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4.7
Measuring principle (FS)
Physical principle
Figure 4-3
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 direction of flow (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,
temperature has no influence on measurement accuracy.
SITRANS FS Ultrasonic flowmeters
In SITRANS FS Ultrasonic flowmeters the ultrasonic transducers are placed at an angle θ in
relation to the pipe axis. The transducers function as transmitters and receivers of the ultrasonic
signals. Measurement is performed by determining the time the ultrasonic signal takes to travel
with and against the flow. The principle can be expressed as follows:
v = K × (t
where
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, that is independent of the temperature.
The proportional flow factor K is determined by wet calibration or calculated by "AUTO"
in case of manual programming of mechanical/geometrical pipe data (Retrofit only). The
transducer angle (θ), distance between sensors (L) and pipe dimension (D
in the figure below.
SITRANS FSS100
Installation Manual, 12/2023, A5E52595000-AA
Velocity distribution along sound path
– t
) / (t
× t
) = K × Δt/t²
B,A
A,B
A,B
B,A
Description
4.7 Measuring principle (FS)
and D
) are shown
i
u
25
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