1Company Confidential

Series 8700 Magnetic Series 8700 Magnetic FlowmetersFlowmetersMagmeter Theory

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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

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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)

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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!

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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

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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

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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

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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

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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