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E-Zine November 2007The magnetic flowmeter signal is proportional to the fluid velocity. Therefore, these flowmeters measure velocity, from which the volumetric flow rate is inferred utilizing the first equation, assuming that the cross-sectional area of the conduit is known. Magnetic flowmeter performance is therefore predicated on how well the average fluid velocity is measured and how well the cross-sectional area is known. Uncertainty in the cross-sectional area can degrade the inferred volumetric flow rate measurement. The construction of the magnetic flowmeter is such that the only wetted parts are the liner and electrodes, both of which can be made from materials that can withstand corrosion. In addition, the straight-through (obstructionless) nature of the design reduces both the loss of hydraulic energy across the flowmeter (pressure drop) and the potential for abrasion from the flowing liquid. Therefore, magnetic flowmeters can measure many corrosive liquids and abrasive slurries. Magnetic flowmeter liners and electrodes can be constructed of materials that do not contaminate the liquid. Therefore, these flowmeters can be applied when liquid contamination is an issue, such as in sanitary applications. Straight run requirements are relatively short, so magnetic flowmeter technology can be applied where limited straight run is available. In addition, magnetic flowmeter technology has no Reynolds number constraints, so it can be applied where the liquid exhibits high or varying viscosity. Magnetic flowmeters that sense velocity and level can measure the flow of liquids in partially filled pipes, such as interceptor sewers and storm water culverts. Magnetic flowmeters with fast response times can measure liquids that flow for relatively short periods of time, such as in batch and fill operations. Magnetic flowmeters measure liquid velocity, from which the volumetric flow rate is inferred. The measurement is linear with liquid velocity and exhibits a relatively large turndown. In addition, the range of accurate flow measurement is relatively large and easy to change after installation. Replacing an existing two-wire instrument with a four-wire instrument typically requires the installation of an extensive amount of conduit/cable to provide power to the new instrument. Two-wire magnetic flowmeters that do not require power wiring can often replace an existing flowmeter using the existing conduit/wiring with little or no rework. Excerpted from The Consumer Guide to Magnetic Flowmeters ISSN 1538-5280 |
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