Principle of Operation
Faraday's Law 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.
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 (Figure 1). 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 (Figure 2).
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?
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 flow meters are ideal for large pipe applications.
Minimum conductivity: 5 to 20 microSiemens/cm
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.
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
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 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.
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.