magnetic flow meter

MAGNETIC FLOW METER

Magnetic flow meter , a device commonly used in many industries to measure the flow rate of fluids in pipelines .The principle behind this type of flow measurement is electromagnetic induction.

Faraday's law of electromagnetic induction

An emf will induce in a conductor when magnetic field linked with the conductor changes.

A conductor of length l moving in a magnetic field of strength B with a speed of v m/s . Then the emf induced across the conductor is given by

e =Blvsinθ

where θ is the angle between magnetic field direction and direction of conductor motion

The direction of induced emf is given by Fleming’s right hand rule.

Fleming’s Right hand rule

If we held our right hand with the thumb,index finger and middle finger mutually perpendicular to each other and if the thumb represents the direction of motion of the conductor in a magnetic field, index finger represents the direction of field ,then the middle finger gives the direction of induced emf .

Working of magnetic flow meter

A fluid with a required minimum conductivity is passed through the flow tube of magnetic flowmeter .The inside of the flow tube assembly is subjected to a magnetic field of required strength .This is achieved by a powering a coil enclosed in flow tube housing .A constant known current is fed in to the coil and coil gets magnetized providing a magnetic field to the flow path .. The liquid should have a minimum conductivity so that it acts as a conductor which induces a voltage while moving in the magnetic field .So the movement of the conductor induces an emf . pick up electrodes are properly embedded in the housing of the flowmeter .They are placed right angle to the flow direction . Pick up electrodes carry the induced voltage which is created due to the movement of fluid .The voltages created are in the order of the millivolts .

Flow and output voltage relation

The induced voltage is proportionally to the flow velocity and is given by the expression,

e =Blvsin θ

l is the length of the conductor or pipe diameter D. As the magnetic field and velocity vector are mutually perpendicular in magnetic flow meter,θ =90°

Or sin θ = sin90=1

e =BlV

e = BDV
Here the terms pipe diameter D, strength of magnetic field B are constant

e= constant×v

Or v= constant×e

Flow =V.A

= Constant × e

Flow = K × millivolt

As magnetic flow meter responds to the velocity of the fluid it is a volumetric flow measuring device and not the mass flow .

Flow Tube Assembly

The part hrough which fluid passes.coils and pickup electrodes are fabricated inside of this part .

Coils

In flowmeter, permanent magnetic field is achieved by passing an exciting current to a properly constructed coils coils are arranged top and bottom portion of the flow meter to create the magnetic e field in a direction perpendicular to the flow . current may be ac or dc pulsating.The exciting current creates a permanent magnetic field through which fluid passes thereby inducing a voltage proportional to fluid flow.

Why pulsating DC ???

Although DC creates magnetic field the permanent polarity of DC current makes polarisation of ions or fluid particles thereby generating electrochemical effect .This causes the ions to move to a specific electrode according to their polarity and accumulates there causing the measurement error.This effect can be over come by giving a pulsating DC which reverses it’s polarity at certain frequency thereby not allowing the ions to deposit on electrodes.

Pick up electrodes

The voltage induced across the conductor (Liquid layer )is picked by electrodes arranged sideways in the flowtube .It is perpendicular to the fluid flow and magnetic field .The inside of the flow tube is electrically isolated from the conducting fluid by teflon lining .only the pick up electrodes is able to make electric circuit with the liquid .All other part of the flow tube is isolated from the mv output .

CONVERTER

Millivolts signal from the electrodes are fed in to the converter unit,for signal conditioning,and transmitting as a standard signal like 4-20ma .Further converter unit serves as a interface unit for transmitting the signal,for supplying excitation current to the flow tube ,and for programming various parameters through front display panel etc .Converter unit either constructed as a seperate unit from flow tube or it is integrated to the flow tube . see below figures.

1.Converter unit integrated with the flow tube

2. converter and flow tube serves as a seperate unit.

Wiring

The following basic wiring and signals are needed for the operation of the flow meter as a sensor and a transmitting device .

Power supply to the converter

Coil exciting current

Millivolts output

Transmitted signal from converter to control room or monitoring devices. ( 4-20 ma )

The below figure gives a overview of the basic connection details for a magnetic flow meter .

magnetic flow meter earthing

Proper earthing should be done for magnetic flowmeter for its correct operation . Magnetic flowmeters output is small electric signal which is of the order of few millivolts . other stray voltages or electrical noises may affect this signal .Industrial environment are subjected to disturbances like these electrical noises These noises are actually unwanted electrical voltages . They circulates through the conducting parts like pipelines.They also pass through the conducting liquid .These noises should not affect the magnetic flowmeter performance .And they should be properly eliminated before they reach to the pick up electrodes .This is done through proper earthing .By this we can eliminate the unwanted electrical noises reaching the pickup electrodes .

Grounding for conducting pipelines

In case of metal pipelines which are conducting , they should be grounded .This can be done by making the ground connections to the flanges . See below figure..

Grounding for non conducting pipelines

For non conducting pipelines,the electric noises through the liquid can be grounded by using grounding rings .The grounding rings is arranged at ends of the flow tube .And they come in contact with the liquid . Electric noises pass through the liquid is grounded through this grounding rings .see figure .