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3 General principles
FLUXUS WD 3.1 Measurement principle
13
UMFLUXUS_WDV1-3EN, 2018-07-16
3.1.3 Measurement of the flow velocity in the NoiseTrek mode
If the proportion of gas or solids in the fluid is high, the damping of the ultrasonic signal can be so high that the complete
propagation of the fluid and therefore a measurement in TransitTime mode is not possible anymore. In this case the
NoiseTrek mode has to be used.
The NoiseTrek mode uses the presence of gas bubbles and/or solid particles in the fluid.
Ultrasonic signals are sent from a transducer into the fluid at short intervals, reflected by the gas bubbles and/or the solids
particles and again received by the same transducer.
The measurement arrangement used in the TransitTime mode does not need to be changed.
The transit time difference

t of 2 consecutive ultrasonic signals is determined. It behaves proportionately to the distance
the gas bubble/solid particle is covering between 2 consecutive pulses and thus, to the average flow velocity of the fluid,
see Fig. 3.4.
The average flow velocity of the fluid is calculated as follows:
v = k
Re
· k
a
·
where
Depending on the attenuation of the ultrasonic signal, the error of measurement in the NoiseTrek mode can be greater
than in the TransitTime mode.
Fig. 3.3: Transit time difference ∆ t
1 – signal in the flow direction
2 – signal against the flow direction
Fig. 3.4: Measurement of the flow velocity in the NoiseTrek mode
v – average flow velocity of the fluid
k
Re
– fluid mechanics calibration factor
k
a
– acoustic calibration factor
∆ t
p
– time difference between 2 consecutive pulses
∆ t – transit time difference of ultrasonic signals S
1
and S
2
( ∆ t = t
2
- t
1
)
1 2
∆ t
flow direction
transducer 1 transducer 1
flow direction
ultrasonic signal S
1
gas bubble or
solids particle
ultrasonic signal S
2
pulse at time t
gas bubble or
solids particle
pulse at time t + ∆ t
p
transit time t
1
transit time t
2

t
2

t
p

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