3 General principles
FLUXUS F60*
3.1 Measurement principle
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UMFLUXUS_F60xV5-0EN, 2017-10-16
3
General principles
For the ultrasonic measurement of the flow rate, the flow velocity of the fluid in a pipe is
determined. Further physical quantities are derived from the flow velocity and from addi-
tional physical quantities, if necessary.
3.1
Measurement principle
The flow velocity of the fluid is measured in the TransitTime mode using the ultrasonic
transit time difference correlation principle. When measuring with a high proportion of
gas or solid particles, the transmitter can toggle into the NoiseTrek mode.
3.1.1
Terms
Flow profile
Distribution of flow velocities over the cross-sectional pipe area. For an optimal measure-
ment, 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 condi-
tions at the inlet of the measuring point.
Reynolds number Re
Coefficient describing the turbulence behavior of a fluid in the pipe. The Reynolds num-
ber Re is calculated from the flow velocity, the kinematic viscosity of the fluid and the in-
ner pipe diameter.
If the Reynolds number exceeds a critical value (usually approx. 2300, if the fluid 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 mixing between the parallel flowing layers of
the fluid.
Turbulent flow
A flow with turbulences (swirling of the fluid). In technical applications, the flow in the pipe
is mostly turbulent.
Transition range
The flow is partly laminar and partly turbulent.
Sound speed c
Speed of the propagating sound. The sound speed depends on the mechanical proper-
ties of the fluid or the pipe material. In pipe materials and other solid materials, a distinc-
tion is made between the longitudinal and transversal sound speed. For the sound speed
of some fluids and materials, see annex C.
Flow velocity v
Average value of all flow velocities of the fluid over the cross-sectional pipe area.