An Electromagnetic flow meter (mag flow meter) is a volumetric flow meter which does not have any moving parts and is ideal for wastewater applications or any dirty liquid which is conductive or water based.
A Electomagnetic flow meter will generally not work with hydrocarbons, distilled water, and many non-aqueous solutions. Magnetic flow meters are also ideal for applications where low-pressure drop and low maintenance are required.
To apply this principle to flow measurement with a Electromagnetic flow meter, it is necessary first to state that the fluid being measured must be electrically conductive for the Faraday principle to apply.
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 wafer-style Electomagnetic flow meter, a magnetic field is established throughout the entire cross-section of the flow tube. If this magnetic field is considered as 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.
Electomagnetic 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 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 magmeter 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.
Electomagnetic Flow Meter 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 best magmeter 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 a 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 Electomagnetic flow meter do 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, flow should always run up and not down. These flow meters are very sensitive to air bubbles. Finally, the magmeter cannot distinguish entrained air from the process fluid; therefore, air bubbles will cause the magmeter to read high.