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Magnetic Flowmeters (Part 3 of 3)

By David W. Spitzer

E-Zine December 2007

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Variation in certain characteristics of magnetic flowmeters has allowed this technology to be applied to many processes. The following sections define flowmeter design categories (in alphabetical order) and how they fit the needs of their intended applications.

Ceramic Magnetic Flowmeters
Ceramic magnetic flowmeters have ceramic liners in place of the typical elastomer liners commonly used in magnetic flowmeters. These liners are more abrasion resistant than their elastomer counterpart and allow higher temperature operation. The electrodes are often embedded in the liner, reducing the number of leak paths.

Electrodeless Magnetic Flowmeters
Electrodeless magnetic flowmeters employ electrodes that are typically embedded in or located behind the liner, and reduce the number of leak paths. The electrodes are usually capacitively-coupled to the flowing liquid and will somewhat compensate for coating and abrasion. Therefore, these flowmeters actually do contain electrodes (perhaps "electrodeless" is a misnomer), but they do not come in contact with the process.

Flow-Low Magnetic Flowmeters
Low flow magnetic flowmeters include sizes below approximately 12 mm (0.5 inch) in size.

Flow-Medium Magnetic Flowmeters
Medium flow magnetic flowmeters include flanged and wafer magnetic flowmeters that are between approximately 12 mm (0.5 inch) and 300-450 mm (12-18 inches) in size. Early designs include flanged magnetic flowmeters that typically had coils located external to the pipe. These flowmeters had rather long face-to-face dimensions and were physically large when compared to pipe size. This made them relatively expensive to fabricate. Later designs that utilized internal coils reduced the face-to-face dimensions and resulted in smaller equipment that was easier to install and more economical to fabricate.

Miniature magnetic flowmeters utilizing wafer designs have further reduced face-to-face dimensions in some sizes to approximately one pipe diameter, making these flowmeters much smaller and less expensive. In general, miniature magnetic flowmeters are designed to handle the majority of magnetic flowmeter applications.

Flow-High Magnetic Flowmeters
These magnetic flowmeters are larger than approximately 300-450 mm (12-18 inches) in size.

High-Noise Magnetic Flowmeters
Application of magnetic flowmeter technology to measure the flow of some liquids, most notably slurries, results in signals that contain large amounts of noise. Various design approaches have been developed to measure the flow of these liquids.

Low Conductivity Magnetic Flowmeters
The application of magnetic flowmeter technology depends upon the process liquid having an electrical conductivity in excess of a minimum value. Low conductivity magnetic flowmeters utilize designs that allow measurement of liquids that exhibit conductivities that are below those that can be measured using typical magnetic flowmeter designs.

Partially-full Magnetic Flowmeters
Partially-full magnetic flowmeters are designed for applications where it is not possible to keep the flowmeter primary full of liquid. These designs utilize measurements of both liquid velocity and liquid level to calculate the flow rate.

Response-Fast Magnetic Flowmeters
In most applications, it is acceptable for magnetic flowmeters to take a few seconds to respond to a change in flow rate. However, some applications require measurement of flows where the duration of the entire flow may be but a few seconds. Fast response magnetic flowmeters are designed to quickly respond and measure liquid flows that occur during these short time periods.

Sanitary Magnetic Flowmeters
Sanitary magnetic flowmeters are designed and fabricated with materials and finishes that allow application to the food industry where they may be cleaned and/or steamed in place.

Two-wire Magnetic Flowmeters
Traditional flowmeters require more power than is available from a two-wire (4-20 mA) power source, so a separate power source is required. This power source is typically routed in an additional conduit/cable that increases the cost and complexity of the installation. Two-wire magnetic flowmeters are designed to operate using the power available from a two-wire signal power source.

Excerpted from The Consumer Guide to Magnetic Flowmeters

ISSN 1538-5280

Spitzer and Boyes, LLC
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