Metek uSonic-2 - Principle of Measurement; Physical Basics; Measurement of Wind Velocity

METEK GmbH uSonic-2
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1 Principle of measurement
The uSonic-2 identifies the orthogonal wind components x, y and the
temperature from the transit times of ultrasonic pulses. Alternatively, instead
of the wind components, the wind velocity and the wind direction are
calculated and displayed. The temperature is determined directly from the
speed of sound, so this “acoustic temperature” is not equal to the “dry
temperature” measured with a usual thermometer or PT100 probe. The
temperature derived from the sound speed depends i.e. on the humidity and
is in good approximation equivalent to the “virtual temperature”.
Each ultrasonic transducer has its individual delay time to convert an
electrical signal into an ultrasonic pulse and reverse. This individual delay
time is compensated by a system calibration. In case of strong mechanical
impact this calibration has to be updated which can be done by the customer
using the optional calibration kit or other adequate helping tools.
The uSonic-2 is constructed with no moving parts so the uSonic-2 is not
subject to mechanical aging. Therefore the uSonic-2 is virtual free of
maintenance. In addition the measured values of the paths are monitored
permanently in view of their quality. In case of deviation out of the acceptance
range the system will stop the calculation of data to inform the user that a new
calibration is required.
Optionally, the uSonic-2 is available with a sensor head heating which allows
operation even under icing conditions.
In this document the name “ultrasonic anemometer” is replaced by its short
form “sonic”.
1.1 Physical basics
The 4 ultrasonic transducers are arranged by the geometry of the sensor
head, that two of them face each other with a distance of approximately 180
mm. Each transducer works as transmitter and receiver alternately, so in total
the 4 transducers create 2 measuring paths, called P1 and P2.
1.2 Measurement of wind velocity
The speed of sound depends on the density of air (scalar variable) and the
wind velocity parallel to the propagating sound pulse (vector variable). By
taking the difference of the travel times of the sonic pulses the scalar
variables eliminate each other and the residual radial wind component can be
determined along this path.