ThesetestmethodsareunderthejurisdictionofASTMCommitteeD-9onElectricalandElectronicInsulatingMaterialsandarethedirectresponsibilityofSubcommitteeD09.12onElectricalTests.
CurrenteditionapprovedOct.10,1999.PublishedNovember1999.OriginallypublishedasD257–25T.LastpreviouseditionD257–93(1998).
123AnnualAnnual4Annual5Annual6AnnualBookBookBookBookBookofofofofofASTMASTMASTMASTMASTMStandards,Standards,Standards,Standards,Standards,VolVolVolVolVol10.01.08.01.10.03.10.02.11.03.
Copyright©ASTM,100BarrHarborDrive,WestConshohocken,PA19428-2959,UnitedStates.
1
conductancetothatobtainediftheelectrodeshadformedtheoppositesidesofasquare.
3.1.4.1Discussion—Surfaceconductivityisexpressedinsiemens.Itispopularlyexpressedassiemens/square(thesizeofthesquareisimmaterial).Surfaceconductivityisthereciprocalofsurfaceresistivity.
3.1.5conductivity,volume,n—thevolumeconductancemultipliedbythatratioofspecimenvolumedimensions(distancebetweenelectrodesdividedbythecross-sectionalareaoftheelectrodes)whichtransformsthemeasuredconduc-tancetothatconductanceobtainediftheelectrodeshadformedtheoppositesidesofaunitcube.
3.1.5.1Discussion—Volumeconductivityisusuallyex-pressedinsiemens/centimetreorinsiemens/metreandisthereciprocalofvolumeresistivity.
3.1.6moderatelyconductive,adj—describesasolidmate-rialhavingavolumeresistivitybetween1and10000000V-cm.
3.1.7resistance,insulation,(Ri),n—theratioofthedcvoltageappliedtotwoelectrodes(onorinaspecimen)tothetotalvolumeandsurfacecurrentbetweenthem.
3.1.7.1Discussion—Insulationresistanceisthereciprocalofinsulationconductance.
3.1.8resistance,surface,(Rappliedtotwoelectrodes(onthes),n—theratioofthedcvoltagesurfaceofaspecimen)tothecurrentbetweenthem.
3.1.8.1Discussion—(Somevolumeresistanceisunavoid-ablyincludedintheactualmeasurement.)Surfaceresistanceisthereciprocalofsurfaceconductance.
3.1.9resistance,volume,(Rv),n—theratioofthedcvoltageappliedtotwoelectrodes(onorinaspecimen)tothecurrentinthevolumeofthespecimenbetweentheelectrodes.
3.1.9.1Discussion—Volumeresistanceisthereciprocalofvolumeconductance.
3.1.10resistivity,surface,(rs),n—thesurfaceresistancemultipliedbythatratioofspecimensurfacedimensions(widthofelectrodesdefiningthecurrentpathdividedbythedistancebetweenelectrodes)whichtransformsthemeasuredresistancetothatobtainediftheelectrodeshadformedtheoppositesidesofasquare.
3.1.10.1Discussion—Surfaceresistivityisexpressedinohms.Itispopularlyexpressedalsoasohms/square(thesizeofthesquareisimmaterial).Surfaceresistivityisthereciprocalofsurfaceconductivity.
3.1.11resistivity,volume,(rofspecimenv),n—thevolumeresistancemultipliedbythatratiovolumedimensions(cross-sectionalareaofthespecimenbetweentheelectrodesdividedbythedistancebetweenelectrodes)whichtransformsthemeasuredresistancetothatresistanceobtainediftheelectrodeshadformedtheoppositesidesofaunitcube.
3.1.11.1Discussion—Volumeresistivityisusuallyex-pressedinohm-centimetres(preferred)orinohm-metres.Volumeresistivityisthereciprocalofvolumeconductivity.4.SummaryofTestMethods
4.1Theresistanceorconductanceofamaterialspecimenorofacapacitorisdeterminedfromameasurementofcurrentorofvoltagedropunderspecifiedconditions.Byusingtheappropriateelectrodesystems,surfaceandvolumeresistance
orconductancemaybemeasuredseparately.Theresistivityorconductivitycanthenbecalculatedwhentherequiredspeci-menandelectrodedimensionsareknown.
5.SignificanceandUse
5.1Insulatingmaterialsareusedtoisolatecomponentsofanelectricalsystemfromeachotherandfromground,aswellastoprovidemechanicalsupportforthecomponents.Forthispurpose,itisgenerallydesirabletohavetheinsulationresis-tanceashighaspossible,consistentwithacceptablemechani-cal,chemical,andheat-resistingproperties.Sinceinsulationresistanceorconductancecombinesbothvolumeandsurfaceresistanceorconductance,itsmeasuredvalueismostusefulwhenthetestspecimenandelectrodeshavethesameformasisrequiredinactualuse.Surfaceresistanceorconductancechangesrapidlywithhumidity,whilevolumeresistanceorconductancechangesslowlyalthoughthefinalchangemayeventuallybegreater.
5.2Resistivityorconductivitymaybeusedtopredict,indirectly,thelow-frequencydielectricbreakdownanddissi-pationfactorpropertiesofsomematerials.Resistivityorcontivityisoftenusedasanindirectmeasureofmoisturecontent,degreeofcure,mechanicalcontinuity,anddeteriora-tionofvarioustypes.Theusefulnessoftheseindirectmeasure-mentsisdependentonthedegreeofcorrelationestablishedbysupportingtheoreticalorexperimentalinvestigations.Ade-creaseofsurfaceresistancemayresulteitherinanincreaseofthedielectricbreakdownvoltagebecausetheelectricfieldintensityisreduced,oradecreaseofthedielectricbreakdownvoltagebecausetheareaunderstressisincreased.
5.3Allthedielectricresistancesorconductancesdependonthelengthoftimeofelectrificationandonthevalueofappliedvoltage(inadditiontotheusualenvironmentalvariables).Thesemustbeknowntomakethemeasuredvalueofresistanceorconductancemeaningful.
5.4Volumeresistivityorconductivitycanbeusedasanaidindesigninganinsulatorforaspecificapplication.Thechangeofresistivityorconductivitywithtemperatureandhumiditymaybegreat(1,2,3,4),7andmustbeknownwhendesigningforoperatingconditions.Volumeresistivityorconductivitydeterminationsareoftenusedincheckingtheuniformityofaninsulatingmaterial,eitherwithregardtoprocessingortodetectconductiveimpuritiesthataffectthequalityofthematerialandthatmaynotbereadilydetectablebyothermethods.
5.5Volumeresistivitiesabove1021V·cm(1019V·m),ob-tainedonspecimensunderusuallaboratoryconditions,areofdoubtfulvalidity,consideringthelimitationsofcommonlyusedmeasuringequipment.
5.6Surfaceresistanceorconductancecannotbemeasuredaccurately,onlyapproximated,becausesomedegreeofvolumeresistanceorconductanceisalwaysinvolvedinthemeasure-ment.Themeasuredvalueisalsoaffectedbythesurfacecontamination.Surfacecontamination,anditsrateofaccumu-lation,isaffectedbymanyfactorsincludingelectrostaticchargingandinterfacialtension.These,inturn,mayaffectthe
7Theboldfacenumbersinparenthesesrefertothelistofreferencesappendedtothesetestmethods.
surfaceresistivity.Surfaceresistivityorconductivitycanbeconsideredtoberelatedtomaterialpropertieswhencontami-nationisinvolvedbutisnotamaterialpropertyintheusualsense.
6.ElectrodeSystems
6.1Theelectrodesforinsulatingmaterialsshouldbeofamaterialthatisreadilyapplied,allowsintimatecontactwiththespecimensurface,andintroducesnoappreciableerrorbecauseofelectroderesistanceorcontaminationofthespecimen(5).Theelectrodematerialshouldbecorrosion-resistantundertheconditionsoftest.Fortestsoffabricatedspecimenssuchasfeed-throughbushings,cables,etc.,theelectrodesemployedareapartofthespecimenoritsmounting.Measurementsofinsulationresistanceorconductance,then,includethecontami-natingeffectsofelectrodeormountingmaterialsandaregenerallyrelatedtotheperformanceofthespecimeninactualuse.
6.1.1Binding-PostandTaper-PinElectrodes,Fig.1andFig.2,provideameansofapplyingvoltagetorigidinsulatingmaterialstopermitanevaluationoftheirresistiveorconduc-tiveproperties.Theseelectrodessimulatetosomedegreetheactualconditionsofuse,suchasbindingpostsoninstrumentpanelsandterminalstrips.Inthecaseoflaminatedinsulatingmaterialshavinghigh-resin-contentsurfaces,somewhatlowerinsulationresistancevaluesmaybeobtainedwithtaper-pinthanwithbindingposts,duetomoreintimatecontactwiththebodyoftheinsulatingmaterial.Resistanceorconductancevaluesobtainedarehighlyinfluencedbytheindividualcontactbetweeneachpinandthedielectricmaterial,thesurfaceroughnessofthepins,andthesmoothnessoftheholeinthedielectricmaterial.Reproducibilityofresultsondifferentspecimensisdifficulttoobtain.
6.1.2MetalBarsinthearrangementofFig.3wereprima-rilydevisedtoevaluatetheinsulationresistanceorconduc-tanceofflexibletapesandthin,solidspecimensasafairlysimpleandconvenientmeansofelectricalqualitycontrol.Thisarrangementissomewhatmoresatisfactoryforobtainingapproximatevaluesofsurfaceresistanceorconductancewhenthewidthoftheinsulatingmaterialismuchgreaterthanitsthickness.
