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AquaProbe FEA100 / FEA200
Electromagnetic flowmeter
Appendix A
A.1 Velocity Profiles Background
Fig. A.1 on page 27 is a vector diagram showing a fully developed turbulent profile of the flow within a pipe.
Such diagrams illustrate the distribution of flow within the pipe. Known as the Flow Profile, it is highest in the
centre falling to zero at either side on the pipe wall. If there is sufficient upstream straight pipe, it can be
assumed that there is a profile of this form. In this case if, for example, the pipe is 600 mm in diameter, the
velocity at the centre line is 2 m/s and the flow is 487 l/s
Mean Velocity Factor
Rapidly Changing Velocities
Flat Part of Curve
1.722 m/s
Max. Velocity Factor
2.00 m/s
Fig. A.1 Turbulent Flow Profile
As the volume flow is known, the mean velocity in the pipe can be calculated – note that it is actually
1.722 m/sec lower than the velocity measured on the centre line. Careful Investigation of this profile or
1
8 th
vector diagram reveals that the mean velocity of 1.722 m/sec occurs at a point 72.5 mm or
/
of the
pipe's diameter in from the edge of the pipe. This point is referred to as the Point of Mean Velocity (for a fully
developed turbulent flow profile only). This is true (provided the profile is turbulent and fully developed) for
all pipes of all sizes and at all flow rates, and is recognized in the British Standard referred to previously.
1
th
Therefore, the best position to measure velocity is at the Point of Mean Velocity, i.e.
/
of the diameter in
8
from the edge of the pipe. By placing the probe at this point a straightforward calculation of volume flow
can be performed – but there is more to be considered ...
OI/FEA100/200–EN Rev. D
27
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