UNIT-II SRM BMI [Compatibility Mode]

8/6/2019 UNIT-II SRM BMI [Compatibility Mode]

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The current crosses it from left to right.

The electrode consists of metallic atomsC. The electrolyte is an aqueous solutioncontaining cations of the electrode metalC+ and anions A-.

CurrentFlowattheElectrodeElectrolyteInterface

Electronsmoveinoppositedirectiontocurrentflow

Cations(C+)moveinsame

directionascurrentflow

Anions(A)moveinopposite

directionofcurrentflow

Chemicaloxidation(currentflowright) reduction(currentflow

left)reactionsattheinterface:

C C++e (5.1)

AA + e

(5.2)

Nocurrentatequilibrium

Electron flow Ion- flow

+ Current flow

Ion+ flow

HalfCellPotential

Whenmetal(C)contactselectrolyte,oxidation(C C++e)orreduction(AA +e

)beginsimmediately.

Localconcentrationofcationsatthesurfacechanges.

Chargebuildsupintheregions.

Electrolytesurroundingthemetalassumesadifferentelectricpotentialfromtherestofthesolution.

Thispotentialdifferenceiscalledthehalfcellpotential(E0).

Separationofchargeattheelectrodeelectrolyteinterfaceresultsinaelectric

doublelayer(bilayer).

Measuringthehalfcellpotentialrequirestheuseofasecondreferenceelectrode.

Byconvention,thehydrogenelectrodeischosenasthereference.


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HalfCellPotentialsofCommonMetalsat25C

Metal PotentialE0(volts)

Al 1.706

Zn 0.763

Cr 0.744

Fe 0.409

Cd 0.401

Ni 0.230

Pb 0.126

H 0.000

AgCl +0.223

Hg2Cl2 +0.268

Cu +0.522

Ag +0.799

Au +1.680

By definition: Hydrogen isbubbled over a platinumelectrode and the potential isdefined as zero.

ElectrodePolarization

Standardhalfcellpotential(E0):

NormallyE0isanequilibriumvalueandassumeszerocurrentacrosstheinterface.

Whencurrentflows,thehalfcellpotential,E0,changes.

Overpotential(Vp):

Differencebetweennonzerocurrentandzerocurrenthalfcellpotentials;alsocalledthe

polarizationpotential(Vp).


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Componentsoftheoverpotential(Vp):

Ohmic(Vr):Duetotheresistanceoftheelectrolyte(voltagedropalongthepathofionicflow).

Concentration(Vc):Duetoaredistributionoftheionsinthevicinityoftheelectrodeelectrolyteinterface(concentration

changes).

Activation(Va):Duetometalionsgoingintosolution(mustovercomeanenergybarrier,theactivationenergy)ordueto

metalplatingoutofsolutionontotheelectrode(asecond

activationenergy).

Vp = Vr + Vc + Va

ElectrodePolarization

Nernst

Equation

Governsthehalfcellpotential:

where

E halfcellpotential

E0 standardhalfcellpotential

(theelectrodeinanelectrolytewithunity

activityatstandardtemperature)

R universalgasconstant[8.31J/(molK)]

T absolutetemperatureinK

n valenceoftheelectrodematerial

F Faradayconstant[96,500C/(mol/valence)]

ionicactivityofcationCn+

(itsavailabilitytoenterintoareaction)

(5.6)


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Ag/AgClFabrication

Electrolyticprocess

LargeAg/AgClelectrodeservesasthecathode.

SmallerAgelectrodetobechloridized

serves

as

the

anode.

A1.5voltbatteryistheenergysource.

Aresistorlimitsthecurrent.

Amilliammetermeasurestheplatingcurrent.

Reactionhasaninitialsurgeofcurrent.

Whencurrentapproachesasteadystate(about10A),theprocessisterminated.

Cathode Anode

A

Electrochemical Cell

Sintered

Ag/Ag

Electrode

Figure 5.3

SinteringProcess

AmixtureofAgandAgClpowderispressedintoapellet

aroundasilverleadwire.

Bakedat400Cforseveralhours.

Knownforgreatendurance(surfacedoesnotflakeoffasin

theelectrolyticallygenerated

electrodes).

