3 General principles
3.1 Measurement principle
FLUXUS F501
2020-06-30, UMFLUXUS_F501V1-3EN
12
3.1.2
Measurement of the flow velocity in the TransitTime mode
The signals are emitted and received by 2 transducers alternatively in and against the flow direction. If the fluid is flowing,
the signals propagating in the fluid are displaced with the flow.
Caused by this displacement, the sound path of the signal in flow direction is reduced and the signal against the flow direction
is increased, see Fig. 3.1 and Fig. 3.2.
This causes a change in the transit times. The transit time of the signal in flow direction is shorter than the transit time
against the flow direction. The transit time difference is proportional to the average flow velocity.
The average flow velocity of the fluid is calculated as follows:
v = k
Re
· k
a
·
where
v
– average flow velocity of the fluid
k
Re
– fluid mechanics calibration factor
k
a
– acoustic calibration factor
Δt
– transit time difference
t
γ
– transit time in the fluid
Fig. 3.1:
Sound path of the signal in the flow direction
c – sound speed
1 – transducer (emitter)
2 – transducer (receiver)
3 – pipe wall
Fig. 3.2:
Sound path of the signal against the flow direction
c – sound speed
1 – transducer (emitter)
2 – transducer (receiver)
3 – pipe wall
t
2 t
-----------
c
β
β
α
γ
c
γ
3
2
1
c
α
c
α
reduction of the
sound path
flow direction of
the fluid
sound path with flow
sound path
without flow
c
β
flow direction of
the fluid
sound path with flow
sound path
without flow
β
α
γ
c
γ
increase of the
sound path
3
1
2
c
α
c
α