Electromagnetic Flowmeters
Measuring electrically conductive fluids with electromagnetic flowmeters
Electromagnetic flowmeters are limited to
measuring the volume flow rate of electrically conductive fluids. A
reasonable measurement accuracy, around ±1.5%, is given, although the
instrument is expensive both in terms of the initial purchase cost and
also in running costs, mainly because of its electricity consumption. A
further reason for its high cost is the need for careful calibration of
each instrument individually during manufacture, as there is
considerable variation in the properties of the magnetic materials used.
The instrument consists of a stainless steel cylindrical tube, fitted
with an insulating liner, which carries the measured fluid. Typical
lining materials used are neoprene, polytetrafluoroethylene (PTFE) and
polyurethane. A magnetic field is created in the tube by placing
mains-energized field coils either side of it, and the voltage induced
in the fluid is measured by two electrodes inserted into opposite sides
of the tube. The ends of these electrodes are usually flush with the
inner surface of the cylinder. The electrodes are constructed from a
material which is unaffected by most types of flowing fluid, such as
stainless steel, platinum–iridium alloys, Hastelloy, titanium and
tantalum. In the case of the rarer metals in this list, the electrodes
account for a significant part of the total cost of the instrument.
The internal diameter of magnetic flowmeters is normally the same as
that of the rest of the flow-carrying pipework in the system. Therefore,
there is no obstruction to the fluid flow and consequently no pressure
loss associated with measurement. Like other forms of flowmeter, the
magnetic type requires a minimum length of straight pipework immediately
prior to the point of flow measurement in order to guarantee the
accuracy of measurement, although a length equal to five pipe diameters
is usually sufficient.
Whilst the flowing fluid must be electrically conductive, the method is
of use in many applications and is particularly useful for measuring the
flow of slurries in which the liquid phase is electrically conductive.
Corrosive fluids can be handled providing a suitable lining material is
used. At the present time, magnetic flowmeters account for about 15% of
new flowmeters sold and this total is slowly growing. One operational
problem is that the insulating lining is subject to damage when abrasive
fluids are being handled, and this can give the instrument a limited
life.
Current developments in electromagnetic flowmeters are producing
physically smaller instruments and employing better coil designs which
reduce electricity consumption and make battery-powered versions
feasible (these are now commercially - available). Also, whereas
conventional electromagnetic flowmeters require a minimum fluid
conductivity of 10 µmho/cm3, some new versions are becoming
available which can cope with fluid conductivities as low as 1 µmho/cm3.
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