SFIDK.PS.029.E5.02 / SW 2.01.07
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1.8.1 Principle of measurement
Velocity distribution along sound path
1.8.2 Measuring principle
A sound wave travelling in the same direction as the liquid flow arrives at point B from point A
in a shorter time than the sound wave travelling against the flow direction (from point B to A).
The difference in sound transit time indicates the flow velocity in the pipe.
Since delay time is measured at short intervals both in and against flow direction, viscosity and
temperature have no influence on measurement accuracy.
In SITRANS F US flowmeters the two ultrasonic transducers are placed at an angle θ in relation
to the pipe axis. The transducers function as both transmitters and receivers of the ultrasonic
signals. Measurement is performed by determining the time it takes the ultrasonic signal to travel
with and against the flow. The principle can be expressed as follows:
v = K ⋅ (t
B,A
– t
A,B
) / (t
B,A
⋅ t
A,B
) = K ⋅ ∆t/t²
v = Average flow velocity
t = Transit time
K = Proportional flow factor
This measuring principle offers the advantage that it is independent of variations in the actual
sound velocity of the liquid, ie independent of temperature. Proportional factor K is determined
by wet calibration ("Wet calibration factor") or “Auto” (SONOKIT only) in case of manual
programming of transducer angle, transducer distance, transducer displacement and pipe
dimensions.
1.8 Measurement
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