6.1.3SilverPaint,Fig.4,Fig.5,andFig.6,isavailablecommerciallywithahighconductivity,eitherair-dryingorlow-temperature-bakingvarieties,whicharesufficientlypo-
FIG.1Binding-PostElectrodesforFlat,SolidSpecimens
FIG.2Taper-PinElectrodes
FIG.3StripElectrodesforTapesandFlat,SolidSpecimens
roustopermitdiffusionofmoisturethroughthemandtherebyallowthetestspecimentobeconditionedaftertheapplicationoftheelectrodes.Thisisaparticularlyusefulfeatureinstudyingresistance-humidityeffects,aswellaschangewithtemperature.However,beforeconductivepaintisusedasanelectrodematerial,itshouldbeestablishedthatthesolventinthepaintdoesnotattackthematerialsoastochangeits
VolumeResistivityg|Ls2tSurfaceResistivity
FIG.4FlatSpecimenforMeasuringVolumeandSurface
ResistancesorConductances
D05(D1+D2)/2L>4tg|La2tVolumeResistivityg|Ls2tSurfaceResistivity
FIG.5TubularSpecimenforMeasuringVolumeandSurface
ResistancesorConductances
electricalproperties.Reasonablysmoothedgesofguardelec-trodesmaybeobtainedwithafine-bristlebrush.However,forcircularelectrodes,sharperedgescanbeobtainedbytheuseofarulingcompassandsilverpaintfordrawingtheoutlinecirclesoftheelectrodesandfillingintheenclosedareasbybrush.Anarrowstripofmaskingtapemaybeused,providedthepressure-sensitiveadhesiveuseddoesnotcontaminatethe
surfaceofthespecimen.Clamp-onmasksalsomaybeusediftheelectrodepaintissprayedon.
6.1.4SprayedMetal,Fig.4,Fig.5,andFig.6,maybeusedifsatisfactoryadhesiontothetestspecimencanbeobtained.Thinsprayedelectrodesmayhavecertainadvantagesinthattheyarereadyforuseassoonasapplied.Theymaybesufficientlyporoustoallowthespecimentobeconditioned,butthisshouldbeverified.Narrowstripsofmaskingtapeorclamp-onmasksmustbeusedtoproduceagapbetweentheguardedandtheguardelectrodes.Thetapeshallbesuchasnottocontaminatethegapsurface.
6.1.5EvaporatedMetalmaybeusedunderthesamecon-ditionsgivenin6.1.4.
6.1.6MetalFoil,Fig.4,maybeappliedtospecimensurfacesaselectrodes.Theusualthicknessofmetalfoilusedforresistanceorconductancestudiesofdielectricsrangesfrom6to80µm.Leadortinfoilisinmostcommonuse,andisusuallyattachedtothetestspecimenbyaminimumquantityofpetrolatum,siliconegrease,oil,orothersuitablematerial,asanadhesive.Suchelectrodesshallbeappliedunderasmoothingpressuresufficienttoeliminateallwrinkles,andtoworkexcessadhesivetowardtheedgeofthefoilwhereitcanbewipedoffwithacleansingtissue.Oneveryeffectivemethodistouseahardnarrowroller(10to15mmwide),andtorolloutwardonthesurfaceuntilnovisibleimprintcanbemadeonthefoilwiththeroller.Thistechniquecanbeusedsatisfactorilyonlyonspecimensthathaveveryflatsurfaces.Withcare,theadhesivefilmcanbereducedto2.5µm.Asthisfilmisinserieswiththespecimen,itwillalwayscausethemeasuredresistancetobetoohigh.Thiserrormaybecomeexcessiveforthelower-resistivityspecimensofthicknesslessthan250µm.Alsothehardrollercanforcesharpparticlesintoorthroughthinfilms(50µm).Foilelectrodesarenotporousandwillnotallowthetestspecimentoconditionaftertheelectrodeshavebeenapplied.Theadhesivemayloseitseffectivenessatelevatedtemperaturesnecessitatingtheuseofflatmetalback-upplatesunderpressure.Itispossible,withtheaidofasuitablecuttingdevice,tocutaproperwidthstripfromoneelectrodetoformaguardedandguardelectrode.Suchathree-terminalspecimennormallycannotbeusedforsurfaceresistanceorconductancemeasurementsbecauseofthegreaseremainingonthegapsurface.Itmaybeverydifficulttocleantheentiregapsurfacewithoutdisturbingtheadjacentedgesoftheelectrode.
6.1.7ColloidalGraphite,Fig.4,dispersedinwaterorothersuitablevehicle,maybebrushedonnonporous,sheetinsulat-ingmaterialstoformanair-dryingelectrode.Maskingtapesorclamp-onmasksmaybeused(6.1.4).Thiselectrodematerialisrecommendedonlyifallofthefollowingconditionsaremet:6.1.7.1Thematerialtobetestedmustacceptagraphitecoatingthatwillnotflakebeforetesting,
6.1.7.2Thematerialbeingtestedmustnotabsorbwaterreadily,and
6.1.7.3Conditioningmustbeinadryatmosphere(Proce-dureB,MethodsD618),andmeasurementsmadeinthissameatmosphere.
6.1.8Mercuryorotherliquidmetalelectrodesgivesatisfac-toryresults.Mercuryisnotrecommendedforcontinuoususeoratelevatedtemperaturesduetotoxiceffects.(Warning—
FIG.6Conducting-PaintElectrodes
Mercurymetalvaporpoisoninghaslongbeenrecognizedasahazardinindustry.ThemaximumexposurelimitsaresetbytheAmericanConferenceofGovernmentalIndustrialHygienists.8Theconcentrationofmercuryvaporoverspillsfrombrokenthermometers,barometers,orotherinstrumentsusingmercurycaneasilyexceedtheseexposurelimits.Mercury,beingaliquidandquiteheavy,willdisintegrateintosmalldropletsandseepintocracksandcrevicesinthefloor.Theuseofacommerciallyavailableemergencyspillkitisrecommendedwheneveraspilloccurs.Theincreasedareaofexposureaddssignificantlytothemercuryvaporconcentrationinair.Mer-curyvaporconcentrationiseasilymonitoredusingcommer-ciallyavailablesniffers.Spotchecksshouldbemadeperiodi-callyaroundoperationswheremercuryisexposedtotheatmosphere.Thoroughchecksshouldbemadeafterspills.)Themetalformingtheupperelectrodesshouldbeconfinedbystainlesssteelrings,eachofwhichshouldhaveitslowerrimreducedtoasharpedgebybevelingonthesideawayfromtheliquidmetal.Fig.7AandFig.7Bshowtwoelectrodearrange-ments.
6.1.9FlatMetalPlates,Fig.4,(preferablyguarded)maybeusedfortestingflexibleandcompressiblematerials,bothatroomtemperatureandatelevatedtemperatures.Theymaybecircularorrectangular(fortapes).Toensureintimatecontactwiththespecimen,considerablepressureisusuallyrequired.Pressuresof140to700kPahavebeenfoundsatisfactory(seematerialspecifications).
6.1.9.1Avariationofflatmetalplateelectrodesystemsisfoundincertaincelldesignsusedtomeasuregreasesorfillingcompounds.Suchcellsarepreassembledandthematerialtobe
AmericanConferenceofGovernmentalandIndustrialHygienists,6500Glen-wayAve.,BuildingD-7,Cincinnati,OH,45211.
8NOTE1—Warning:See6.1.8
FIG.7MercuryElectrodesforFlat,SolidSpecimens
testediseitheraddedtothecellbetweenfixedelectrodesortheelectrodesareforcedintothematerialtoapredeterminedelectrodespacing.Becausetheconfigurationoftheelectrodesinthesecellsissuchthattheeffectiveelectrodeareaandthedistancebetweenthemisdifficulttomeasure,eachcellconstant,K,(equivalenttotheA/tfactorfromTable1)canbederivedfromthefollowingequation:
K53.6pC511.3C
(1)
NOTE1—Warning:See6.1.8
FIG.7MercuryCellforThinSheetMaterial(continued)where:
Khasunitsofcentimetres,and
Chasunitsofpicofaradsandisthecapacitanceoftheelectrodesystemwith
airasthedielectric.SeeTestMethodsD150formethodsofmeasurementforC.
6.1.10ConductingRubberhasbeenusedaselectrodema-terial,asinFig.4,andhastheadvantagethatitcanquicklyandeasilybeappliedandremovedfromthespecimen.Astheelectrodesareappliedonlyduringthetimeofmeasurement,theydonotinterferewiththeconditioningofthespecimen.Theconductive-rubbermaterialmustbebackedbyproperplatesandbesoftenoughsothateffectivecontactwiththespecimenisobtainedwhenareasonablepressureisapplied.
NOTE1—Thereisevidencethatvaluesofconductivityobtainedusingconductive-rubberelectrodesarealwayssmaller(20to70%)thanvaluesobtainedwithtinfoilelectrodes(6).Whenonlyorder-of-magnitudeaccuraciesarerequired,andthesecontacterrorscanbeneglected,aproperlydesignedsetofconductive-rubberelectrodescanprovidearapidmeansformakingconductivityandresistivitydeterminations.
6.1.11Wateriswidelyemployedasoneelectrodeintestinginsulationonwiresandcables.Bothendsofthespecimenmustbeoutofthewaterandofsuchlengththatleakagealongtheinsulationisnegligible.Guardringsmaybenecessaryateachend.Itmaybedesirabletoaddasmallamountofsodiumchloridetothewatertoensurehighconductivity.Measure-mentsmaybeperformedattemperaturesuptoabout100°C.7.ChoiceofApparatusandTestMethod
7.1PowerSupply—Asourceofverysteadydirectvoltageisrequired(seeX1.7.3).Batteriesorotherstabledirectvoltagesuppliesmaybeused.
7.2GuardCircuit—Whethermeasuringresistanceofaninsulatingmaterialwithtwoelectrodes(noguard)orwitha
three-terminalsystem(twoelectrodesplusguard),considerhowtheelectricalconnectionsaremadebetweenthetestinstrumentandthetestsample.Ifthetestspecimenisatsomedistancefromthetestinstrument,orthetestspecimenistestedunderhumidconditions,orifarelativelyhigh(1010to1015ohms)specimenresistanceisexpected,spuriousresistancepathscaneasilyexistbetweenthetestinstrumentandtestspecimen.Aguardcircuitisnecessarytominimizeinterferencefromthesespuriouspaths(seealsoX1.9).
7.2.1WithGuardElectrode—Usecoaxialcable,withthecoreleadtotheguardedelectrodeandtheshieldtotheguardelectrode,tomakeadequateguardedconnectionsbetweenthetestequipmentandtestspecimen.Coaxialcable(againwiththeshieldtiedbacktotheguard)fortheunguardedleadisnotmandatoryhere(orin7.2.2),althoughitsuseprovidessomereductioninbackgroundnoise(seealsoFig.8).
7.2.2WithoutGuardElectrode—Usecoaxialcable,withthecoreleadtooneelectrodeandtheshieldterminatedabout1cmfromtheendofthecorelead(seealsoFig.9).
