Niagara 6600 Series, Installation, Operation & Maintenance Manual

Page: 6 / 44

3
Installation, Operation & Maintenance Manual M600 NIAGARA
The principle of operation is based on Faraday's law of Electromagnetic Induction. In an Electromagnetic flowmeter,
the liquid passing through the device represents a conductor moving through a magnetic field. It induces a voltage
perpendicular to both the magnetic field and the direction of flow. The voltage is linearly proportional to the mean
velocity as is represented in the following empirical equation:
e = l*B*v Where, e = Induced Voltage
l = Conductor Length
B = Magnetic Field Strength
v = Velocity of the conductive fluid
The voltage generated is detected by a pair of insulated electrodes as the conductive liquid moves through the
magnetic field.
l :The length corresponds to the distance between the electrodes. The internal distance between the electrodes
inside the measuring pipe remains constant for a particular model.
B :The strength of the magnetic field is constant by design for a particular model and meter size.
e : The induced voltage is therefore linear in a relation with the pipe internal diameter, strength of the magnetic
field and the mean flow velocity. With l and B being constant for a particular size, the voltage generated (e.m.f.) is
directly proportional to the mean flow velocity.
Voltage e (e.m.f. generated ) α v (velocity)
Since, v = Flow Rate / Cross sectional area => Q in ft
3
/min / (π x D
i
2
/(4x144))
Where D
i
= Internal Pipe Diameter Inches
Substituting in the main equation:
e = l x B x Q in ft
3
/min / (π x D
i
2
/(4 x 144)) => Q x l x B x 4 x 144/ (π x D
i
2 )
Variable Constant
Therefore, for a given nominal diameter or cross section, the e.m.f. generated is proportional to flowrate, i.e.
e (e.m.f. generated) α Q (volumetric flowrate)
This equation holds true only when the pipe is completely filled, as we are
assuming that the entire section of the pipe is always full. The measurement
is not reliable if this condition is not fulfilled.
B
l
v
e = l.B.v e
l
B
v
Where, = Induced Voltage.
= Conductor Length.
= Magnetic Field Strength.
= Conductor Velocity.