Magnetic Flow Meters Based on Faraday’s Laws of Electromagnetic Induction Whenever A conductor moves through a magnetic field of given field strength, a voltage is induced on the conductor proportional to the relative velocity between the conductor and the magnetic field. They offer true noninvasive measurements. They can measure reverse flows and are insensitive to viscosity, density, and flow disturbances. Electromagnetic flow meters can rapidly respond to flow changes and they are linear devices for a wide range of measurements.
Consists of electrically insulated or non-conducting pipe, with a pair of electrodes mounted opposite to each other and a flush with the inside walls of the pipe. A magnetic coil wound around the pipe so that magnetic field is generated in a plane mutually perpendicular to the axis of the flow meter body and to the plane of the electrodes. Electrically conductive flowing liquids works as the conductor. Here, Length = distance between the electrodes= Pipe Diameter. As the liquid passes through the pipe section, it passes through the magnetic field set up by magnetic coils, thus inducing a voltage in the liquid which is detected by a pair of electrodes mounted on the pipe wall. Amplitude of the induced voltage is proportional to the fluid velocity.
Electromagnetic Flow meters are based on Faraday’s law of Electromagnetic Induction. In Electromagnetic Flow meter, 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 standardised output signal or displayed in appropriate Engineering Units on LED/LCD Display.
The flux density of the electromagnetic field in a given Flow meter and the distance between the electrodes are constant, therefore, the induced voltage is only a function of liquid velocity.
E = K.B.V D Where E - Induced voltage - Volume flow is calculated by equation :
K - Flow Tube Constant - Q = vD2π/4
B - Magnetic field velocity
V - Mean flow velocity - Therefore Q = E.D. π
D - Electrode Spacing - K.B.4
The induced voltage is not affected by the physical properties of liquid like temperature, viscosity, pressure, density and conductivity, as long as conductivity of the measure liquid is above minimum threshold level. For reliable measurement the pipe must be completely full of liquid. The electromagnetic field coil assembly is excited by pulsed DC technique, which eliminates the interfering noise and provides automatic zero correction.
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