7.3DirectMeasurements—Thecurrentthroughaspecimenatafixedvoltagemaybemeasuredusinganyequipmentthathastherequiredsensitivityandaccuracy(610%isusuallyadequate).Current-measuringdevicesavailableincludeelec-trometers,d-camplifierswithindicatingmeters,andgalva-nometers.TypicalmethodsandcircuitsaregiveninAppendixX3.Whenthemeasuringdevicescaleiscalibratedtoreadohmsdirectlynocalculationsarerequired.
7.4ComparisonMethods—AWheatstone-bridgecircuitmaybeusedtocomparetheresistanceofthespecimenwiththatofastandardresistor(seeAppendixX3).7.5PrecisionandBiasConsiderations:
7.5.1General—Asaguideinthechoiceofapparatus,thepertinentconsiderationsaresummarizedinTable2,butitisnotimpliedthattheexamplesenumeratedaretheonlyonesapplicable.Thistableisnotintendedtoindicatethelimitsofsensitivityanderrorofthevariousmethodsperse,butratherisintendedtoindicatelimitsthataredistinctlypossiblewithmodernapparatus.Inanycase,suchlimitscanbeachievedorexceededonlythroughcarefulselectionandcombinationoftheapparatusemployed.Itmustbeemphasized,however,thattheerrorsconsideredarethoseofinstrumentationonly.ErrorssuchasthosediscussedinAppendixX1areanentirelydifferentmatter.Inthislatterconnection,thelastcolumnofTable2liststheresistancethatisshuntedbytheinsulationresistancebetweentheguardedelectrodeandtheguardsystemforthevariousmethods.Ingeneral,thelowersuchresistance,thelessprobabilityoferrorfromundueshunting.
NOTE2—Nomatterwhatmeasurementmethodisemployed,thehighestprecisionsareachievedonlywithcarefulevaluationofallsourcesoferror.Itispossibleeithertosetupanyofthesemethodsfromthecomponentparts,ortoacquireacompletelyintegratedapparatus.Ingeneral,themethodsusinghigh-sensitivitygalvanometersrequireamorepermanentinstallationthanthoseusingindicatingmetersorrecorders.Themethodsusingindicatingdevicessuchasvoltmeters,galvanometers,d-camplifiers,andelectrometersrequiretheminimumofmanualadjustmentandareeasytoreadbuttheoperatorisrequiredtomakethereadingataparticulartime.TheWheatstonebridge(Fig.X1.4)andthepotentiometermethod(Fig.X1.2(b))requiretheundividedattentionoftheoperatorin
TABLE1CalculationofResistivityorConductivityATypeofElectrodesorSpecimen
rv5
Circular(Fig.4)RectangularSquare
Tubes(Fig.5)Cables
VolumeResistivity,V-cm
VolumeConductivity,S/cmgv5
p~D11g!2
A5
4AtRvtAGvrv5
2pLRvD2lnD1
A5(a+g)(b+g)A5(a+g)2A5pD0(L+g)D2D1
gv5
2pLRvln
SurfaceConductivity,S(persquare)
ggs5GsPCircular(Fig.4)RectangularSquare
Tubes(Figs.5and6)
SurfaceResistivity,V(persquare)
Pps5gRsP5pD0P52(a+b+2g)P54(a+g)P52pD2Nomenclature:A5theeffectiveareaofthemeasuringelectrodefortheparticulararrangementemployed,P5theeffectiveperimeteroftheguardedelectrodefortheparticulararrangementemployed,Rv5measuredvolumeresistanceinohms,
Gv5measuredvolumeconductanceinsiemens,Rs5measuredsurfaceresistanceinohms,
Gs5measuredsurfaceconductanceinsiemens,t5averagethicknessofthespecimen,
D0,D1,D2,g,L5dimensionsindicatedinFigs.4and6(seeAppendixX2forcorrectiontog),a,b,5lengthsofthesidesofrectangularelectrodes,andln5naturallogarithm.
AAlldimensionsareincentimetres.
keepingabalance,butallowthesettingataparticulartimetobereadatleisure.
7.5.2DirectMeasurements:
7.5.2.1Galvanometer-Voltmeter—Themaximumpercent-ageerrorinthemeasurementofresistancebythegalvanometer-voltmetermethodisthesumofthepercentageerrorsofgalvanometerindication,galvanometerreadability,andvoltmeterindication.Asanexample:agalvanometerhavingasensitivityof500pA/scaledivisionwillbedeflected25divisionswith500Vappliedtoaresistanceof40GV(conductanceof25pS).Ifthedeflectioncanbereadtothenearest0.5division,andthecalibrationerror(includingAyrtonShunterror)is62%oftheobservedvalue,theresultantgalvanometererrorwillnotexceed%.Ifthevoltmeterhasanerrorof62%offullscale,thisresistancecanbemeasuredwithamaximumerrorof66%whenthevoltmeterreadsfullscale,and610%whenitreadsone-thirdfullscale.Thedesirabilityofreadingsnearfullscalearereadilyapparent.7.5.2.2Voltmeter-Ammeter—Themaximumpercentageer-rorinthecomputedvalueisthesumofthepercentageerrorsinthevoltages,VxandVs,andtheresistance,Rs.TheerrorsinVsandRsaregenerallydependentmoreonthecharacteristicsoftheapparatususedthanontheparticularmethod.ThemostsignificantfactorsthatdeterminetheerrorsinVsareindicatorerrors,amplifierzerodrift,andamplifiergainstability.Withmodern,well-designedamplifiersorelectrometers,gainstabil-ityisusuallynotamatterofconcern.Withexistingtechniques,thezerodriftofdirectvoltageamplifiersorelectrometerscannotbeeliminatedbutitcanbemadeslowenoughtobe
FIG.8ConnectionstoGuardedElectrodeforVolumeandSurfaceResistivityMeasurements(VolumeResistancehook-upshown)
relativelyinsignificantforthesemeasurements.Thezerodrift
FIG.9ConnectionstoUnguardedElectrodesforUnguarded
SurfaceMeasurements
isvirtuallynonexistentforcarefullydesignedconverter-typeamplifiers.Consequently,thenullmethodofFig.X1.2(b)istheoreticallylesssubjecttoerrorthanthosemethodsemploy-inganindicatinginstrument,provided,however,thatthepotentiometervoltageisaccuratelyknown.TheerrorinRsomeextentdependentontheamplifiersensitivity.Forsistomea-surementofagivencurrent,thehighertheamplifiersensitivity,thegreaterlikelihoodthatlowervalued,highlyprecisewire-woundstandardresistorscanbeused.Suchamplifierscanbeobtained.Standardresistancesof100GVknownto62%,areavailable.If10-mVinputtotheamplifierorelectrometergivesfull-scaledeflectionwithanerrornotgreaterthan2%offullscale,with500Vapplied,aresistanceof5000TVcanbemeasuredwithamaximumerrorof6%whenthevoltmeterreadsfullscale,and10%whenitreads1⁄3scale.
7.5.2.3Comparison-Galvanometer—Themaximumper-centageerrorinthecomputedresistanceorconductanceisgivenbythesumofthepercentageerrorsinRreadings,andthes,thegalvanom-eterdeflectionsoramplifierassumptionthatthecurrentsensitivitiesareindependentofthedeflections.Thelatterassumptioniscorrecttowellwithin62%overtheusefulrange(above1⁄10full-scaledeflection)ofagood,moderngalvanometer(probably1⁄3scaledeflectionforadccurrentamplifier).TheerrorinRsdependsonthetypeofresistorused,butresistancesof1MVwithalimitoferroraslowas0.1%areavailable.Withagalvanometerord-ccurrentamplifierhavingasensitivityof10nAforfull-scaledeflection,500Vappliedtoaresistanceof5TVwillproducea1%deflection.Atthisvoltage,withtheprecedingnotedstandardresistor,andwithFs5105,dswouldbeabouthalfoffull-scaledeflection,withareadabilityerrornotmorethan61%.Ifd⁄xisapproxi-mately14offull-scaledeflection,thereadabilityerrorwouldnotexceed%,andaresistanceoftheorderof200GV
couldbemeasuredwithamaximumerrorof651⁄2%.
7.5.2.4VoltageRate-of-Change—Theaccuracyofthemea-surementisdirectlyproportionaltotheaccuracyofthemeasurementofappliedvoltageandtimerateofchangeoftheelectrometerreading.Thelengthoftimethattheelectrometerswitchisopenandthescaleusedshouldbesuchthatthetimecanbemeasuredaccuratelyandafull-scalereadingobtained.Undertheseconditions,theaccuracywillbecomparablewiththatoftheothermethodsofmeasuringcurrent.
7.5.2.5ComparisonBridge—Whenthedetectorhasad-equatesensitivity,themaximumpercentageerrorinthecom-puterresistanceisthesumofthepercentageerrorsinthearms,A,B,andN.Withadetectorsensitivityof1mV/scaledivision,500Vappliedtothebridge,andRN51GV,aresistanceof1000TVwillproduceadetectordeflectionofonescaledivision.AssumingnegligibleerrorsinR62%andwiththebridgeAandRbalancedB,withRGVknowntowithintoN51onedetector-scaledivision,aresistanceof100TVcanbemea-suredwithamaximumerrorof66%.
8.Sampling
8.1Refertoapplicablematerialsspecificationsforsam-plinginstructions.
9.TestSpecimens
9.1InsulationResistanceorConductanceDetermination:9.1.1Themeasurementisofgreatestvaluewhenthespeci-menhastheform,electrodes,andmountingrequiredinactualuse.Bushings,cables,andcapacitorsaretypicalexamplesforwhichthetestelectrodesareapartofthespecimenanditsnormalmountingmeans.
9.1.2Forsolidmaterials,thetestspecimenmaybeofanypracticalform.Thespecimenformsmostcommonlyusedareflatplates,tapes,rods,andtubes.TheelectrodearrangementsofFig.2maybeusedforflatplates,rods,orrigidtubeswhoseinnerdiameterisabout20mmormore.TheelectrodearrangementofFig.3maybeusedforstripsofsheetmaterialorforflexibletape.Forrigidstripspecimensthemetalsupportmaynotberequired.TheelectrodearrangementsofFig.6maybeusedforflatplates,rods,ortubes.Comparisonofmaterialswhenusingdifferentelectrodearrangementsisfrequentlyinconclusiveandshouldbeavoided.
