Siemens SITRANS F Operating Instructions Manual page 22

Description
3.5 Principle of operation
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 travel 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
where
v = Average flow velocity
t = Travel 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
Figure 3-2
The ultrasonic signal is sent directly between the transducers. The advantage gained by
sending signals from point to point is an extremely good signal strength.
22
– t ) / (t × t ) = K × Δt/t²
B,A A,B A,B B,A
and D ) shown in the figure below.
i u
1A
1B
Measuring principle
SITRANS FUS380/FUE380
Operating Instructions, 05/2018, A5E00730100-AE