Dynasonics MFX Series Insertion MagProbe Flow Meter - Manual (Page 7)

Rev. 01/11 -1.5- MFX
PART 1 - INTRODUCTION
The MagProbe is a point-velocity measuring device used primarily
for measuring electrically conductive liquids, such as water and
water-based liquids, in closed piping systems. The unit operates
utilizing Faraday’s principle of magnetic conduction, whereby a
moving conductor (the liquid) has a voltage imposed on it that is
directly proportional to two variables—the strength of a local
magnetic field and the velocity of the moving conductor.
Imposed voltage Į Magnetic field X Fluid velocity
If the strength of the magnetic field is held constant, then the
magnitude of the voltage will be proportional to the velocity of the
moving conductor. The equation then simplifies to Imposed voltage
Į Fluid velocity. Most modern magnetic meters, including the MFX,
apply tri-stated, alternating polarity DC pulses to an integral
electromagnet. See Figure 1.3. Voltage measurements are made
with the magnet off, to measure ambient background noise, and
then with the magnet on
in both polarities. The
magnitude difference in
voltage measured is
proportional to flow. Once
fluid velocity is measured,
then various volumetric
flow measurements can
be obtained if the pipe
internal diameter (I.D.) is
known.
Point-velocity flow meters measure the fluid velocity at a specified
depth into the fluid stream, typically ǩ of the pipe I.D., which has
been proven to be the nominal velocity point when symmetrical flow
profiles are present. This assumption requires the probe to be
certain numbers of pipe diameter downstream of any piping
condition (elbows, valves, thermo-wells, tees, etc.) that can cause
flow abnormalities. Typically, a minimum of 15 pipe diameters of
straight pipe is required to develop a symmetrical flow profile.
Systems where symmetrical flow profiles are not present can still be
measured accurately, but flow profiling must be performed to
determine proper probe insertion depth. A diagram of the MagProbe
tip is illustrated in Figure 1.4.
General
Figure 1.3 - Magnet Excitation