9.2VolumeResistanceorConductanceDetermination:9.2.1Thetestspecimenmayhaveanypracticalformthatallowstheuseofathirdelectrode,whennecessary,toguardagainsterrorfromsurfaceeffects.Testspecimensmaybeintheformofflatplates,tapes,ortubes.Fig.4andFig.7illustratetheapplicationandarrangementofelectrodesforplateorsheetspecimens.Fig.5isadiametralcrosssectionofthreeelec-trodesappliedtoatubularspecimen,inwhichelectrodeNo.1istheguardedelectrode,electrodeNo.2isaguardelectrodeconsistingofaringateachendofelectrodeNo.1,andelectrodeNo.3istheunguardedelectrode(7,8).Formaterialsthathavenegligiblesurfaceleakage,theguardringsmaybeomitted.Convenientandgenerallysuitabledimensionsappli-cabletoFig.4inthecaseoftestspecimensthatare3mminthicknessareasfollows:D35100mm,D576mm,oralternatively,Dmm,D2588mm,andD1525mm.Foragivensensitivity,3550thelarger2538mm,andD1specimen
TABLE2ApparatusandConditionsforUse
Reference
Method
Section
Voltmeter-ammeter(galvanometer)Comparison(galvanometer)
Voltmeter-ammeter(dcamplifica-tion,electrometer)
X3.1X3.4X3.2
Figure
MaximumOhmsDetectableat500V10121012101510101710171015;100MV·F101515MaximumOhmsMeasurableto66%at500V
10111011101310101510151014101413TypeofMeasurement
OhmsShuntedbyInsulationResistance
fromGuardtoGuardedElectrode
10to10510to105102to109102to103103to10110(effective)105to106unguarded104to1010Comparison(Wheatstonebridge)Voltagerate-of-changeMegohmmeter(typical)
X1X3X2(a)
(Position1)X2(a)
(Position2)X2(b)X2(b)
X3.5X4X3.3X5
commercialinstruments
deflectiondeflectiondeflectiondeflectiondeflectionnull
null
deflectiondirect-reading
allowsmoreaccuratemeasurementsonmaterialsofhigherresistivity.
9.2.2MeasuretheaveragethicknessofthespecimensinaccordancewithoneofthemethodsinTestMethodsD374pertainingtothematerialbeingtested.Theactualpointsofmeasurementshallbeuniformlydistributedovertheareatobecoveredbythemeasuringelectrodes.
9.2.3ItisnotnecessarythattheelectrodeshavethecircularsymmetryshowninFig.4althoughthisisgenerallyconve-nient.Theguardedelectrode(No.1)maybecircular,square,orrectangular,allowingreadycomputationoftheguardedelec-trodeareaforvolumeresistivityorconductivitydeterminationwhensuchisdesired.Thediameterofacircularelectrode,thesideofasquare,ortheshortestsideofarectangularelectrode,shouldbeatleastfourtimesthespecimenthickness.ThegapwidthshouldbegreatenoughsothatthesurfaceleakagebetweenelectrodesNo.1andNo.2doesnotcauseanerrorinthemeasurement(thisisparticularlyimportantforhigh-input-impedanceinstruments,suchaselectrometers).Ifthegapismadeequaltotwicethespecimenthickness,assuggestedin9.3.3,sothatthespecimencanbeusedalsoforsurfaceresistanceorconductancedeterminations,theeffectiveareaofelectrodeNo.1canbetaken,usuallywithsufficientaccuracy,asextendingtothecenterofthegap.If,underspecialconditions,itbecomesdesirabletodetermineamoreaccuratevaluefortheeffectiveareaofelectrodeNo.1,thecorrectionforthegapwidthcanbeobtainedfromAppendixX2.Elec-trodeNo.3mayhaveanyshapeprovidedthatitextendsatallpointsbeyondtheinneredgeofelectrodeNo.2byatleasttwicethespecimenthickness.
9.2.4Fortubularspecimens,electrodeNo.1shouldencircletheoutsideofthespecimenanditsaxiallengthshouldbeatleastfourtimesthespecimenwallthickness.Considerationsregardingthegapwidtharethesameasthosegivenin9.2.3.ElectrodeNo.2consistsofanencirclingelectrodeateachendofthetube,thetwopartsbeingelectricallyconnectedbyexternalmeans.Theaxiallengthofeachofthesepartsshouldbeatleasttwicethewallthicknessofthespecimen.ElectrodeNo.3mustcovertheinsidesurfaceofthespecimenforanaxiallengthextendingbeyondtheoutsidegapedgesbyatleasttwicethewallthickness.Thetubularspecimen(Fig.5)maytaketheformofaninsulatedwireorcable.Ifthelengthofelectrodeismorethan100timesthethicknessoftheinsulation,theeffectsoftheendsoftheguardedelectrodebecomenegligible,andcarefulspacingoftheguardelectrodesisnotrequired.Thus,thegapbetweenelectrodesNo.1andNo.2maybeseveralcentimetrestopermitsufficientsurfaceresistancebetweentheseelectrodeswhenwaterisusedaselectrodeNo.1.Inthiscase,nocorrectionismadeforthegapwidth.
9.3SurfaceResistanceorConductanceDetermination:9.3.1Thetestspecimenmaybeofanypracticalformconsistentwiththeparticularobjective,suchasflatplates,tapes,ortubes.
9.3.2ThearrangementsofFig.2andFig.3weredevisedforthosecaseswherethevolumeresistanceisknowntobehighrelativetothatofthesurface(2).However,thecombinationofmoldedandmachinedsurfacesmakestheresultobtainedgenerallyinconclusiveforrigidstripspecimens.Thearrange-mentofFig.3issomewhatmoresatisfactorywhenappliedtospecimensforwhichthewidthismuchgreaterthanthethickness,thecutedgeeffectthustendingtobecomerelativelysmall.Hence,thisarrangementismoresuitablefortestingthinspecimenssuchastape,thanfortestingrelativelythickerspecimens.ThearrangementsofFig.2andFig.3shouldneverbeusedforsurfaceresistanceorconductancedeterminationswithoutdueconsiderationsofthelimitationsnotedpreviously.9.3.3ThethreeelectrodearrangementsofFig.4,Fig.6andFig.7maybeusedforpurposesofmaterialcomparison.Theresistanceorconductanceofthesurfacegapbetweenelec-trodesNo.1andNo.2isdetermineddirectlybyusingelectrodeNo.1astheguardedelectrode,electrodeNo.3astheguardelectrode,andelectrodeNo.2astheunguardedelectrode(7,8).TheresistanceorconductancesodeterminedisactuallytheresultantofthesurfaceresistanceorconductancebetweenelectrodesNo.1andNo.2inparallelwithsomevolumeresistanceorconductancebetweenthesametwoelectrodes.Forthisarrangementthesurfacegapwidth,g,shouldbeapproximatelytwicethespecimenthickness,t,exceptforthinspecimens,wheregmaybemuchgreaterthantwicethematerialthickness.
9.3.4Specialtechniquesandelectrodedimensionsmayberequiredforverythinspecimenshavingsuchalowvolumeresistivitythattheresultantlowresistancebetweentheguardedelectrodeandtheguardsystemwouldcauseexcessiveerror.9.4LiquidInsulationResistance—Thesamplingofliquidinsulatingmaterials,thetestcellsemployed,andthemethods
ofcleaningthecellsshallbeinaccordancewithTestMethodD1169.
10.SpecimenMounting
10.1Inmountingthespecimensformeasurements,itisimportantthatthereshallbenoconductivepathsbetweentheelectrodesorbetweenthemeasuringelectrodesandgroundthatwillhaveasignificanteffectonthereadingofthemeasuringinstrument(9).Insulatingsurfacesshouldnotbehandledwithbarefingers(acetaterayonglovesarerecommended).Forrefereetestsofvolumeresistivityorconductivity,thesurfacesshouldbecleanedwithasuitablesolventbeforeconditioning.Whensurfaceresistanceistobemeasured,thesurfacesshouldbecleanedornotcleanedasspecifiedoragreedupon.11.Conditioning
11.1ThespecimensshallbeconditionedinaccordancewithPracticeD618.
11.2Circulating-airenvironmentalchambersorthemethodsdescribedinPracticesE104orD5032maybeusedforcontrollingtherelativehumidity.
12.Procedure
12.1InsulationResistanceorConductance—Properlymountthespecimeninthetestchamber.Ifthetestchamberandtheconditioningchamberarethesame(recommendedproce-dure),thespecimensshouldbemountedbeforethecondition-ingisstarted.Makethemeasurementwithasuitabledevicehavingtherequiredsensitivityandaccuracy(seeAppendix).Unlessotherwisespecified,thetimeofelectrificationshallbe60sandtheapplieddirectvoltageshallbe50065V.
12.2VolumeResistivityorConductivity—Measurethedi-mensionsoftheelectrodesandwidthofguardgap,g.Makethemeasurementwithasuitabledevicehavingtherequiredsensitivityandaccuracy.Unlessotherwisespecified,thetimeofelectrificationshallbe60s,andtheapplieddirectvoltageshallbe50065V.
12.3SurfaceResistanceorConductance:
12.3.1Measuretheelectrodedimensionsandthedistancebetweentheelectrodes,g.MeasurethesurfaceresistanceorconductancebetweenelectrodesNo.1and2withasuitabledevicehavingtherequiredsensitivityandaccuracy.Unlessotherwisespecified,thetimeofelectrificationshallbe60s,andtheapplieddirectvoltageshallbe50065V.
12.3.2WhentheelectrodearrangementofFig.3isused,Pistakenastheperimeterofthecrosssectionofthespecimen.Forthinspecimens,suchastapes,thisperimetereffectivelyreducestotwicethespecimenwidth.
12.3.3WhentheelectrodearrangementsofFig.6areused(andthevolumeresistanceisknowntobehighcomparedtothesurfaceresistance),Pistakentobethelengthoftheelectrodesorcircumferenceofthecylinder.
13.Calculation
13.1Calculatethevolumeresistivity,rv,andthevolumeconductivity,gCalculatev,usingtheequationsinTable1.
13.2thesurfaceresistivity,rconductivity,gs,andthesurfaces,usingtheequationsinTable1.
