3 General principles
FLUXUS H721 3.1 Measurement principle
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UMFLUXUS_H721V1-5EN, 2022-05-15
3 General principles
The FLUXUS H721 is used in the oil and gas industry on single and multiproduct pipelines of liquid hydrocarbons. Its
purpose is the measurement of the volumetric flow rate and the standard volumetric flow rate by means of the transit time
difference principle. Furthermore, the mass flow rate and other fluid parameters (e.g., API gravity, density, standardized
density, kinematic viscosity) can be determined from the sound speed (transit time measurement) and the temperature.
The FLUXUS H721 can also be used for leakage detection, fluid detection (using the fluid table) and interface detection
(slope).
3.1 Measurement principle
The ultrasonic transducers are mounted on the pipe which is completely filled with the fluid. The ultrasonic signals are
alternatively emitted and received by the transducers. The physical quantity is determined from the transit time of the
ultrasonic signals.
Transit time difference principle
The signals are emitted and received by two 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 is reduced in flow direction and increased in the opposite
direction.
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
Fig. 3.1: Path of the ultrasonic signal in the flowing fluid
v – average flow velocity of the fluid
k
Re
– fluid mechanic calibration factor
k
a
– acoustic calibration factor
Δt – transit time difference
t
γ
– transit time in the fluid
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