Page 447 - The Mechatronics Handbook

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at all and so the flowmeter will stop operating. The vortex flowmeter is also sensitive to changes in
upstream flow velocity profile and other disturbances such as swirl, and so should be installed with a
sufficient straight length of pipe upstream and downstream of the measurement point. The flowmeter
should not be used in applications where pipe vibration or local sources of electrical interference are
high, since this will corrupt the vortex signal being detected and possibly give false readings under no-
flow conditions.

The Electromagnetic Flowmeter
The operation of the electromagnetic flowmeter is based on Faraday’s law of induction, i.e., when a
conductor is moving perpendicular to a magnetic field, the voltage induced across the conduction is
proportional its velocity. In the case of the electromagnetic flowmeter, the conductor is the fluid being
metered, while the induced voltage is measured using electrodes in the pipe wall. Since in most applica-
tions the pipe wall of the flowmeter is made from a conductive material such as a stainless steel, an inner
nonconducting liner is required to insulate the electrodes and prevent the generated voltage signal being
dissipated into the pipe wall. Coils on the outside of the pipe are used to generate a magnetic field across
the fluid, with simpler AC coil excitation methods which suffer from zero drift problems being increas-
ingly replaced by pulsed DC excitation techniques which do not.
The electromagnetic flowmeter has a number of advantages over traditional flow measurement tech-
niques, and some characteristics of an ideal flowmeter. The flowmeter has no moving parts, does not
obstruct the pipe at all, is available in a very wide range of pipe sizes, and may be used to measure
bidirectional flows. A measurement accuracy of typically ±0.5% of reading over a range of at least 10:1
is possible. The flowmeter’s accuracy is also unaffected by changes in fluid viscosity and density, and may
be used to meter difficult mixtures such as slurries and paper pulp.
The major limitation of the electromagnetic flowmeter is that it can only be used with fluids with a
conductivity of typically greater than 5 µS/cm, although special designs are available for use with liquids
with conductivities of as low as 0.1 µS/cm. The flowmeter is, therefore, not suitable for use with gases,
steam, or nonconducting liquids such as oil. The flowmeter’s calibration is also sensitive to changes in
flow velocity profile although requiring a shorter straight length of pipe upstream of the meter than the
orifice plate or turbine meter. Although electromagnetic flowmeters do not require significant mainte-
nance, care must be taken during operation to ensure that the liner does not become damaged, and that
significant deposits do not build-up on the electrodes, since these can cause changes in the calibration
or in some cases cause the flowmeter to stop functioning altogether. Even if these effects are minimized,
electromagnetic flowmeters will require periodic recalibration using either traditional techniques or an
electronic calibrator now available as an accessory from most manufacturers.

The Ultrasonic Flowmeter

The dream of producing a universal non-invasive flowmeter has been the catalyst for the many different
ultrasonic flowmeter configurations, which have been investigated over the last 40 years [9]. However,
most ultrasonic flowmeters commercially available today can be placed into one of two categories—
Doppler and transit-time.
The ultrasonic Doppler flowmeter is based on the Doppler shift principle. Ultrasound at a frequency of
typically 1 MHz is transmitted at an angle into the moving fluid being monitored. Some of this energy will
be reflected back by acoustic discontinuities such as particles, bubbles, or turbulent eddies. The difference
in frequency between the transmitted and received signals (the Doppler frequency shift) is directly propor-
tional to the velocity of the flow.
The ultrasonic transducers, which are used to transmit and receive the ultrasound, are usually located
in a single housing that can be fixed onto the outside of the pipe, and so a simple clamp-on flowmeter,
which is easy to install and completely noninvasive, is possible.

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