eFunda: Introduction to Magnetic Flowmeters
engineering fundamentals Magnetic Flowmeter
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Overview

Magnetic flowmeters, also known as electromagnetic flowmeters or induction flowmeters, obtain the flow velocity by measuring the changes of induced voltage of the conductive fluid passing across a controlled magnetic field.

A typical magnetic flowmeter places electric coils around (inline model) / near (insertion model) the pipe of the flow to be measured and sets up a pair of electrodes across the pipe wall (inline model) or at the tip of the flowmeter (insertion model). If the targeted fluid is electrically conductive, i.e., a conductor, its passing through the pipe is equivalent to a conductor cutting across the magnetic field. This induces changes in voltage reading between the electrodes. The higher the flow speed, the higher the voltage.

The operation principle of inline magnetic flowmeters

The operation principle of insertion magnetic flowmeters

Further Information
According to Faraday's law of electromagnetic induction: any change in the magnetic field with time induces an electric field perpendicular to the changing magnetic field:

where E is the voltage of induced current, B is the external magnetic field, A is the corss section area of the coil, N is the number of turns of the coil, is the magnetic flux, and finally the negative sign indicates that the current induced will create another magnetic field opposing to the buildup of magnetic field in the coil based on Lenz's law.

When applying the above equation to magnetic flowmeters, the number of turns N and the strength of the magnetic field B are fixed. The Faraday's law becomes

where D is the distance between the two electrodes (the length of conductor), and V is the flow velocity.

If we combine all fixed parameters N, B, and D into a single factor , we have

It is clear that the voltage developed is proportional to the flow velocity.

A prerequisite of using magnetic flowmeters is that the fluid must be conductive. The electrical conductivity of the fluid must be higher than 3 µS/cm in most cases. A lining of nonconductive material is often used to prevent the voltage from dissipating into the pipe section when it is constructed from conductive material.

Common Specifications

Common specifications for commercially available magnetic flowmeterss are listed below:

  Fluid Phase:

Score Phase Condition
 Liquid  Clean 
  Corrosive  
  Dirty 
  Viscous 
 Slurry  Abrasive 
  Fibrous 
 Liquid  Non-Newtonian 
  Open Channel 
: Recommended
: Limited applicability

  Line Size: Inline models: 10 ~ 1200 mm (0.4 ~ 48 inch)
Insertion models: 75 mm (3 in) and up
  Turndown Ratio: 100: 1

Pros and Cons

  • Pros:
  - Minimum obstruction in the flow path yields minimum pressure drop
  - Low maintenance cost because of no moving parts
  - High linearity
  - Two and multi beam models have higher accuracy than other comparably priced flowmeters
  - Can be used in hazardous environments or measure corrosive or slurry fluid flow
  • Cons:
  - Requires electrical conductivity of fluid higher than 3 µS/cm in most cases
  - Zero drifting at no/low flow (may be avoided by low flow cut-off; new designs improve on this issue)

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