ABB FSM4000 Series, Operating Instruction

Page: 9 / 136

FSM4000 ELECTROMAGNETIC FLOWMETER | OI/FSM4000-EN REV. E 9
Measuring principle
Measurements performed by the electromagnetic flowmeter are
based on Faraday’s law of induction. A voltage is generated in a
conductor when it moves through a magnetic field.
1
Magnet coil
2
Measuring tube in electrode plane
3
Measuring electrode
Figure 1: Electromagnetic flowmeter diagram
v D B U × ×
~
1
v
D
qv
×
×
=
4
2
π
qv U
~
1
U
1
Measuring span
B Magnetic induction
D Electrode spacing
v Average flow velocity
qv Volume flow rate
With the device-relevant application of this measuring principle,
a conductive measuring medium flows through a tube in which a
magnetic field is generated perpendicular to the flow direction
(see Figure 1).
The voltage induced in the measuring medium is tapped by two
diametrically opposed electrodes. This measurement voltage is
proportional to the magnetic induction, the electrode spacing
and the average medium velocity v.
Taking into account that the magnetic induction and the
electrode spacing are constant values results in a proportion
between the measurement voltage U
1
and the average medium
velocity.
From the calculation of the volume flow rate follows that the
measurement voltage is linear and proportional to the volume
flow rate
The induced voltage is converted by the transmitter to
standardized, analog and digital signals.
Design
An electromagnetic flowmeter system consists of a flowmeter
sensor and a transmitter. The flowmeter sensor (model SE41F,
SE21W, SE21F, SE21) is installed in the specified pipeline while
the transmitter (S4) is mounted locally or at a central location.
Device designs
Figure 2: Device designs
The µP transmitter is mounted at a separate location from the
sensor. Up to 50 m signal cable length for a minimum
conductivity of 20 µS/cm. For sensors with preamplifiers, the
signal cable length is increased to 200 m. The electrical
connection between the transmitter and the sensor primary is
provided by a signal cable and a magnetic coil cable in the
terminal box.
The transmitter is available in the design:
• Field mount housing model S4
The flowmeter sensor comes with aluminum or stainless steel
housing:
• Aluminum housing: model FSM4000-SE41F
• Stainless steel housing: model FSM4000-SE21W / -SE21F
/ -SE21
Note
Older model flowmeter sensors can be connected to S4
transmitters. For additional information, see
Checks before
commissioning on page 46or S4 operation with older sensor on
page 109.