Resistivity Meter - Construction Materials Testing Equipment · 2016-04-23 · Description When...
Transcript of Resistivity Meter - Construction Materials Testing Equipment · 2016-04-23 · Description When...
product manual11.10
H-4385
Resistivity Meter
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Description
When combined with appropriate electrodes (pins) and test leads, the H-4385 can be used to measure earth resistance or the resistance-to-earth of a buried electrode, such as a ground rod or an anode, for example.
Depending on the application, 4-Electrode, 3-Electrode or 2-Electrode, the H-4385 can be used to determine the following:
• Theaverageearthresistivitytoaspecificdepth(withtheapplicationofanappropriatemultipliertoconvertresistancetoresistivity,basedontheelectrodeseparationdistance) – 4-Electrode Application
• Theresistivityofasoilsample,orofaliquid,viaanelectrolyte(soil/liquid)box(withtheapplicationofanappropriatemultipliertoconvertresistancetoresistivity,dependingontheboxgeometry)–4-ElectrodeApplication
• Theresistance-to-earthofaburiedelectrode,suchasagroundrod,orananode,forexample – 3-Electrode Application
• Theresistancebetweentwoburiedelectrodes,suchastwogroundrods,ortwoanodes – 2-Electrode Application
ThecurrentsourceintheH-4385,whichsuppliescurrentbetweentheC1andC2terminals(withaloadapplied),isa12V(rms)crystal-controlled97Hzsquarewaveoscillatorandthevoltmeterinsidetheunit,whichsensesthepotentialdifference(voltagedrop)appearingacrosstheP1andP2terminals,employsaverynarrowband-passfiltercenteredat97HzWhatthismeansisthatresistancemeasurementstakenbytheH-4385areunaffectedbyanystrayinterferencesignals(havingfrequenciesotherthan97Hz)thatmaybepresentintheearthduringameasurement.
TheH-4385hasaresistancemeasurementrangefrom0.01Ohm(0.01Ω)to1.1MOhm(1.1MΩ)thatisachievedbymeansofasetof8rangesettingsandasystemofinternal“standard”resistors.Externalresistances(resistancevaluesundertest)arecomparedagainsttheinternal“standards,viaanullbalancingsystem,resultinginadeterminationoftheexternalresistancevalues.
TheH-4385runsonasetofreplaceableC-sizealkalinebatteries,sothereisnorequirementtoperiodicallyre-chargetheunitortoplugtheunitintoapowersource.Pleaseseethe“Maintenance”sectionforinformationonhowtoreplacethebatteries.
Operating Instructions Thetestleadconnectionsthataremadetothe“Current”terminals,C1andC2,andthe“Potential”terminals,P1andP2,dependontheparticularapplicationoftheH-4385.Thevariousconnectionrequirements,aswellasthevariousaccessoriesrequiredineachapplication are detailed in the “Applications” section.
Regardingtheapplicationsthatinvolveinsertingelectrodes(pins)intotheearth,itisrecommendedthattheelectrodes(pins)befirmlydrivenintotheearth(pinscannotbeloose).Also,indrysoils,itisrecommendedthatthesoilaroundthepinsbemoistenedinorder that reliable (low resistance) contact is made to the surrounding soil.
How to take a resistance reading:
1) Connectthetestleadsandsetuptheelectrodes(pins)asoutlinedintheApplicationssectionforyourparticularapplication.
2) Whentheapproximateresistance(sayofthelocalsoil)isnotknown,movetherangeselectorswitch(labeled“OhmsMultiplyBy”)tothe100Ksettingandpositionthe“BalanceDial”knobat“10”.
3) Pullthe“NullSensitivity”switchdowntothe“Low”positionandnotethatthenullindicatingmeterneedlemovestotheright,indicatingtoohigharesistancesetting.
4) Whileholdingthe“NullSensitivity”switchinthe“Low”position,stepdownthroughtheresistanceranges(10K,1K,100Ωetc.)untiltheneedlemovestotheleftofthenullposition (left of the center position) and then step back up one range.
5) Adjustthepositionofthe“BalanceDial”untiltheneedleispositionedatthenull(center) location on the meter.
6) Multiplythe“BalanceDial”settingbytherangesetting(settingontheswitchlabeled“OhmsMultiplyBy”)toobtaintheresistancevalue.Forexample,fora“BalanceDial”settingof4.5andarangeswitchsettingof100Ω,theresistancevalueis450Ω
7) Applytheresistancevaluetothecalculationofresistivityusingtheappropriateformulaforyourapplication(seetheApplicationssectionbelow).
Note:Youcanincreasethesensitivityoftheresistancereadingbyholding the“NullSensitivity”switchinthe“High”positionandfinetuningthe balance,afterfindingthebalancepointinthe“Low”sensitivityposition.
Applications
4-electrode Applications
1.EarthResistivityMeasurement
Thisapplicationuses4electrodes(pins).Theelectrodesaredrivendownintotheearththesamedistanceandareevenlyspacedinastraightline. AschematicofthisarrangementisillustratedinFigure1below.
TheH-4385canbeusedinconjunctionwiththe4-lead(color-coded)testreelandfourheavy-duty(stainlesssteel)electrodes(soilpins)–(H-4388).ThetestleadsareconnectedtotheH-4385asshowninFigure1.Withthisarrangement,theH-4385effectivelymeasurestheearth’saverageresistancetoadepthequaltotheelectrodespacing (S).
Dr.FrankWenneroftheU.S.BureauofStandardsdevelopedthetheorybehindthistestin1915[1].Heshowedthat,iftheelectrode(pin)depth(d)iskeptsmallrelativetotheseparationbetweentheelectrodes(S),theearth’saverageresistivitytoadepthequaltotheelectrodespacing(S)canbeobtainedbyapplyingthefollowingformula:
ρ = 2 π S R
whereRistheresistancevalueinohmsasdeterminedusingtheH-4385,istheresistivity in ohm.cm, πistheconstant3.1416,andSistheelectrodeseparationincm.
