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Magnetic Flow Meters: How They Work & What Is It
An electromagnetic flow meter is a volumetric flow meter that does not have any moving parts. It is ideal for wastewater applications or any dirty liquid which is conductive or water based. Magnetic flow meters are also ideal for applications where low-pressure drop and low maintenance are required.
Magnetic flow meters will generally not work with hydrocarbons, distilled water, and many non-aqueous solutions.
How does a magnetic flow meter work?
Magnetic flow meters use a magnetic field to generate and channel liquid flow through a pipe. A voltage signal is created when a conductive liquid flows through the flowmeter’s magnetic field. The faster the flow of the fluid, the greater the voltage signal generated. Electrode sensors located on the flow tube walls pick up the voltage signal and send it to the electronic transmitter, which processes the signal to determine liquid flow.
What is the operating principle of a magnetic flow meter?
The operation of a magnetic flow meter or mag meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.
As applied to the design of magnetic flow meters, Faraday’s Law indicates that signal voltage (E) is dependent on the average liquid velocity (V) the magnetic field strength (B) and the length of the conductor (D) (which in this instance is the distance between the electrodes).
Vortex Flow Meter
A vortex flow meter is a flow measurement device best suited for flow measurements where the introduction of moving parts presents problems. They are available in industrial grade, brass or all plastic construction. Sensitivity to variations in the process conditions are low and with no moving parts have relatively low wear compared to other types of flow meters.
Vortex flow meters operate under the vortex shedding principle, where an oscillating vortexes occur when a fluid such as water flow past a bluff (as opposed to streamlined) body.
The frequency that the vortexes are shed depend on the size and shape of the body. It is ideal for applications where low maintenance costs are important. Industrial size vortex meters are custom built and require appropriate sizing for specific applications.
The Working Principles of Turbine Flow Meters
Turbine flow meters are employed for both lower viscosity gases and liquids with turn down ratios from 7 to 30:1. The use of an appropriate zero drag electronic pick off in place of the standard magnetic type is the reason behind achieving this extended range. The turbine experiences too much extra drag at low fluid velocities due to the standard magnetic type. As a result, the rotational speed is slowed down.
The typical accuracy and repeatability of turbine flow meters are ±0.5% and ±0.1%, respectively. However, it is possible to achieve accuracy of ±0.25% and repeatability of ±0.05%.
Turbine flow meters (Figure 2) are inherently sensitive to Reynolds number. They will not be linear at Reynolds numbers nearing or in the laminar region. Therefore, they should be carefully used in the case of fluids with a broad temperature and viscosity range such as some oils. It is not a problem at constant conditions, but is often hard to achieve practically.
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