Rosemount 8732C, Руководство пользователя

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Rosemount Model 8732C Integral Mount Magnetic Flowmeter System
D-2
PRIMARY AND
SECONDARY DEVICES The Series 8700 System consists of primary and secondary devices.
The primary Series 8700 Flowtube is a pipe section with coils and
electrodes. The secondary Series 8700 Transmitter generates the coil
drive signal. The drive signal creates a magnetic field in the flowtube.
The Series 8700 interprets the voltage generated at the electrodes and
transmits a standardized signal to the readout or control system.
Series 8700 Magnetic
Flowmeter Flowtube
The primary flowtube function is to produce a voltage proportional to
the velocity of the liquid being measured. The field coils, energized by a
pulsed dc current, develop the magnetic field. The process fluid
functions as a moving conductor. Voltage is induced in the liquid as it
flows through the magnetic field. The electrodes make a direct
electrical connection with the conductive process fluid and detect
voltages present. The liquid must be contained in an insulated material
so that the voltage is not shorted. This is accomplished with a
nonconductive flowtube material, or with a nonconductive lining for
metal flowtubes. Because the magnetic field is typically created outside
the flowtube, both the flowtube and flowtube liner must be
nonmagnetic to prevent interference with the field.
Series 8712 Magnetic
Flowmeter Transmitter
Signal conversion, conditioning, and transmission are principle Series
8700 System functions. The transmitter translates the millivolt
flowtube output detected on the electrodes into a usable signal for
process control or monitoring. The pulsed dc design of the Series 8700
System makes this signal particularly resistant to electrical
interference. For a remote-mounted transmitter, this design allows
the coil drive and electrode cables to share a single, dedicated conduit.
The conductivity of the process fluid is a factor in specifying a cable
run of up to 1,000 feet between the flowtube and a remote-mounted
transmitter.
ELECTRICAL DESIGN The pulsed dc design of the Series 8700 System represents an advanced
application of Faraday’s law. Because ideal environments (free from
extraneous voltage and noise) seldom exist, this design primarily avoids
conflict rather than compensates for it.
Unwanted noise has many sources. These include:
• Electrochemical voltage resulting from electrolytic reaction
between the metal electrode and the ion-conducting process fluid.
• Inductive coupling of the magnetic field to the internal electrode
wiring and the process fluid (commonly called quadrature voltage
with ac systems).
• Capacitive coupling of the coil voltage or outside power systems to
the electrode circuit.
• Transmission losses or phase shifts resulting from fluid
impedance and transmission cable capacitance.
• Stray voltage or current loops within the process fluid.
Pulsed dc
Field Coil Advantages Pulsed dc systems are immune to electrochemical noise without
associated quadrature and other induced voltages inherent in an ac
design. Pulsed dc systems power the coils with a controlled amplitude
low frequency square wave. The flow signal is a matching square wave
with an amplitude proportional to velocity of the liquid conductor.
There is no need to compensate for voltage and frequency variations on
the ac power line.