3.4
Measuring principle
Physical principle
Figure 3-3
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 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 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, 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 (SONOKIT only). The transducer
angle (θ), distance between sensors (L) and pipe dimension (D
i
and D
u
) are shown in the figure
below.
Description
3.4 Measuring principle
SITRANS FUS060 with PROFIBUS PA
20
Operating Instructions, 09/2021, A5E02225054-AD