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Magnetic flow meter is based on faradays law of
electromagnetic induction.
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For early magmeters, 1-5 microsiemens per
centimeter minimum fluid conductivity required for their operation. But newer
design required between 0.05 and 0.1 μs/cm.
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A conductive fluid flows through a pipe of
diameter (D) through a magnetic density (B) generated by the coils, the amount
of voltage (E) develops across the electrodes (faradays law) will be proportional
to the velocity(v) of the liquid.
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The magnetic field density and the pipe dia are
fixed values, they can be combined into a calibration factor (K)
E=KV
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Magmeter can measure flow in both directions, as
a reversing direction will change the polarity but not the magnitude of the
signal.
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The electrodes develops millivolt signal, this
signal is typically converted into (4-20ma) or frequency output (0-10,000Hz)
·
Magmeter signal is a weak one, the lead wire
should be shielded and twisted if the transmitter is remote.
·
The magmeters coils can be powered by either AC
or DC.
·
AC
powered produced sine wave. The main problem with AC magmeter design in
that noise can vary with process conditions and frequent rezeoring is required
to maintain accuracy.
·
DC powered magmeter produced pulse waves, a low frequency (7-30Hz) dc pulse is used to
exite the magnetic coils in between the pulses, the transmitter sees only the
noise signal. Therefore the noise can be continuously eliminated after each
cycle. (eliminate zero shift)
·
DC powered magmeter, addition to being more accurate
and able to measure lower flows, it is less bulky, easier to install, use less
energy and having lower cost than AC meters.
·
Its accuracy does not depend on viscosity since
its measure by volume. So it can be used for highly viscous, slurries or
liquids with varying viscosities.
·
Liner material used for: - hard rubber, soft
rubber, PTFE, PFA and ceramics.
·
Common electrode materials are stainless steel
1.4571, 1.4539, hasteloy, tantalum and platinum.
