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A typical magnetic flow meter

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Magnetic Flow Meter Definition
Magnetic Flow Meter: A magnetic flowmeter (Mag Meter) is a device that can measure a water-based or conductive volumetric flow with no moving parts.
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Inline Magmeter Operating Principle
Diagram showing the operating principles of a magnetic flowmeter

Inline Magmeter Operating Principle
Diagram showing the operating principles of an insertion type flowmeter

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Magnetic Flow Meters - Introduction

What is a Magnetic Flow Meter?

A magnetic flow meter (mag flowmeter) 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. Magnetic flowmeters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions). Magnetic flowmeters are also ideal for applications where low pressure drop and low maintenance are required.

Principles of Operation
The operation of a magnetic flowmeter 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.

Faraday's Formula:
E is proportional to V x B x D where:

E = The voltage generated in a conductor
V = The velocity of the conductor
B = The magnetic field strength
D = The length of the conductor

To apply this principle to flow measurement with a magnetic flowmeter, 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 flowmeters, 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).In the case of wafer-style magnetic flowmeters, a magnetic field is established throughout the entire cross-section of the flow tube (Figure 1). If this magnetic field is considered as the measuring element of the magnetic flowmeter, it can be seen that the measuring element is exposed to the hydraulic conditions throughout the entire cross-section of the flowmeter. With insertion-style flowmeters, the magnetic field radiates outward from the inserted probe (Figure 2).

Selecting a Magmeter
The key questions which need to be answered before selecting a magnetic flowmeter 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?
What is the minimum and maximum flow rate for the flow meter?
What is the minimum and maximum process pressure?
What is 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?
Insertion Magmeters
Insertion type meters offered by Omega Engineering have a standard 2" NPT or fit into a specific size fitting. The FMG-550 Series are designed for 2" to 48" in size with a flow rate of 0.05 to 10 m/sec (0.15 to 33 ft/sec). The FMG-550 Series offers an analog output with an integral display for flow rate and totalization. The FMG3000 series offers corrosion resistant materials for pipes from 0.5 to 8". These insertion type flowmeters are ideal for large pipe applications.
Minimum conductivity: 5 to 20 microSiemens/cm
Installation Considerations: Select a location for the sensor where the flow profile is fully developed and not affected by any disturbances. A minimum of 10 pipe diameters of straight run upstream and 5 diameters downstream is recommended. Some situations may require 20 pipe diameters or more upstream to insure a fully developed turbulent flow profile. The insertion magmeter is sensitive to air bubbles at the electrodes. If there is any question that the pipe is absolutely full, mount the sensor at a 45 to 135 angle.
Grounding requirements: Magnetic flow sensors are sensitive to electrical noise which is present in most piping systems. In plastic piping systems, the fluid carries significant levels of static electricity that must be grounded for best magmeter performance. Instructions are included with the installation manual on how to best ground the magnetic flow meter.

In Line Magmeters
The in line type magnetic flow meters offer a higher accuracy. They can be as accurate as 0.5% of the flow rate. The insertion styles offer a 0.5 to 1% accuracy. Omega's FMG-600 series 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".
Minimum conductivity:  5 microSiemens/cm
Installation Considerations:   In line flow meters 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 vertical pipe runs, flow should always run up and not down. These flowmeters are very sensitive to air bubbles. The magmeter cannot distinguish entrained air from the process fluid; therefore, air bubbles will cause the magmeter to read high.

Low Flow Magmeters
These low flow mag flow meters are also in line and offer 3/8" to ½" NPT connections. The FMG200 series offer flow rates down to 0.38 LPM (.1 GPM). A digital display with relay and analog outputs are standard.

 Types of Magnetic Flow Meters
FMG-606 In Line Mag Meter
These cost-effective magnetic flow meters combine superior noise immunity with high purity Alumina ceramic detection tubes, allowing precise flow measurements in a wide range of applications. These magmeters are available in pipe sizes from 1 /2 " up to 16" to accommodate many materials: chemicals, corrosives, foods, pulp and other solid matter slurry flows. The FMG-600 Series electromagnetic flowmeter unique noise-suppression circuitry utilizes the optimum square-waved excitation frequency, eliminating various noises generated by slurries, making this flowmeter ideal for pulp, food and grout applications.

FMG-550 Insertion Style Magnetic Flow Meter
The FMG-550 Series fits pipe sizes from 2 to 48". The standard connection is a 1¼" NPT. It features an integral display for flow rate or totalization. The latest in bipolar pulsed dc technology and the best features of an insertion magnetic flow meter sensor are packed into the FMG-550 Series insertion magmeter. The FMG-551 is a magnetic flow meter to meet your flow measurements challenges. Simple installation, easy maintenance, and state-of-the-art microprocessor technology make the FMG-551 the best alternative to traditional full-line mag flowmeters. The FMG-551 electromagnetic flowmeter generates an isolated current output and an isolated frequency output. The current output provides a universal signal to recorders, valves, and a host of process control and data acquisition devices.
FMG3000 and FMG3100 Series Insertion Style Magnetic Flow Meter
The FMG3000 and FMG3100 Series are for 0.5 to 8 in. Pipes. These are blind transmitter with either a 4 to 20mA or frequency output. All versions of this magmeter are constructed of corrosion-resistant materials to provide long-term reliability with minimal maintenance costs.
FMG200 Series Low Flow Magmeters
The FMG201 Series is a small plastic-bodied magnetic flow meter capable of measuring the pulsating flows from air and solenoid-driven metering pumps, the meter is primarily designed for electrically-conductive chemicals and fluids of varying viscosities. The FMG201 electromagnetic flowmeter is an ideal flow measurement device for low flow applications. In addition to measuring and displaying flow rate and total flow, this magmeter unit has a variety of outputs. For continuous transmission of a flow signal, there is a 4 to 20 mA, and 0 to 5 volt signal, and a frequency signal proportional to flow. In addition, there are relay alarm outputs for low and high flows with user-set flow levels.
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