The industry's most authoritative handbook on flow measurement provides a road map to the field of flow measurement. This best-seller discusses strategies for problem solving and puts the whole array of types of flowmeters at the reader's disposal. The text includes laminar flow elements, critical flowmeters, statistics for measurement, laboratory primary standards, and uncertainty in flow measurement. Emphasis is placed on the importance of accuracy in measurements and ways of ensuring accuracy and avoiding equipment damage through correct forecast of operating conditions, flowmeter selection, installation, calibration, and maintenance. Fundamental considerations such as mixed-phase flow, piping effects, and flow conditioning are examined at length. The problem of attaining a meaningful flow signal through linearization, compensation, and totalization is discussed. Join the thousands of engineers, technicians, managers, and salespeople that have found this reference text an invaluable resource.
TABLE OF CONTENTS Chapter 11 - Magnetic Flowmeters
| Magnetic flowmeters (magmeters) are designed to measure the flow of electrically conductive liquids in a closed pipe. They are volumetric flow-measuring devices and have been commercially available since the mid-1950s. Sizes range from 1/25 inch to about 120 inches from most manufacturers. This covers a flow range of about 0.003 gallon per minute to about 1,100,000 gallons per minute. Early units were flanged devices that bolted to adjacent pipe flanges in much the same way as an ordinary section of flanged pipe. They were large, heavy, and expensive, but they offered several advantages over other flowmeters available at that time. | Advantages:
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With the obstructionless design, there are no moving parts to wear and no pressure
drop other than that offered by a section of pipe with equal length and inside
diameter. The only wetted parts are the electrodes and an insulating liner. These
can be selected for compatibility with the most corrosive of chemicals as well as
to meet sanitary requirements for food applications such as milk and other liquid
dairy products. The output signal is linear and directly proportional to the flow
velocity. Accuracy over a wide range (typically 30 to 1) has evolved from 1% of
full scale reading to 0.5% of rate as typical. Higher accuracies are available for
special applications. The basic fundamentals of operation, selection, installation,
and maintenance will be discussed in this chapter. The discussion will be basic
and generic. Specifics on availability of features and options can be obtained from
the various magmeter manufacturers.
Operating Principle
Magnetic flowmeters operate on the principle of Michael Faraday's law of electromagnetic
induction. Without getting into the mathematics of this theory, it can
be simply stated as follows:
| E = constant × B × L × v | (11-1) |
| where: | |||
| E | = | magnitude of the voltage | |
| B | = | magnetic field density | |
| L | = | path length | |
 | = | average velocity of the medium |
Equation (11-1) implies that a voltage is developed when a conductor is passed
through a magnetic field. It further states that the voltage developed is proportional
to the density of the magnetic field, the length of the conductor, and the
velocity of the conductor moving through the field. There is nothing in the equation
about temperature, pressure, density, or viscosity because the magmeter
develops its signal independent of these parameters.
