A magnetic 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).
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 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).
In the case of wafer-style magnetic flow meters, 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.
How to select and install a magnetic flow meter?
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?
- 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?
There are three common types of magnetic flow meters. The specific requirements of your application will determine which magnetic flow meter is best:
- Insertion magnetic flow meter – Best for large pipe applications
- In-line magnetic flow meter – Best for applications that require high accuracy or require higher flow rates
- Low-flow magnetic meters – Best for low flow applications
Choose the right mag flow meter
Insertion Magmeters
These versatile, simple-to-install flow meters deliver accurate flow measurement over a wide dynamic range in pipe sizes ranging from 0.5 to 8", satisfying the requirements of many diverse applications. FMG3000 Series magmeters offer a variety of output options for use with OMEGA flow instrumentation that has a frequency or 4 to 20 mA output. Flow velocity measurement is compensated for temperature via an integrated temperature sensor.
In-Line Magmeters
The FMG90B Series electromagnetic flow meters are designed for measurement of conductive liquids. The FMG90B magmeters have no moving parts and a PTFE lining, they can handle applications involving wastewater, pulp, food, and slurries.
Low Flow Magmeters
The FMG-2000 has no moving parts, and electrodes are designed to discourage fouling. This magmeter requires no maintenance in applications where debris would impede mechanical meters. There are no parts to wear out. Minimal straight pipe requirements allow FMG-2000 Series meters to be used in piping configurations where there is little space between the meter and an elbow. FMG-2000 Series meters are rated IP68 for applications where the meter may be under water up to a depth of 3 m (10') for prolonged periods of time. Rate and total indication are standard. Rate and total units and pulse output are settable via the front panel touch key pad by the user.
How do you install a magnetic flow meter?
Insertion Magnetic Flow Meters 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 magnetic flow meter 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 deg 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 magnetic flow meter performance. Instructions are included with the installation manual on how to best ground the magnetic flow meter.
Magnetic flow meters are highly sensitive to air bubbles, which can cause the magnetic flow meter to read high. The direction of the flow meter should ensure that the flow meter is completely filled with water. If there is any question that the pipe may not be absolutely full, mount the sensor at a 45 to 135-degree angle to avoid impacting the accuracy of the flow measurement.
In-Line Magnetic Flow Meters 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 flow meters are very sensitive to air bubbles. The magnetic flow meter cannot distinguish entrained air from the process fluid; therefore, air bubbles will cause the mag flow meter to read high.Low Flow magnetic flow meters
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 (0.1 GPM). A digital display with relay and analog outputs are standard.
For many years, sensors used in high reliability applications, such as those used in aerospace and military applications, relied upon connectors such as the Mil-C-5015 or MIL-C-38999 connectors that provided high-reliability and secure connections but at a high price. With the expanded use of industrial automation systems, however, the number of sensors used have increased dramatically, driving the need for a reliable, cost effective connection system for these sensors.
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