3.5
Principle of operation
Physical principle
Figure 3-1
Velocity distribution along sound path
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, temperature
has no influence on measurement accuracy.
SITRANS F US flowmeters
In SITRANS F US 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
B,A
– t
A,B
) / (t
A,B
× t
B,A
) = K × Δt/t²
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, i.e. independent of the temperature.
The mechanical/geometrical pipe data is transducer angle (θ), distance between sensors (L)
and pipe dimension (D
i
and D
u
) shown in the figure below.
Description
3.5 Principle of operation
FUS380/FUE380
18
Operating Instructions, 09/2016, A5E00730100-AB