3 General Principles
UMFLUXUS_G8V4-6-1EN, 2018-10-10
13
3.2.2
Measurement of the Flow Velocity
The signals are emitted and received by two transducers alternatively in and against the flow direction. If the fluid moves,
the signals propagating in the fluid are displaced with the flow. This displacement causes a reduction in distance for the
signal in the flow direction and an increase in distance for the signal against the flow direction in the section of the receiv-
ing transducer (see Fig. 3.2 and Fig. 3.3). This causes a change in the transit times. The transit time of the signal in the
flow direction is shorter than the transit time against the flow direction. This transit time difference is proportional to the av-
erage flow velocity.
The average flow velocity of the fluid is calculated as follows:
V = k
Re
. k
a
.
with
v
– average flow velocity of the fluid
k
Re
– fluid mechanics correction factor
k
a
– acoustic calibration factor
∆
t
– transit time difference
t
fl
– transit time in fluid
Fig. 3.2: Sound path of the signal in the flow direction.
Fig. 3.3: Sound path of the signal against the flow direction
t
2 t
fl
------------
transducer (emitter)
flow direction
of the fluid
sound path without flow
sound path with flow
pipe wall
α
β
γ
c
α
c
α
c
γ
c
β
transducer (receiver)
reduction in distance
in the transducer
flow direction
of the fluid
sound path without flow
sound path with flow
α
β
γ
c
α
c
α
c
γ
c
β
increase in distance
in the transducer
pipe wall
transducer (receiver)
transducer (emitter)
