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4.0 FUNCTIONAL DESCRIPTION
The Flowmeter body houses two signal electrodes and the flux producing magnet coils, as shown
schematically in Figure 4-1. All Flowmeter intraconnection wiring is terminated at a printed circuit
board assembly located in the base of the flowmeter housing.
The flowmeter provides two output signals to the associated signal converter:
• an electrode signal that contains the flow rate information
• the reference signal which is proportional to the magnet excitation current
(theoretically, this reference signal is proportional to the flux density in the
metering section).
The reference voltage is derived from a precision current transformer and its load impedance network
that is connected in series with the magnet coils. Changes in magnet drive voltage, which cause a
variation of flow signal, will simultaneously cause a proportional variation of the reference voltage.
The converter circuitry will provide an exact ratio and thereby provide immunity to power supply
variation. The magnet coils are excited by the 120 VAC 60 Hz power line. In some sizes, the line
voltage is stepped down via an additional transformer.
4.1 Basic Operating Principle
4.1.1 Signal Voltage Generation
The operating principle of the flowmeter is based upon Faraday's Law of Induction which states that
the voltage induced across any conductor as it moves at right angles through a magnetic field will be
proportional to the velocity of that conductor. This principle finds common application in direct and
alternating current generators. Essentially, the magnetic flowmeter constitutes a modified form of a
generator.
FIGURE 4-1. BASIC OPERATING PRINCIPLE
10DS3111A INSTRUCTION MANUAL
Figure 4-1 graphically illustrates the basic
operating principle. A magnetic field, "B", is
being generated in a plane which is perpen-
dicular to the axis of the meter pipe. A disk
of the metered liquid can be considered as a
conductor. The transverse length "D" is
equal to the meter pipe diameter. Since the
velocity "V" of the liquid disk is directed
along the axis of the meter pipe, a signal
voltage, "E
", will be induced within this liq-
s
uid which is mutually perpendicular to the
direction of the liquid velocity and the flux
linkages of the magnetic field; i.e., in the
axial direction of the meter electrodes. This
electrode voltage is the summation of all
incremental voltages developed within each
liquid particle that passes under the influ-
ence of the magnetic field.
4-1
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