Flow Meters

Toshiba Magnetic Flow Meters: 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.Toshiba Magnetic flowmeters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions). Toshiba Magnetic flowmeters are also ideal for applications where low pressure drop and low maintenance are required. Competitive prices can be found at the Automatic Controls web site at www.autoctrls.com. Also downloadable data sheets can be found at: Magnetic Flow Meter Principle 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. 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). Magmeter Selection 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 Toshiba have a standard 2" NPT or fit into a specific size fitting. The FMG2552 is designed for 2" to 12" in size with a flow rate of 0.1 to 6 m/sec (0.3 to 20 ft/sec). The FMG2552 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 1 to 2% accuracy. Toshiba's FMG-400 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 16". 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. Grounding requirements: The FMG-400 Series uses 100 ohm or less ground resistance. The grounding terminal of the FMG400 Series flowmeter should be grounded with a heavy copper braid or wire to ground the terminal and make it as short as possible. 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.