The gust speed reported is the highest instantaneous
wind speed measured during the selected averaging interval. Cup
anemometers average over a much longer interval than 42 ms, so the
gusts measured with a sonic anemometer will have a larger peak to
mean ratio than one would see with a cup anemometer.
2
2
S =
u
+ v
1
θ = tan
(v/u)
5
2 THEORY
(6)
(7)