14.Report
14.1Reportthefollowinginformation:
14.1.1Adescriptionandidentificationofthematerial(name,grade,color,manufacturer,etc.),
14.1.2Shapeanddimensionsofthetestspecimen,14.1.3Typeanddimensionsofelectrodes,
14.1.4Conditioningofthespecimen(cleaning,predrying,hoursathumidityandtemperature,etc.),
14.1.5Testconditions(specimentemperature,relativehu-midity,etc.,attimeofmeasurement),
14.1.6Methodofmeasurement(seeAppendixX3),14.1.7Appliedvoltage,
14.1.8Timeofelectrificationofmeasurement,
14.1.9Measuredvaluesoftheappropriateresistancesinohmsorconductancesinsiemens,
14.1.10Computedvalueswhenrequired,ofvolumeresis-tivityinohm-centimetres,volumeconductivityinsiemenspercentimetre,surfaceresistivityinohms(persquare),orsurfaceconductivityinsiemens(persquare),and
14.1.11Statementastowhetherthereportedvaluesare“apparent”or“steady-state.”
15.PrecisionandBias
15.1Precisionandbiasareinherentlyaffectedbythechoiceofmethod,apparatus,andspecimen.ForanalysisanddetailsseeSections7and9,andparticularly7.5.1-7.5.2.5.
16.Keywords
16.1DCresistance;insulationresistance;surfaceresistance;surfaceresistivity;volumeresistance;volumeresistivity
APPENDIXES
(NonmandatoryInformation)
X1.FACTORSAFFECTINGINSULATIONRESISTANCEORCONDUCTANCEMEASUREMENTS
X1.1InherentVariationinMaterials—Becauseofthevariabilityoftheresistanceofagivenspecimenundersimilartestconditionsandthenonuniformityofthesamematerialfromspecimentospecimen,determinationsareusuallynotreproducibletocloserthan10%andoftenareevenmorewidelydivergent(arangeofvaluesfrom10to1maybeobtainedunderapparentlyidenticalconditions).
X1.2Temperature—Theresistanceofelectricalinsulatingmaterialsisknowntochangewithtemperature,andthevariationoftencanberepresentedbyafunctionoftheform:(18)
R5Bem/T
(X1.1)
where:
R5resistance(orresistivity)ofaninsulatingmaterialor
system,
B5proportionalityconstant,m5activationconstant,and
T5absolutetemperatureinkelvin(K).
ThisequationisasimplifiedformoftheArrheniusequationrelatingtheactivationenergyofachemicalreactiontotheabsolutetemperature;andtheBoltzmannprinciple,agenerallawdealingwiththestatisticaldistributionofenergyamonglargenumbersofminuteparticlessubjecttothermalagitation.Theactivationconstant,m,hasavaluethatischaracteristicofaparticularenergyabsorptionprocess.Severalsuchprocessesmayexistwithinthematerial,eachwithadifferenteffectivetemperaturerange,sothatseveralvaluesofmwouldbeneededtofullycharacterizethematerial.Thesevaluesofmcanbedeterminedexperimentallybyplottingthenaturallogarithmofresistanceagainstthereciprocaloftheabsolutetemperature.Thedesiredvaluesofmareobtainedfromsuchaplotbymeasuringtheslopesofthestraight-linesectionsoftheplot.Thisderivesfrom(EqX1.1),foritfollowsthatbytakingthenaturallogarithmofbothsides:
1nR5lnB1m1
T(X1.2)
Thechangeinresistance(orresistivity)correspondingtoachangeinabsolutetemperaturefromT1toT2,basedonEqX1.1,andexpressedinlogarithmicform,is:
ln~R11
DT2/R1!5mST22T1D5mST1T2
D(X1.3)
Theseequationsarevalidoveratemperaturerangeonlyifthematerialdoesnotundergoatransitionwithinthistempera-turerange.Extrapolationsareseldomsafesincetransitionsareseldomobviousorpredictable.Asacorollary,deviationofaplotofthelogarithmofRagainst1/Tfromastraightlineisevidencethatatransitionisoccurring.Furthermore,inmakingcomparisonsbetweenmaterials,itisessentialthatmeasure-mentsbemadeovertheentirerangeofinterestforallmaterials.
NOTEX1.1—Theresistanceofanelectricalinsulatingmaterialmaybeaffectedbythetimeoftemperatureexposure.Therefore,equivalenttemperatureconditioningperiodsareessentialforcomparativemeasure-ments.
NOTEX1.2—Iftheinsulatingmaterialshowssignsofdeteriorationafterconditioningatelevatedtemperatures,thisinformationmustbeincludedwiththetestdata.
X1.3TemperatureandHumidity—Theinsulationresis-tanceofsoliddielectricmaterialsdecreasesbothwithincreas-ingtemperatureasdescribedinX1.2andwithincreasing
humidity(1,2,3,4).Volumeresistanceisparticularlysensitivetotemperaturechanges,whilesurfaceresistancechangeswidelyandveryrapidlywithhumiditychanges(2,3).Inbothcasesthechangeisexponential.Forsomematerialsachangefrom25to100°Cmaychangeinsulationresistanceorconduc-tancebyafactorof100000,oftenduetothecombinedeffectsoftemperatureandmoisturecontentchange;theeffectoftemperaturechangealoneisusuallymuchsmaller.Achangefrom25to90%relativehumiditymaychangeinsulationresistanceorconductancebyasmuchasafactorof1000000ormore.Insulationresistanceorconductanceisafunctionofboththevolumeandsurfaceresistanceorconductanceofthespecimen,andsurfaceresistancechangesalmostinstanta-neouslywithchangeofrelativehumidity.Itis,therefore,absolutelyessentialtomaintainbothtemperatureandrelativehumiditywithincloselimitsduringtheconditioningperiodandtomaketheinsulationresistanceorconductancemeasurementsinthespecifiedconditioningenvironment.Anotherpointnottobeoverlookedisthatatrelativehumiditiesabove90%,surfacecondensationmayresultfrominadvertantfluctuationsinhu-midityortemperatureproducedbytheconditioningsystem.Thisproblemcanbeavoidedbytheuseofequivalentabsolutehumidityataslightlyhighertemperature,asequilibriummoisturecontentremainsnearlythesameforasmalltempera-turechange.Indeterminingtheeffectofhumidityonvolumeresistanceorconductance,extendedperiodsofconditioningarerequired,sincetheabsorptionofwaterintothebodyofthedielectricisarelativelyslowprocess(10).Somespecimensrequiremonthstocometoequilibrium.Whensuchlongperiodsofconditioningareprohibitive,useofthinnerspeci-mensorcomparativemeasurementsnearequilibriummaybereasonablealternatives,butthedetailsmustbeincludedinthetestreport.
X1.4TimeofElectrification—Measurementofadielectricmaterialisnotfundamentallydifferentfromthatofaconductorexceptthatanadditionalparameter,timeofelectrification,(andinsomecasesthevoltagegradient)isinvolved.Therelation-shipbetweentheappliedvoltageandthecurrentisinvolvedinbothcases.Fordielectricmaterials,thestandardresistanceplacedinserieswiththeunknownresistancemusthavearelativelylowvalue,sothatessentiallyfullvoltagewillbeappliedacrosstheunknownresistance.Whenapotential
differenceisappliedtoaspecimen,thecurrentthroughitgenerallydecreasesasymptoticallytowardalimitingvaluewhichmaybelessthan0.01ofthecurrentobservedattheendof1min(9,11).Thisdecreaseofcurrentwithtimeisduetodielectricabsorption(interfacialpolarization,volumecharge,etc.)andthesweepofmobileionstotheelectrodes.Ingeneral,therelationofcurrentandtimeisoftheformI(t)5At−m,aftertheinitialchargeiscompletedanduntilthetrueleakagecurrentbecomesasignificantfactor(12,13).InthisrelationAisaconstant,numericallythecurrentatunittime,andmusually,butnotalways,hasavaluebetween0and1.Dependinguponthecharacteristicsofthespecimenmaterial,thetimerequiredforthecurrenttodecreasetowithin1%ofthisminimumvaluemaybefromafewsecondstomanyhours.Thus,inordertoensurethatmeasurementsonagivenmaterialwillbecompa-rable,itisnecessarytospecifythetimeofelectrification.Theconventionalarbitrarytimeofelectrificationhasbeen1min.Forsomematerials,misleadingconclusionsmaybedrawnfromthetestresultsobtainedatthisarbitrarytime.Aresistance-timeorconductance-timecurveshouldbeobtainedundertheconditionsoftestforagivenmaterialasabasisforselectionofasuitabletimeofelectrification,whichmustbespecifiedinthetestmethodforthatmaterial,orsuchcurvesshouldbeusedforcomparativepurposes.Occasionally,amaterialwillbefoundforwhichthecurrentincreaseswithtime.Inthiscaseeitherthetimecurvesmustbeusedoraspecialstudyundertaken,andarbitrarydecisionsmadeastothetimeofelectrification.X1.5MagnitudeofVoltage:
X1.5.1Bothvolumeandsurfaceresistanceorconductanceofaspecimenmaybevoltage-sensitive(4).Inthatcase,itisnecessarythatthesamevoltagegradientbeusedifmeasure-mentsonsimilarspecimensaretobecomparable.Also,theappliedvoltageshouldbewithinatleast5%ofthespecifiedvoltage.ThisisaseparaterequirementfromthatgiveninX1.7.3,whichdiscussesvoltageregulationandstabilitywhereappreciablespecimencapacitanceisinvolved.
X1.5.2Commonlyspecifiedtestvoltagestobeappliedtothecompletespecimenare100,250,500,1000,2500,5000,10000and15000V.Ofthese,themostfrequentlyusedare100and500V.Thehighervoltagesareusedeithertostudythevoltage-resistanceorvoltage-conductancecharacteristicsofmaterials(tomaketestsatorneartheoperatingvoltagegradients),ortoincreasethesensitivityofmeasurement.X1.5.3Specimenresistanceorconductanceofsomemate-rialsmay,dependinguponthemoisturecontent,beaffectedbythepolarityoftheappliedvoltage.Thiseffect,causedbyelectrolysisorionicmigration,orboth,particularlyinthepresenceofnonuniformfields,maybeparticularlynoticeableininsulationconfigurationssuchasthosefoundincableswherethetest-voltagegradientisgreaterattheinnerconductorthanattheoutersurface.Whereelectrolysisorionicmigrationdoesexistinspecimens,theelectricalresistancewillbelowerwhenthesmallertestelectrodeismadenegativewithrespecttothelarger.Insuchcases,thepolarityoftheappliedvoltageshallbespecifiedaccordingtotherequirementsofthespeci-menundertest.
