3 General principles
FLUXUS G831ST-HT
3.1 Measurement principle
13
UMFLUXUS_G831ST-HTV1-0EN, 2021-06-01
3.1.2
Measurement of the flow velocity
The flow velocity of the fluid is measured using the correlation principle. 2 pairs of ultrasonic transducers of the same type
are mounted one after the other at a distance Δx on the pipe which is completely filled with the fluid. The transducer pairs
form the measuring barriers A and B. Ultrasonic signals are alternately emitted by the emitters A
1
and B
1
and received by
the respective receivers A
2
and B
2
. The ultrasonic signals are modulated regarding amplitude and phase by the swirls of
the turbulent flowing fluid. Since the swirls move with the flow, they pass the measuring barriers A and B with a time
offset Δt, so that the modulation patterns of the ultrasonic signals of measuring barrier A and B are also offset by Δt. This
time offset Δt is measured by means of cross correlation of the modulation signals. The average flow velocity can be
calculated using the fluid mechanics calibration factor.
v = k
Re
·
where
v
– average flow velocity of the fluid
k
Re
– fluid mechanics calibration factor
Δx
– distance between measuring barriers
Δt
– time offset of the modulation patterns
Fig. 3.1:
Measurement arrangement of the correlation principle
A
1
– emitter of measuring barrier A
A
2
– receiver of measuring barrier A
B
1
– emitter of measuring barrier B
B
2
– receiver of measuring barrier B
Δx – distance between measuring barriers
Fig. 3.2:
Time offset of the modulation signals Δt
A – modulation signal of measuring barrier A
B – modulation signal of measuring barrier B
x
t
-------
A
1
B
1
A
2
B
2
Δx
flow direction
of the fluid
A
B
Δt
time
m
od
ula
tio
n
