Flexim FLUXUS F704**-A2 Series, Руководство пользователя

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3 General Principles FLUXUS F70x
UMFLUXUS_F7V4-6-2EN, 2017-10-01 11
3 General Principles
For the ultrasonic measurement of the flow rate, the flow velocity of the medium flowing in a pipe is determined. Further
physical quantities (e.g., volumetric flow rate, mass flow rate, heat 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 medium 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
In the TransitTime mode, the flow velocity of the medium is measured using the transit time difference correlation principle
(see section 3.2.2). If the proportion of gas or solid particles is too high, the transmitter can toggle to the NoiseTrek mode
(see section 3.2.3).
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 medium in the pipe. The Reynolds number Re is calculated from the
flow velocity, the kinematic viscosity of the medium and the inner pipe diameter.
If the Reynolds number exceeds a critical value (usually approx. 2 300, if the medium flows in a pipe), a transition from a
laminar 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 medium.
Turbulent flow
A flow in which turbulence (swirling of the medium) occurs. In technical applications, the flow in the pipe is mostly turbu-
lent.
Transition range
The flow is partly laminar and partly turbulent.
Transit time difference ∆ t
Difference of the transit times of the signals. In the TransitTime method, the transit time difference of the signals in and
against the flow direction is measured, in the NoiseTrek mode - the time difference of the signal from the transducer to the
particle and from the particle to the transducer. The flow velocity of the medium in the pipe is determined 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