X1.6ContourofSpecimen:
X1.6.1Themeasuredvalueoftheinsulationresistanceorconductanceofaspecimenresultsfromthecompositeeffectofitsvolumeandsurfaceresistancesorconductances.Sincetherelativevaluesofthecomponentsvaryfrommaterialtomaterial,comparisonofdifferentmaterialsbytheuseoftheelectrodesystemsofFig.1,Fig.2,andFig.3isgenerallyinconclusive.Thereisnoassurancethat,ifmaterialAhasahigherinsulationresistancethanmaterialBasmeasuredbytheuseofoneoftheseelectrodesystems,itwillalsohaveahigherresistancethanBintheapplicationforwhichitisintended.X1.6.2Itispossibletodevisespecimenandelectrodeconfigurationssuitablefortheseparateevaluationofthevolumeresistanceorconductanceandtheapproximatesurfaceresistanceorconductanceofthesamespecimen.Ingeneral,thisrequiresatleastthreeelectrodessoarrangedthatonemayselectelectrodepairsforwhichtheresistanceorconductancemeasuredisprimarilythatofeitheravolumecurrentpathorasurfacecurrentpath,notboth(7).
X1.7DeficienciesintheMeasuringCircuit:
X1.7.1Theinsulationresistanceofmanysoliddielectricspecimensisextremelyhighatstandardlaboratoryconditions,approachingorexceedingthemaximummeasurablelimitsgiveninTable2.Unlessextremecareistakenwiththeinsulationofthemeasuringcircuit,thevaluesobtainedaremoreameasureofapparatuslimitationsthanofthematerialitself.Thuserrorsinthemeasurementofthespecimenmayarisefromundueshuntingofthespecimen,referenceresistors,orthecurrent-measuringdevice,byleakageresistancesorconductancesofunknown,andpossiblyvariable,magnitude.X1.7.2Electrolytic,contact,orthermalemf’smayexistinthemeasuringcircuititself;orspuriousemf’smaybecausedbyleakagefromexternalsources.Thermalemf’sarenormallyinsignificantexceptinthelowresistancecircuitofagalva-nometerandshunt.Whenthermalemf’sarepresent,randomdriftsinthegalvanometerzerooccur.Slowdriftsduetoaircurrentsmaybetroublesome.Electrolyticemf’sareusuallyassociatedwithmoistspecimensanddissimilarmetals,butemf’sof20mVormorecanbeobtainedintheguardcircuitofahigh-resistancedetectorwhenpiecesofthesamemetalareincontactwithmoistspecimens.Ifavoltageisappliedbetweentheguardandtheguardedelectrodesapolarizationemfmayremainafterthevoltageisremoved.Truecontactemf’scanbedetectedonlywithanelectrometerandarenotasourceoferror.Theterm“spuriousemf’’issometimesappliedtoelectrolyticemf’s.Toensuretheabsenceofspuriousemf’sofwhateverorigin,thedeflectionofthedetectingdeviceshouldbeobservedbeforetheapplicationofvoltagetothespecimenandafterthevoltagehasbeenremoved.Ifthetwodeflectionsarethesame,ornearlythesame,acorrectioncanbemadetothemeasuredresistanceorconductance,providedthecorrec-tionissmall.Ifthedeflectionsdifferwidely,orapproachthedeflectionofthemeasurement,itwillbenecessarytofindandeliminatethesourceofthespuriousemf(5).Capacitancechangesintheconnectingshieldedcablescancauseseriousdifficulties.
X1.7.3Whereappreciablespecimencapacitanceisin-volved,boththeregulationandtransientstabilityoftheappliedvoltageshouldbesuchthatresistanceorconductancemeasure-mentscanbemadetoprescribedaccuracy.Short-timetran-sients,aswellasrelativelylong-timedriftsintheappliedvoltagemaycausespuriouscapacitivechargeanddischargecurrentswhichcansignificantlyaffecttheaccuracyofmea-surement.Inthecaseofcurrent-measuringmethodsparticu-larly,thiscanbeaseriousproblem.ThecurrentinthemeasuringinstrumentduetoavoltagetransientisI05CxdV/dt.Theamplitudeandrateofpointerexcursionsdependuponthefollowingfactors:
X1.7.3.1Thecapacitanceofthespecimen,
X1.7.3.2Themagnitudeofthecurrentbeingmeasured,X1.7.3.3Themagnitudeanddurationoftheincomingvoltagetransient,anditsrateofchange,
X1.7.3.4Theabilityofthestabilizingcircuitusedtopro-videaconstantvoltagewithincomingtransientsofvariouscharacteristics,and
X1.7.3.5Thetime-constantofthecompletetestcircuitascomparedtotheperiodanddampingofthecurrent-measuringinstrument.
X1.7.4Changesofrangeofacurrent-measuringinstrumentmayintroduceacurrenttransient.WhenRCequationofthistransientis
m[Lt]RxandCm[Lt]x,theI5~V0/Rx!@I2e2t/RmCx#
(X1.4)
where:
VR05appliedvoltage,
Rx5apparenteffectiveresistanceinputresistanceofthespecimen,
m5ofthemeasuringinstru-C5ment,
Cxcapacitanceofthespecimenat1000Hz,
tm55inputtimeaftercapacitanceRisswitchedofthemeasuringintothecircuit.instrument,andmFornotmorethan5%errorduetothistransient,
RmCx#t/3
(X1.5)
Microammetersemployingfeedbackareusuallyfreeofthissourceoferrorastheactualinputresistanceisdivided,effectively,bytheamountoffeedback,usuallyatleastby1000.X1.8ResidualCharge—InX1.4itwaspointedoutthatthecurrentcontinuesforalongtimeaftertheapplicationofapotentialdifferencetotheelectrodes.Conversely,currentwillcontinueforalongtimeaftertheelectrodesofachargedspecimenareconnectedtogether.Itshouldbeestablishedthatthetestspecimeniscompletelydischargedbeforeattemptingthefirstmeasurement,arepeatmeasurement,ameasurementofvolumeresistancefollowingameasurementofsurfaceresis-tance,orameasurementwithreversedvoltage(9).Thetimeofdischargebeforemakingameasurementshouldbeatleastfour
timesanypreviouschargingtime.Thespecimenelectrodesshouldbeconnectedtogetheruntilthemeasurementistobemadetopreventanybuild-upofchargefromthesurroundings.X1.9Guarding:
X1.9.1Guardingdependsoninterposing,inallcriticalinsulatedpaths,guardconductorswhichinterceptallstraycurrentsthatmightotherwisecauseerrors.Theguardconduc-torsareconnectedtogether,constitutingtheguardsystemandforming,withthemeasuringterminals,athree-terminalnet-work.Whensuitableconnectionsaremade,straycurrentsfromspuriousexternalvoltagesareshuntedawayfromthemeasur-ingcircuitbytheguardsystem.
X1.9.2ProperuseoftheguardsystemforthemethodsinvolvingcurrentmeasurementisillustratedinFigs.X1.1-X1.3,inclusive,wheretheguardsystemisshownconnectedtothejunctionofthevoltagesourceandcurrent-measuringinstrumentorstandardresistor.InFig.X1.4fortheWheatstone-bridgemethod,theguardsystemisshowncon-nectedtothejunctionofthetwolower-valued-resistancearms.Inallcases,tobeeffective,guardingmustbecomplete,andmustincludeanycontrolsoperatedbytheobserverinmakingthemeasurement.Theguardsystemisgenerallymaintainedatapotentialclosetothatoftheguardedterminal,butinsulatedfromit.Thisisbecause,amongotherthings,theresistanceofmanyinsulatingmaterialsisvoltage-dependent.Otherwise,thedirectresistancesorconductancesofathree-terminalnetworkareindependentoftheelectrodepotentials.Itisusualtogroundtheguardsystemandhenceonesideofthevoltagesourceandcurrent-measuringdevice.Thisplacesbothterminalsofthespecimenaboveground.Sometimes,oneterminalofthespecimenispermanentlygrounded.Thecurrent-measuringdeviceusuallyisthenconnectedtothisterminal,requiringthatthevoltagesourcebewellinsulatedfromground.
X1.9.3Errorsincurrentmeasurementsmayresultfromthefactthatthecurrent-measuringdeviceisshuntedbytheresistanceorconductancebetweentheguardedterminalandtheguardsystem.Thisresistanceshouldbeatleast10to100timestheinputresistanceofthecurrentmeasuringdevice.Insomebridgetechniques,theguardandmeasuringterminalsarebroughttonearlythesamepotentials,butastandardresistorinthebridgeisshuntedbetweentheunguardedterminalandtheguardsystem.Thisresistanceshouldbeatleast1000timesthatofthereferenceresistor.
FIG.X1.1Voltmeter-AmmeterMethodUsingaGalvanometer
FIG.X1.2Voltmeter-AmmeterMethodUsingDCAmplification
FIG.X1.3ComparisonMethodUsingaGalvanometer
FIG.X1.4ComparisonMethodUsingaWheatstoneBridge
X2.EFFECTIVEAREAOFGUARDEDELECTRODE
X2.1General—CalculationofvolumeresistivityfromthemeasuredvolumeresistanceinvolvesthequantityA,theeffectiveareaoftheguardedelectrode.Dependingonthematerialpropertiesandtheelectrodeconfiguration,Adiffersfromtheactualareaoftheguardedelectrodeforeither,orboth,ofthefollowingreasons.
X2.1.1Fringingofthelinesofcurrentintheregionoftheelectrodeedgesmayeffectivelyincreasetheelectrodedimen-sions.
X2.1.2Ifplaneelectrodesarenotparallel,oriftubularelectrodesarenotcoaxial,thecurrentdensityinthespecimenwillnotbeuniform,andanerrormayresult.Thiserrorisusuallysmallandmaybeignored.
X2.2Fringing:
X2.2.1Ifthespecimenmaterialishomogeneousandisotro-pic,fringingeffectivelyextendstheguardedelectrodeedgebyanamount(14,19):
~g/2!2d
(X2.1)
where:
d5t$~2/p!lncosh@~p/4!~g/t!#%,
(X2.2)
andgandtarethedimensionsindicatedinFig.4andFig.6.Thecorrectionmayalsobewritten
g@12~2d/g!#5Bg
(X2.3)
whereBisthefractionofthegapwidthtobeaddedtothe
diameterofcircularelectrodesortothedimensionsofrectan-gularorcylindricalelectrodes.
