Wind |
Cup anemometer |
Measurement of Wind
direction and Wind velocity
1. Range of Application
The Ultrasonic Anemometer 2D is
designed to detect the horizontal components of wind velocity and wind
direction as well as the virtual temperature in two dimensions. Due to
its very short measurement intervals, the instrument is ideal for the inertia
free measurement of gusts and peak values.
The accuracy of the air temperature measurement
(virtual temperature) surpasses that one of the classic method where the
temperature transmitter is used in a weather and thermal radiation shield.
The measured data are available as analogue
signals or as a data telegram over a serial interface. The anemometer is
equipped with an automatic heating for the instrument body as well as for
the sensors so that the measuring results, in case of critical ambient
temperatures, are not affected by ice, snow or rainfall.
2. Mode of Operation
The Ultrasonic Anemometer 2D consists
of 4 ultrasonic transformers, in pairs of 2 which are opposite each other
at a distance of 200 mm.
The two measurement paths thus formed
are vertical to each other.
The transformers act both as acoustic
transmitters and acoustic receivers.
The respective measurement paths and their
measurement direction are selected via the electronic control. When a measurement
starts, a sequence of 8 individual measurements in all 4 directions of
the measurement paths is carried out at maximum speed.
The measurement directions (acoustic propagation
directions) rotate clockwise, first from south to north, then from west
to east, from north to south and finally from east to west.
The mean values are formed from the 8
individual measurements of the path directions and used for further calculations.
A measurement sequence takes approx. 20
msec at +20°C.
3. Measurement Principle
3.1 Wind velocity and direction
The speed of propagation of the sound
in calm air is superposed by the velocity components of an air flow in
wind direction.
A wind velocity component in the direction
of the propagation of the sound supports the speed of propagation, thus
leading to an increase in the speed. A wind velocity component opposite
to the direction of propagation, on the contrary, leads to a reduction
of the speed of propagation.
The speed of propagation resulting from
the superposition leads to different propagation times of the sound at
different wind velocities and directions over a fixed measurement path.
As the speed of sound is very dependent
on the air temperature, the propagation time of the sound is measured on
both of the measurement paths in both directions. In this way, the
influence of the temperature dependent speed of sound on the measurement
result can be eliminated by subtracting the reciprocals of the measured
propagation times.
By combining the two measuring paths which
are at right angles to each other, one obtains the measurement results
of the sum and the angle of the wind velocity vector in the form of rectangular
components.
After the rectangular velocity components
have been measured over the measurement path, they are then transformed
by the µ-processor of the anemometer into polar coordinates and output
as sum and angle of wind velocity.
As previously mentioned, the speed of the
propagation of sound is highly dependent on the air temperature, but is
hardly affected by air pressure and humidity. Thus these physical properties
of gases can be used to measure air temperature.
As this is a measurement of gas temperature
which is made without thermal coupling to a measurement sensor, it is called
the "virtual temperature".
The advantages of this measured variable
is, on the one hand, its inertia free reaction to the actual gas temperature,
and, on the other, the avoidance of measurement errors such as those which
occur when a solid state temperature sensor is heated up by radiation.
Measuring sensors in a weather and thermal
radiation shield show values which are, on the one hand, too high, due
to sun irradiation, and on the other hand too low, due to evaporation cooling
with rain and wind.
The measuring errors of those thermometers
in practice can be up to ± 2 °K.
The 2D-Anemometer achieves a measuring
accuracy of ± 1 °K over the entire temperature range from 40
°C to + 70°C, thus offering a very precise determination of the
air temperature without the disadvantages caused by the use in a weather
and thermal radiation shield.
Technical Data:
Wind
velocity
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Wind direction
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Data output digital
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Data output analogue
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General
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Virtual Temperature
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Info
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