An electromagnetic flow meter is a type of flow meter that uses Faraday’s law of electromagnetic induction to measure the flow rate of a conductive fluid. It operates based on the principle that when a conductive fluid flows through a magnetic field, a voltage is induced across the fluid that is proportional to the velocity of the fluid.
An electromagnetic flow meter consists of a flow tube made of a non-magnetic material such as plastic or ceramic, with two electrodes mounted on opposite sides of the tube. A magnetic field is generated perpendicular to the flow tube, and when a conductive fluid flows through the tube, the voltage induced across the electrodes is proportional to the flow rate of the fluid.
The advantages of electromagnetic flow meters include their high accuracy, their ability to measure flow rates in both liquids and gases, their ability to handle corrosive and abrasive fluids, and their ability to measure bidirectional flow. They are commonly used in industries such as water and wastewater treatment, chemical and petrochemical, and food and beverage.
However, electromagnetic flow meters may not be suitable for applications where the fluid is non-conductive or has a low conductivity, such as oils or hydrocarbons. Additionally, they may require periodic calibration to maintain their accuracy, and their accuracy may be affected by changes in fluid temperature and conductivity.
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).
Also, In the case of a wafer-style Electromagnetic flow meter, a magnetic field is established throughout the entire cross-section of the flow tube. If this magnetic field is considered the measuring element of the magnetic flow meter, it can be seen that the measuring element is exposed to the hydraulic conditions throughout the entire cross-section of the flow meter. With insertion-style flow meters, the magnetic field radiates outward from the inserted probe.
Electromagnetic Flow Meter Selection
The key questions which need to be answered before selecting a magnetic flow meter are:
- Is the fluid conductive or water-based?
- Is the fluid or slurry abrasive?
- Do you require an integral display or a remote display?
- Do you require an analog output?
- The minimum and maximum flow rate for the Electromagnetic flow meter?
- What is the minimum and maximum process pressure?
- The minimum and maximum process temperature?
- Is the fluid chemically compatible with the flow meter wetted parts?
- What is the size of the pipe?
- Is the pipe always full?
First of all, select a location for the Electromagnetic flow meter sensor where the flow profile is fully developed and not affected by any disturbances. consequently, a minimum of 10 pipe diameters of straight run upstream and 5 diameters downstream is recommended.
Furthermore, some situations may require 20 pipe diameters or more upstream to insure a fully developed turbulent flow profile. Also, the insertion mag meter is sensitive to air bubbles at the electrodes. Finally, if there is any question that the pipe is absolutely full, mount the sensor at a 45 to 135 deg angle.
Electromagnetic Flow meters are sensitive to electrical noise which is present in most piping systems. Another type is in plastic piping systems, the fluid carries significant levels of static electricity that must be grounded for the best mag meter performance. In conclusion, Instructions are included with the installation manual on how to best ground the magnetic flow meter.
The in-line type Electromagnetic flow meter offers higher accuracy. Also, they can be as accurate as 0.5% of the flow rate. The insertion styles offer a 0.5 to 1% accuracy.
Also, In-line flange and wafer-style meters offer higher flow rates of 1 to 10 m/sec. These in-line meters are offered in pipe sizes up to 12″.
An In-line electromagnetic flow meter does not require as much straight pipe as the insertion styles. A minimum of 5 to 10 pipe diameters of straight run upstream and 1 to 2 diameters downstream is recommended.
In addition, in vertical pipe runs, the flow should always run up and not down. These flow meters are very sensitive to air bubbles. Finally, the mag meter cannot distinguish entrained air from the process fluid; therefore, air bubbles will cause the mag meter to read high.
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