X2.2.2Laminatedmaterials,however,aresomewhataniso-tropicaftervolumeabsorptionofmoisture.Volumeresistivityparalleltothelaminationsisthenlowerthanthatintheperpendiculardirection,andthefringingeffectisincreased.Withsuchmoistlaminates,dapproacheszero,andtheguardedelectrodeeffectivelyextendstothecenterofthegapbetweenguardedandunguardedelectrodes(14).
X2.2.3Thefractionofthegapwidthgtobeaddedtothediameterofcircularelectrodesortotheelectrodedimensionsofrectangularorcylindricalelectrodes,B,asdeterminedbytheprecedingequationford,isasfollows:
g/t0.10.20.30.4
B0.960.920.880.85
g/t1.01.21.52.0
B0.0.590.510.41
0.50.60.80.810.770.71
2.53.00.340.29
NOTEX2.1—Thesymbol“ln”designateslogarithmtothebasee52.718....Whengisapproximatelyequalto2t,disdeterminedwithsufficientapproximationbytheequation:
d50.586t(X2.4)
NOTEX2.2—Fortestsonthinfilmswhent< X3.TYPICALMEASUREMENTMETHODS X3.1Voltmeter-AmmeterMethodUsingaGalvanometer:X3.1.1AdcvoltmeterandagalvanometerwithasuitableshuntareconnectedtothevoltagesourceandtothetestspecimenasshowninFig.X1.1.Theappliedvoltageismeasuredbyadcvoltmeter,havingarangeandaccuracythatwillgiveminimumerrorinvoltageindication.Innocaseshallavoltmeterbeusedthathasanerrorgreaterthan62%offullscale,norarangesuchthatthedeflectionislessthanonethirdoffullscale(forapivot-typeinstrument).Thecurrentismeasuredbyagalvanometerhavingahighcurrentsensitivity(ascalelengthof0.5misassumed,asshorterscalelengthswillleadtoproportionatelyhighererrors)andprovidedwithaprecisionAyrtonuniversalshuntforsoadjustingitsdeflectionthatthereadabilityerrordoesnot,ingeneral,exceed62%oftheobservedvalue.Thegalvanometershouldbecalibratedtowithin62%.Currentcanbereaddirectlyifthegalvanometerisprovidedwithanadditionalsuitablefixedshunt. X3.1.2Theunknownresistance,Rx,orconductance,Gx,iscalculatedasfollows: Rx51/Gx5Vx/Ix5Vx/KdF (X3.1) where: K5galvanometersensitivity,inamperesperscaledivi-sion, d5deflectioninscaledivisions,F5ratioofthetotalcurrent,Ix,tothegalvanometer current,and Vx5appliedvoltage. X3.2Voltmeter-AmmeterMethodUsingDCAmplificationorElectrometer: X3.2.1Thevoltmeter-ammetermethodcanbeextendedtomeasurehigherresistancesbyusingdcamplificationoranelectrometertoincreasethesensitivityofthecurrentmeasuringdevice(6,15,16).Generally,butnotnecessarily,thisisachievedonlywithsomesacrificeinprecision,dependingontheapparatusused.Thedcvoltmeterandthedcamplifieror electrometerareconnectedtothevoltagesourceandthespecimenasillustratedinFig.X1.2.TheappliedvoltageismeasuredbyadcvoltmeterhavingthesamecharacteristicsasprescribedinX3.1.1.Thecurrentismeasuredintermsofthevoltagedropacrossastandardresistance,Rs. X3.2.2InthecircuitshowninFig.X1.2(a)thespecimencurrent,Ix,producesacrossthestandardresistance,Rs,avoltagedropwhichisamplifiedbythedcamplifier,andreadonanindicatingmeterorgalvanometer.Thenetgainoftheamplifierusuallyisstabilizedbymeansofafeedbackresis-tance,Rf,fromtheoutputoftheamplifier.Theindicatingmetercanbecalibratedtoreaddirectlyintermsofthefeedbackvoltage,Vf,whichisdeterminedfromtheknownvalueoftheresistanceofRf,andthefeedbackcurrentpassingthroughit.Whentheamplifierhassufficientintrinsicgain,thefeedbackvoltage,Vs,differsfromthevoltage,IxRs,byanegligibleamount.AsshowninFig.X1.2(a)thereturnleadfromthevoltagesource,Vx,canbeconnectedtoeitherendofthefeedbackresistor,Rf.WiththeconnectionmadetothejunctionofRsandRf(switchindottedpositionl),theentireresistanceofRsisplacedinthemeasuringcircuitandanyalternatingvoltageappearingacrossthespecimenresistanceisamplifiedonlyasmuchasthedirectvoltageIxRs,acrossRs.WiththeconnectionmadetotheotherendofRf(switchposition2),theapparentresistanceplacedinthemeasuringcircuitisRstimestheratioofthedegeneratedgaintotheintrinsicgainoftheamplifier;anyalternatingvoltageappearingacrossthespeci-menresistanceisthenamplifiedbytheintrinsicamplifiergain.X3.2.3InthecircuitshowninFig.X1.2(b),thespecimencurrent,Ix,producesavoltagedropacrossthestandardresistance,Rswhichmayormaynotbebalancedoutbyadjustmentofanopposingvoltage,Vs,fromacalibratedpotentiometer.Ifnoopposingvoltageisused,thevoltagedropacrossthestandardresistance,Rs,isamplifiedbythedcamplifierorelectrometerandreadonanindicatingmeterorgalvanometer.Thisproducesavoltagedropbetweenthe measuringelectrodeandtheguardelectrodewhichmaycauseanerrorinthecurrentmeasurementunlesstheresistancebetweenthemeasuringelectrodeandtheguardelectrodeisatleast10to100timesthatofRs.Ifanopposingvoltage,Vs,isused,thedcamplifierorelectrometerservesonlyasaverysensitive,high-resistancenulldetector.Thereturnleadfromthevoltagesource,Vx,isconnectedasshown,toincludethepotentiometerinthemeasuringcircuit.Whenconnectionsaremadeinthismanner,noresistanceisplacedinthemeasuringcircuitatbalanceandthusnovoltagedropappearsbetweenthemeasuringelectrodeandtheguardelectrode.However,asteeplyincreasingfractionofRsisincludedinthemeasuringcircuit,asthepotentiometerismovedoffbalance.Anyalternatingvoltageappearingacrossthespecimenresistanceisamplifiedbythenetamplifiergain.Theamplifiermaybeeitheradirectvoltageamplifieroranalternatingvoltageamplifierprovidedwithinputandoutputconverters.Inducedalternatingvoltagesacrossthespecimenoftenaresufficientlytroublesomethataresistance-capacitancefilterprecedingtheamplifierisrequired.Theinputresistanceofthisfiltershouldbeatleast100timesgreaterthantheeffectresistancethatisplacedinthemeasurementcircuitbyresistanceRRs. X3.2.4Theresistancelatedasfollows: x,ortheconductance,Gx,iscalcu-Rx51/Gx5Vx/Ix5~Vx/Vs!Rs (X3.2) where:VIx5appliedRx5specimenvoltage,Vs5standardcurrent, s5 voltagedropresistance,acrossand Rs,indicatedbytheamplifieroutputmeter,theelectrometerorthecalibratedpotentiometer. X3.3VoltageRate-of-ChangeMethod: X3.3.1Ifthespecimencapacitanceisrelativelylarge,orcapacitorsaretobemeasured,theapparentresistant,Rthechargingvoltage,Vx,canbedeterminedfrom0,thespecimencapacitancevalue,Cofvoltage,0(capacitanceofCdV/dt,usingthexat1000Hz),andtherate-of-changecircuitofFig.X3.1(17).TomakeameasurementthespecimenischargedbyclosingSS2,withtheelectrometershortingswitchS1closed.When1issubsequentlyopened,thevoltageacrossthespecimenwillfallbecausetheleakageandabsorptioncurrentsmustthenbesuppliedbythecapacitanceC0ratherthanbyVThedropinvoltageacrossthespecimenwillbeshownbythe0.electrometer.Ifarecorderisconnectedtotheoutputoftheelectrometer,therateofchangeofvoltage,dV/dt,canbereadfromtherecordertraceatanydesiredtimeafterS2isclosed(60 FIG.X3.1VoltageRate-of-ChangeMethod susuallyspecified).Alternatively,thevoltage,DV,appearingontheelectrometerinatime,Dt,canbeused.Sincethisgivesanaverageoftherate-of-changeofvoltageduringDt,thetimeDtshouldbecenteredatthespecifiedelectrificationtime(timesinceclosingS2). X3.3.2Iftheinputresistanceoftheelectrometerisgreaterthantheapparentspecimenresistanceandtheinputcapaci-tanceis0.01orlessofthatofthespecimen,theapparentresistanceatthetimeatwhichdV/dtorDV/Dtisdeterminedis Rx5V0/Ix5V0dt/C0dVmor,V0Dt/C0DVm (X3.3) dependingonwhetherornotarecorderisused.WhentheelectrometerinputresistanceorcapacitancecannotbeignoredorwhenVmismorethanasmallfractionofV0thecompleteequationshouldbeused. Rs5$V0@~Rx1Rm!