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1Company Confidential
Series 8700 Magnetic Series 8700 Magnetic FlowmetersFlowmetersMagmeter Theory
2
Michael Faraday (1791 - 1867)
Magmeter TheoryMagmeter TheoryFaraday’s Law of Electromagnetic InductionFaraday’s Law of Electromagnetic Induction
• A voltage will be induced in a conductor moving in a magnetic field (E)
• The magnitude of that induced voltage is proportional – to the velocity of the conductor (V)
– to the length of the conductor (D)
– to the strength of the magnetic field (B)
E = k * B * D * V
3
Magmeter TheoryMagmeter TheoryFaraday’s Law and the FlowtubeFaraday’s Law and the Flowtube
SensingElectrodes
FieldCoils
Lining
ConductiveProcess Medium
VariableFlow Rate
(ft/s)SST Tube
Flange
“E”
“E”
Magnetic Field “B”
Faraday’s Law: E=kBDV
k=Proportionality ConstantB=Magnetic Field StrengthD=Distance between ElectrodesV=Velocity of Process FluidE=Induced voltage (linear with velocity)
4
Magmeter TheoryMagmeter TheoryFaraday’s Law and the Flow RateFaraday’s Law and the Flow Rate
• Magmeters calculate fluid velocity (V) by measuring the induced voltage (E) on the electrodes
• Q = V * A– Volumetric flow rate (Q) is
velocity (V) times cross-sectional area (A)
Av
s
E = k * B * D * V
Not effected by changes in fluid
conductivity!
5
Magmeter TheoryMagmeter TheoryFluid ConductivityFluid Conductivity
• Magmeters Require a Conductive Process Fluid– No Gases
– No Entrapped Air, Foam, or Two-Phase Flow
– Minimum 5 µSiemens/cm• Conductance is the reciprocal of Resistance and is
measured in Siemens (formerly Mhos)
• 10 Siemens = 0.1 Ohm
• 5 µSiemens/cm = 200 kOhm
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Coil ExcitationCoil Excitation CharacteristicsCharacteristics
• DC (0.5% of rate accuracy)– More accurate
– Continuous automatic zeroing
– Magnetic field immune to power line disruptions
• AC (1% of rate accuracy)– Poor zero stability
– AC power required at flowtube
– High maintenance
– High power consumption
– Good process noise rejection
8
0
+
-
AMPLITUDE
One Measurement Cycle =1/10 Line Frequency (6 Hz)
V Noise
E
E
1 2 3 4 5 6 7
FlowSignal
(E + VNoise) - (-E + VNoise) = 2E
Magnetic field transition begins
Transients present due to transitionof magnetic field
Magnetic field and flow signal stabilize
Flow signal sampled
Magnetic field transition begins
Transients present due to transitionof magnetic field
Magnetic field and flow signal stabilize
Flow signal sampled; difference takenfrom previous sample. Flow ratecalculated.
1
1-2
2-3
3-4
4
4-5
5-6
6-7
EVENT SEQUENCE
Output Stage of transmitter is updated 12 times per second regardless of coil drive frequency
High-Signal coil current 5.0 ANormal 8705/8711 coil current 0.5 A
Coil ExcitationCoil ExcitationPulsed DC Flow Signal DiagramPulsed DC Flow Signal Diagram
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Positive (A1)quadraturevoltage signal
Negative (A2)quadrature
voltage signal
Positiveflow signal
Time
Negativeflow signal
90 degreesphasedifferential
Transmittersensing positive
Transmittersensing negative
+
0
-
When A1 = A2, quadrature voltage cancels
In phasenoise signal
Coil ExcitationCoil ExcitationAC Flow Signal DiagramAC Flow Signal Diagram
10
Coil ExcitationCoil ExcitationMagmeters with AC ExcitationMagmeters with AC Excitation
Measurement Signal
Inductively coupled interference voltages
High Excitation frequency results in fast response time for a safe and stable measurement
Disadvantage: phase shift between signal and noise varies over time
Result: Zero point drift
11
Coil ExcitationCoil ExcitationZero Point DriftZero Point Drift
AC-magmeter DC-magmeter
% Errorof actualflow rate
5/11/985/10/98 5/12/98 5/13/98
10
6
14
2
-2
-6
-10
0
-14
12
Magmeter Advantages / LimitationsMagmeter Advantages / Limitations• Advantages
– Obstructionless, no pressure drop
– Measures in both directions
– Immune to changes in process variables
– Output linear with flowrate
– High accuracy (0.5% of rate) & high rangeability (30:1)
– Process flow profile has minimal effect on measurement accuracy
– Compatible with wide range of process fluids
• Limitations– Measures conductive fluids only; no gases
– 4-wire device