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Theories Of Electromagnetic Flow Meters

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Electromagnetic or magnetic flow meters are designed to measure the flow of electrically conductive liquids in a closed pipe. With this technology, the fluid must have a conductivity that is ≥ 5μS/cm. Mag meters are volumetric flow measuring devices. The principle of operation of the magnetic flowmeter is based on Michael Faraday’s Law of Electromagnetic Induction.

Advantages of Magnetic Flow Meters

We’ll soon explore the technology used in electromagnetic flow meters in an effort to understand how they function. First, let’s look at some of the advantages of these devices:

  • They have no moving parts, decreasing the frequency of repairs and prolonging their lifespan.
  • They use very little power – some models use only 15 Watts.
  • They are corrosion-resistant, making them an effective tool for monitoring the flows of acids and other aggressive fluids. 
  • The mag meter may be fabricated from abrasion-resistant materials for measuring slurries and other erosive fluids.
  • They can be used to measure bi-directional flows and provide excellent range-ability for measuring both low and high volumetric flow rates.
  • They are cost-effective, with accuracies of ±0.5% or better

How Does an Electromagnetic Flowmeter Work?

Magnetic flow measurement

There are two primary components to an electromagnetic flow meter (also known as a magmeter): The sensor and the transmitter. The sensor is placed inline and detects the induced voltage generated by the fluid as it flows past the electrodes mounted in thesensor. This information is then interpreted by the transmitter, which converts the induced voltage into a flow measurement, and transmits that information to your control system. 

Induced voltage

Induced voltage is the principle by which magmeters detect flow. The sensor includes electromagnetic coils and a pair of electrodes. The transmitter powers these coils, forming a magnetic field between them. Once fluid begins to pass through the coils, the magnetic field generated by the coils,  in accordance with Faraday’s Law below, generates a voltage proportional to the fluid velocity.  The transmitter translates this information into a flow rate using the inputted internal diameter of the pipe.  

Faraday’s Law of Induction

We can translate induced current into a measurement of flow using Faraday’s Law of Induction:

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The above equation indicates that a voltage is induced when a conductor (conductive fluid) is passed through a magnetic field. The voltage developed is proportional to the density of the field, the distance between electrodes, and the velocity of the conductive fluid moving through the field. 

Special attention is required when the pipe is constructed of conductive material. A lining of nonconductive material is used to prevent the voltage from being dissipated into the pipe section.

The magnetic field density is fixed for each magnetic flowmeter size. Therefore, velocity is left as the only variable, which means that a magnetic flowmeter is a velocity-measuring device.

Common Use Cases

Magnetic flow meters are used to measure liquid velocity and volumetric flow rate. They can be used to monitor and diagnose a number of issues and conditions related to changes in flow rate. They are particularly effective at measuring corrosive and errosive fluids, including slurries. They can also be used either indoors or outdoors and also in hazardous environments where explosive gasses are present. Additionally, they can be used to measure a wide range of flow rates; typically covering a range of more than 100:1 from the maximum to the minimum detectable flow rate. As such, they’re 

used in a variety of industries, including:

  • Pulp and paper
  • Oil and gas
  • Food and beverage
  • Municipal / Government (water, wastewater, etc.)
  • And more

When choosing whether or not to use a magnetic mass flowmeter in a given application, the first parameter to consider is the electrical conductivity. Applications comprise conductive fluids (liquids and slurries). For a list of known fluids, SmartMeasurement™ has constructed a conductivity table.

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