Electromagnetic Flow meters are based on Faraday’s law of Electromagnetic Induction.
Magnetic field is generated by a set of coils. As the conductive liquid passes through the electromagnetic field, an electric voltage is induced in the liquid, which is directly proportional to its velocity. This induced voltage is perpendicular to both the liquid flow direction and the electromagnetic field direction. The voltage sensed by the electrodes is further processed by the transmitter to give standardized output signal or displayed in appropriate engineering unit. The flux density of the electromagnetic field in each flow meter and the distance between the electrodes are constant. Therefore, the induced voltage is only a function of liquid velocity.
E = KxBxVxD
Where E – Induced voltage
K – Flow tube constant
B –Magnetic field strength
V –Mean flow velocity
D – Electrode spacing
Volume flow is calculated by: Q = VxD2xΠ/4
The induced voltage is not affected by the physical properties of the liquid like temperature, viscosity, pressure and density, as long as the conductivity of the liquid is above minimum threshold level. For reliable measurement, the pipe must be full of the liquid. The electromagnetic field coil assembly is excited by pulsed DC technic, which eliminates the interfering noise and provides automatic zero correction.