3 General Principles FLUXUS WD100, WD200
UMFLUXUS_F5WDV1-1EN, 2016-04-08 11
3 General Principles
For the ultrasonic measurement of the flow rate, the flow velocity of the fluid flowing in a pipe is determined. Further phys-
ical quantities (e.g., volumetric flow rate, mass flow rate) are derived from the flow velocity and from additional physical
quantities, if necessary.
3.1 Measurement System
The measurement system consists of a transmitter, the ultrasonic transducers with the transducer cables and the pipe on
which the measurement is conducted.
The ultrasonic transducers are mounted on the outside of the pipe. Ultrasonic signals are sent through the fluid and re-
ceived by the transducers. The transmitter controls the measuring cycle, eliminates the disturbance signals and analyzes
the useful signals. The measured values can be displayed, used for calculations and transmitted.
3.2 Measurement Principle
The flow velocity of the fluid is measured using the transit time difference correlation principle (see section 3.2.2).
3.2.1 Terms
Flow profile
Distribution of the flow velocities over the cross-sectional pipe area. For an optimal measurement, the flow profile has to
be fully developed and axisymmetrical. The shape of the flow profile depends on whether the flow is laminar or turbulent
and is influenced by the conditions in the supply line of the measuring point (see chapter 5).
Reynolds number Re
Coefficient describing the turbulence behavior of a fluid in the pipe. The Reynolds number Re is calculated from the flow
velocity, the kinematic viscosity of the fluid and the inner pipe diameter.
If the Reynolds number exceeds a critical value (usually approx. 2 300, if the fluid flows in a pipe), a transition from a lam-
inar flow to a turbulent flow takes place.
Laminar flow
A flow without any turbulence. There is no disruption between the parallel flowing layers of the fluid.
Turbulent flow
A flow in which turbulence (swirling of the fluid) occurs. In technical applications, the flow in the pipe is mostly turbulent.
Transition range
The flow is partly laminar and partly turbulent.
Transit time difference ∆t
Difference of the transit times of the signals in and against the flow direction. The flow velocity of the fluid in the pipe is de-
termined from the transit time difference (see Fig. 3.2, Fig. 3.3 and Fig. 3.4).
Fig. 3.1: Example of a measurement setup
transducers
transmitter
pipe
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