SilverpowderisaddedtoincreaseconductivitysinceAgCl

isnotagoodconductor.


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CalomelElectrode Calomelismercurouschloride

(Hg2Cl2).

Approachesperfectlynonpolarizingbehavior

UsedasareferenceinpHmeasurements.

Calomelpasteisloadedintoaporousglassplugattheendofaglasstube.

ElementalHgisplacedontopwithaleadwire.

TubeisinsertedintoasaturatedKClsolutioninasecondglasstube.

Asecondporousglassplugformsaliquidliquidinterfacewiththeanalytebeingmeasured.

Hg2Cl2

Hg

K Cl

Electrolyte being measured

porous glass plug

Lead Wire

Electrode

Circuit

Model

Ehcisthehalfcellpotential

Cdisthecapacitanceoftheelectricdoublelayer

(polarizableelectrode

properties).

Rdisresistancetocurrentflowacrosstheelectrode

electrolyteinterface(non

polarizableelectrode

properties).

Rsistheseriesresistanceassociatedwiththe

conductivityoftheelectrolyte.

Athighfrequencies:Rs

Atlowfrequencies:Rd+Rs

Figure 5.4


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Ag/AgClElectrodeImpedance

Figure 5.5 Impedance as a function of frequency for Ag electrodes coated withan electrolytically deposited AgCl layer. The electrode area is 0.25 cm2.Numbers attached to curves indicate the number of mA

s for each deposit.

Nichel &CarbonLoadedSilicone

Figure 5.6

Electrode area is 1.0 cm2


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SkinAnatomy

Figure 5.7

ElectrodeSkinInterfaceModel

A body-surface electrode is placed against skin, showingthe total electrical equivalent circuit obtained in this

situation. Each circuit element on the right is atapproximately the same level at which the physical processthat it represents would be in the left-hand diagram.

Sweat glandsand ducts

Electrode

Epidermis

Dermis andsubcutaneous layer R

u

Re

Ese

Ehe

Rs

RdCd

EP

RPCPCe

Gel

Motionartifact:

Gelisdisturbed,thechargedistributionis

perturbedchanging

thehalfcell

potentialsatthe

electrodeandskin.

Minimizedbyusingnonpolarizable

electrodeand

mechanicalabrasion

ofskin.

Skinregeneratesin24hours.


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TRANSDUCER Atransducerisadevice,usuallyelectrical,electronic,electromechanical,electromagnetic,photonic,orphotovoltaicthatconvertsonetypeofenergyor

physicalattributetoanotherforvariouspurposesincludingmeasurementorinformationtransfer(forexample,pressuresensors).

Thetermtransduceriscommonlyusedintwosenses;thesensor,usedtodetectaparameterinoneformandreportitinanother(usuallyanelectricalordigitalsignal),andtheaudioloudspeaker,whichconvertselectricalvoltagevariationsrepresentingmusicorspeech,tomechanicalconevibrationandhence

vibratesairmoleculescreatingacousticalenergy.

Whatistransducer?

Non-electrical physical quantity: temperature, sound or light

Electrical signal


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TRANSDUCERS

TRANSDUCERS


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TRANSDUCERS

RESISTIVE TRANSDUCERS


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RESISTIVE TRANSDUCERS

CAPACITIVE TRANSDUCERS


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INDUCTIVE TRANSDUCERS

INDUCTIVE TRANSDUCERS


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TEMPERATURE TRANSDUCERS



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Selection criteria

Some of the charateristics for designing medical equipmentare:Accuracy

Frequency response

Hysteresis

Isolation

Linearity

Sensitivity

Signal to noise ratio

Simplicity

Stability

precision

DIRECT WRITING RECORDERS


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HEATED STLUS SYSTEM

SERVOLOOP FEEDBACK PEN MOTOR

SYSTEM


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INKJET WRITING MECHANISM

SELF-BALANCING

POTENTIOMETRIC RECORDER


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THERMAL ARRAY RECORDER

ELECTROSTATIC RECORDER


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INSTRUMENTATION TAPE

RECORDERS

Extensive use in hospitals

DC input to several kHZ signal

Tapes can be erased number of times

Tapes can be played any number of times

UNIT-II SRM BMI [Compatibility Mode]

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