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1.5
Reducing the influence of varying flow profiles
When a liquid flows through a pipe, its velocity tends to vary from zero along the pipe wall
up to its maximum through the centre of the pipe.
The velocity profile depends on the actual flow velocity together with the viscosity of the
liquid.
When the flow rate is low, the velocity profile takes the shape of a parab-
ola. This is called laminar flow. If the flow rate is gradually increased, the
flow profile also changes gradually, still maintaining the laminar flow pro-
file, until it reaches the critical velocity. At this flow rate, the flow profile
will, via a transition phase, change from laminar to turbulent flow with the
formation of eddies and chaotic motion, which do not contribute to the
volume flow rate. The two types of flow profile are illustrated on the figure
to the right. Precisely when the liquid reaches the critical velocity and
changes to turbulent flow, depends on the diameter of the pipe, and on
the viscosity and density of the liquid. For cream, for example, the visco-
sity strongly depend on the temperature and the percentage of fat, and it
is therefore not possible to predict the flow profile at any given flow rates.
The technique used to avoid an unknown flow profile from affecting the accuracy of a
measurement, consists essentially of measuring the average velocity of the liquid through-
out the full cross section of the pipe, hence registering all the liquid passing through it.
Measures over
The average measurement is achieved in a PD 340 by using a
the entire cross
square measurement chamber, where each of the two electrodes are
section of the
designed to measure across the complete cross section of flow.
chamber
This is in contrast to flow meters having a circular cross section and
point-type electrodes. Here the 'field of view' between the electrodes
Field of view
is limited to only involving the liquid passing directly between the
electrodes. Its calibration is therefore conditional on a particular
known flow profile.
Manual
Flow Transmitter, PD 340
9/55
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