/Rm#Vm%/~C01Cm!dVm/dt (X3.4) where:CR05capacitanceinputofCm5Vm5inputresistanceCofxatthe1000Hz, V05appliedcapacitancem5 electrometervoltage,readingand oftheelectrometer,electrometer,5voltagedecreaseonCx. X3.4ComparisonMethodUsingaGalvanometerorDCAmplifier(1): X3.4.1Astandardresistance,Rs,andagalvanometerordcamplifierareconnectedtothevoltagesourceandtothetestspecimenasshowninFig.X3.1.ThegalvanometeranditsassociatedAyrtonshuntisthesameasdescribedinX3.1.1.Anamplifierofequivalentdirectcurrentsensitivitywithanappropriateindicatormaybeusedinplaceofthegalvanometer.Itisconvenient,butnotnecessary,andnotdesirableifbatteriesareusedasthevoltagesource(unlessahigh-inputresistancevoltmeterisused),toconnectavoltmeteracrossthesourceforacontinuouscheckofitsvoltage.Theswitchisprovidedforshortingtheunknownresistanceintheprocessofmeasure-ment.Sometimesprovisionismadetoshorteithertheun-knownorstandardresistancebutnotbothatthesametime.X3.4.2Ingeneral,itispreferabletoleavethestandardresistanceinthecircuitatalltimestopreventdamagetothecurrentmeasuringinstrumentincaseofspecimenfailure.Withtheshuntsettotheleastsensitivepositionandwiththeswitchopen,thevoltageisapplied.TheAyrtonshuntisthenadjustedtogiveasnearmaximumscalereadingaspossible.Attheendoftheelectrificationtimethedeflection,dsettox,andtheshuntratio,Fx,arenoted.Theshuntisthentheleastsensitivepositionandtheswitchisclosedtoshorttheunknownresistance.Againtheshuntisadjustedtogiveasnearmaxi-mumscalereadingaspossibleandthegalvanometerormeterdeflection,ds,andtheshuntratio,Fofthegalvanometers,arenoted.Itisassumedthatthecurrentsensitivitiesoramplifierareequalfornearlyequaldeflectionsdxandds. X3.4.3Theunknownresistance,Rx,orconductance,Gx,iscalculatedasfollows: R51/Gx5Rs@~dsFs/dxFx!–1# (X3.5) where: 5ratiosofthetotalcurrenttothegalvanometerordcamplifierwithRxinthecircuit,andshorted,respectively. X3.4.4IncaseRsisshortedwhenRxisinthecircuitortheratioofFstoFxisgreaterthan100,thevalueofRxorGxiscomputedasfollows:FxandFsRx51/Gx5R~dsFs/dxFx! (X3.6) X3.5ComparisonMethodsUsingaWheatstoneBridge(2):X3.5.1ThetestspecimenisconnectedintoonearmofaWheatstonebridgeasshowninFig.X1.4.Thethreeknownarmsshallbeofashighresistanceaspracticable,limitedbytheerrorsinherentinsuchresistors.Usually,thelowestresistance,RA,isusedforconvenientbalanceadjustment,witheitherRBorRNbeingchangedindecadesteps.Thedetectorshallbeadcamplifier,withaninputresistancehighcomparedtoanyofthesearms. X3.5.2Theunknownresistance,Rx,orconductance,Gx,iscalculatedasfollows: Rx51/Gx5RBRN/RA (X3.7) indicatingmeterofFig.X1.2(a)canbereplacedbyarecordingmilliammeterormillivoltmeterasappropriatefortheamplifierused.Therecordermaybeeitherthedeflectiontypeorthenull-balancetype,thelatterusuallyhavingasmallererror.Null-balance-typerecordersalsocanbeemployedtoperformthefunctionofautomaticallyadjustingthepotentiometershowninFig.X1.2(b)andtherebyindicatingandrecordingthequantityundermeasurement.Thecharacteristicsofamplifier,recorderbalancingmechanism,andpotentiometercanbemadesuchastoconstituteawellintegrated,stable,electromechani-cal,feedbacksystemofhighsensitivityandlowerror.Suchsystemsalsocanbearrangedwiththepotentiometerfedfromthesamesourceofstablevoltageasthespecimen,therebyeliminatingthevoltmetererror,andallowingasensitivityandprecisioncomparablewiththoseoftheWheatstone-BridgeMethod(X3.5). X3.7Direct-ReadingInstruments—Thereareavailable,andingeneraluse,instrumentsthatindicateresistancedirectly,byadeterminationoftheratioofvoltageandcurrentinbridgemethodsorrelatedmodes.Someunitsincorporatevariousadvancedfeaturesandrefinementssuchasdigitalreadout.Mostdirectreadinginstrumentsareself-contained,portable,andcompriseastabledcpowersupplywithmulti-testvoltagecapability,anulldetectororanindicator,andallrelevantauxiliaries.Measurementaccuraciesvarysomewhatwithtypeofequipmentandrangeofresistancescovered;forthemoreelaborateinstrumentsaccuraciesarecomparabletothoseobtainedwiththevoltmeter-ammetermethodusingagalva-nometer(X3.1).Thedirect-readinginstrumentsdonotneces-sarilysupplantanyoftheothertypicalmeasurementmethodsdescribedinthisAppendix,butdooffersimplicityandconve-nienceforbothroutineandinvestigativeresistancemeasure-ments. whereRA,RB,andRNareasshowninFig.X1.4.WhenarmAisarheostat,itsdialcanbecalibratedtoreaddirectlyinmegohmsaftermultiplyingbythefactorRBRNwhichforconveniencecanbevariedindecadesteps. X3.6Recordings—Itispossibletorecordcontinuouslyagainsttimethevaluesoftheunknownresistanceorthecorrespondingvalueofcurrentataknownvoltage.Generally,thisisaccomplishedbyanadaptationofthevoltmeter-ammetermethod,usingdcamplification(X3.2).Thezerodriftofdirectcoupleddcamplifiers,whileslowenoughforthemeasure-mentsofX3.2,maybetoofastforcontinuousrecording.Thisproblemcanberesolvedbyperiodicchecksofthezero,orbyusinganacamplifierwithinputandoutputconverter.The REFERENCES (1)Curtis,H.L.,“InsulatingPropertiesofSolidDielectric,”Bulletin,NationalInstituteofStandardsandTechnology,VolII,1915,ScientificPaperNo.234,pp.369–417. (2)Field,R.F.,“HowHumidityAffectsInsulation,PartI,D-CPhenom-ena,”GeneralRadioExperimenter,Vol20,Nos.2and3,July–August1945. (3)Field,R.F.,“TheFormationofIonizedWaterFilmsonDielectricsUnderConditionsofHighHumidity,”JournalofAppliedPhysics,Vol5,May1946. (4)Herou,R.,andLaCoste,R.,“SurLaMe´sureDesResistivitiesetL’EtudedeConditionnementdesIsolantesenFeuilles,”ReportIEC15-GT2(France)April4,1963. (5)Thompson,B.H.,andMathes,K.N.,“ElectrolyticCorrosion—MethodsofEvaluatingMaterialsUsedinTropicalService,”Transac-tions,AmericanInstituteofElectricalEngineers,Vol,June1945,p.287. (6)Scott,A.H.,“AnomalousConductanceBehaviorinPolymers,”Reportofthe1965ConferenceonElectricalInsulation,NRC-NAS. (7)Amey,W.G.,andHamberger,F.,Jr.,“AMethodforEvaluatingtheSurfaceandVolumeResistanceCharacteristicsofSolidDielectricMaterials,”Proceedings,AmericanSocietyforTestingandMaterials,Vol49,1949,pp.1079–1091. (8)Witt,R.K.,Chapman,J.J.,andRaskin,B.L.,“MeasuringofSurfaceandVolumeResistance,”ModernPlastics,Vol24,No.8,April1947,p.152. (9)Scott,A.H.,“InsulationResistanceMeasurements,”FourthElectricalInsulationConference,Washington,DC,February19–22,1962. (10)Kline,G.M.,Martin,A.R.,andCrouse,W.A.,“SorptionofWaterby Plastics,”Proceedings,AmericanSocietyforTestingandMaterials,Vol40,1940,pp.1273–1282. (11)Greenfield,E.W.,“InsulationResistanceMeasurements,”Electrical Engineering,Vol66,July1947,pp.698–703. (12)Cole,K.S.,andCole,R.H.,“DispersionandAbsorptionin Dielectrics,IIDirectCurrentCharacteristics,”JournalofChemicalPhysics,Vol10,1942. (13)Field,R.F.,“InterpretationofCurrent-TimeCurvesasAppliedto InsulationTesting,”AIEEBostonDistrictMeeting,April19–20,1944. (14)Lauritzen,J.I.,“TheEffectiveAreaofaGuardedElectrode,”Annual Report,ConferenceonElectricalInsulation.NAS-NRCPublication1141,1963. (15)Turner,E.F.,Brancato,E.L.,andPrice,W.,“TheMeasurementof InsulationConductivity,”NRLReport5060,NavalResearchLabo-ratory,Feb.25,1958. (16)Dorcas,D.S.,andScott,R.N.,“InstrumentationforMeasuringthe D-CConductivityofVeryHighResistivityMaterials,”ReviewofScientificInstruments,Vol35,No.9,September19. (17)Endicott,H.S.,“InsulationResistance,Absorption,andTheirMea-surement,”AnnualReport,ConferenceonElectricalInsulation,NAS-NRCPublication,1958. (18)Occhini,E.,andMaschio,G.,“ElectricalCharacteristicsofOil-ImpregnatedPaperasInsulationforHV-DCCables,”IEEETransac-tionsonPowerApparatusandSystems,VolPAS-86,No.3,March1967. (19)Endicott,H.S.,“Guard-GapCorrectionforGuarded-ElectrodeMea-surementsandExactEquationsfortheTwo-FluidMethodofMea-suringPermittivityandLoss,”JournalofTestingandEvaluation,Vol4,No.3,May1976,pp.188–195. TheAmericanSocietyforTestingandMaterialstakesnopositionrespectingthevalidityofanypatentrightsassertedinconnectionwithanyitemmentionedinthisstandard.Usersofthisstandardareexpresslyadvisedthatdeterminationofthevalidityofanysuchpatentrights,andtheriskofinfringementofsuchrights,areentirelytheirownresponsibility.Thisstandardissubjecttorevisionatanytimebytheresponsibletechnicalcommitteeandmustbereviewedeveryfiveyearsandifnotrevised,eitherreapprovedorwithdrawn.YourcommentsareinvitedeitherforrevisionofthisstandardorforadditionalstandardsandshouldbeaddressedtoASTMHeadquarters.Yourcommentswillreceivecarefulconsiderationatameetingoftheresponsibletechnicalcommittee,whichyoumayattend.IfyoufeelthatyourcommentshavenotreceivedafairhearingyoushouldmakeyourviewsknowntotheASTMCommitteeonStandards,attheaddressshownbelow.ThisstandardiscopyrightedbyASTM,100BarrHarborDrive,POBoxC700,WestConshohocken,PA19428-2959,UnitedStates.Individualreprints(singleormultiplecopies)ofthisstandardmaybeobtainedbycontactingASTMattheaboveaddressorat610-832-9585(phone),610-832-9555(fax),orservice@astm.org(e-mail);orthroughtheASTMwebsite(www.astm.org).
因篇幅问题不能全部显示，请点此查看更多更全内容