Typically,theelectrode(pin)spacingisnotmeasuredincentimetersbut,rather,infeet(intheU.S.)orinmeters(inmostothercountries).
U.S.Example(electrodespacingmeasuredinfeet):
Sincethereare30.38centimetersin1foot,theaboveformulacanbewrittenas:
ρ (Ω.cm) = 2πx30.38x(electrodespacinginfeet)xR(ohms),or
ρ (Ω.cm)=191.5x(electrodespacinginfeet)xR(ohms)
So,forexample,iftheH-4385producesaresistancevalueof15ohmsforanelectrode(pin)spacingof20feet,theearth’saverageresistivityvaluetoadepthof20feetwouldbe:
ρ=191.5x20x15=57,450ohm.cm
Iftheresistivityvalueisrequiredtobeexpressedinohm.m,ratherthanohm.cm,theohm.cmvalueisdividedby100.Intheaboveexample,theresistivitywouldbe574.5ohm.m
MetricExample(electrodespacingmeasuredinmeters):
Sincethereare100centimetersin1meter,theaboveformulacanbewrittenas:
ρ (Ω.cm) = 2π x 100 x (electrode spacing in meters) x R (ohms)
or, ρ (Ω.cm) = 628.32 x (electrode spacing in meters) x R (ohms)
So,forexample,iftheH-4385producesaresistancevalueof15ohmsforanelectrode(pin)spacingof7meters,theearth’saverageresistivityvaluetoadepthof7meters would be:
ρ =628.32x7x15=65,973.6ohm.cm
Iftheresistivityvalueisrequiredtobeexpressedinohm.m,ratherthanohm.cm,theohm.cmvalueisdividedby100.Intheaboveexample,theresistivitywouldbe659.73ohm.m
Note:Theaboveformulaisaccurateonlyiftheelectrodedepth(d)issmallrelativetotheelectrodespacing(S).An“S”valueequalto,orgreaterthan,20timesthe“d”valueisrecommended.Thismeansthatifdis1foot,forexample,thenShastobeatleast20feet(or,ifdis0.3meter,thenShastobeatleast6meters)
TheWennerMethodisthebasisofanASTMStandardTestMethod
(ASTMG57-78).
Figure1
2. Soil Sample (or Liquid) Resistivity Measurement
Thisapplicationalsouses4electrodes,however,inthiscase,theelectrodesareanintegralpartofanelectrolytebox,whichismorecommonlyreferredtoasasoilbox. Forthisapplication,theH-4385canbeusedinconjunctionwithoneaHumboldtsoilbox(H-4386orH-4386SM)and4testleads(H-4388).
AschematicofthetestarrangementisillustratedinFigure2below.
Forthisapplication,thetestleadsareconnectedtotheH-4385asshowninFigure2.
With this arrangement, the H-4385 determines the resistance of the soil sample, or of the liquidthatfillstheelectrolytebox.
Ingeneral,foraparticularvolumeofsoilsample(orliquid),asdefinedbythegeometricconstraintsoftheelectrolytebox,thesample’sresistivitycanbecalculatedfromtheresistancevaluedeterminedusingtheH-4385byapplyingthefollowingformula:
ρ=RA/L
where ρistheresistivityinohm.cm,Ristheresistanceinohms,Aisthecross-sectionalareaofthecurrentelectrodesincmsquared,andListheseparationbetweenthepotential electrodes in cm.
Consequently,theratioA/Lrepresentsamultiplicationfactorthatneedstobeappliedtotheresistancereadinginordertoobtainthesample’sresistivityvalue.
Conveniently,forbothsoilboxes(H-4386andH4386SM),theA/Lratioisexactly1cm.
Consequently,whenH-4386andH4386SMsoilboxesareused,theresistancereadinginohmsdeterminedusingtheH-4385becomestheresistivityvalueinohm.cm.
Iftheresistivityvalueisrequiredtobeexpressedinohm.m,ratherthanohm.cm,theohm.cmvalueisdividedby100.Forexamplearesistivityvalueof2500ohm.cmwouldbeequivalentto25ohm.m
Figure2
3-Electrode Application
The3-ElectrodeApplicationcanbeusedtomeasuretheresistance-to-earthofaburiedelectrode, such as a buried ground rod or a buried anode.
Inthiscase,twooftheelectrodesarepinsdrivenintotheearthandthethirdelectrode is the test electrode itself, for example, a ground rod or an anode.
Aresistance-to-earthmeasurementwillactuallycomprise3components;theresistanceof the electrode itself (the resistance of the ground rod or anode material, for example) including test leads, the electrode-earth contact resistance and the resistance of the surroundingearth.Typically,theresistanceofthesurroundingearthisthelargestcomponent of the resistance-to-earth measurement.
Thethreeelectrodesarepositionedinastraightline(ideally)asindicatedinFigure3below.
AsindicatedinFigure3,ajumperwireisconnectedbetweentheC1andP1terminalsontheH-4385andtestleadsconnectthe“PotentialElectrode”andthe“CurrentElectrode”toterminalsP2andC2,respectively.Finally,atestleadconnectstheelectrodeundertest(agroundrodorananode,forexample)totheC1terminal.
Withthisconfiguration,theH-4385passesacurrentbetweenthetestelectrodeandthe“CurrentElectrode”andgeneratesaresistancereadingbasedonthevoltagedroppedbetween the electrode under test and the “Potential Electrode”.
Themagnitudeoftheresistancereadingwillbeafunctionoftheseparation(distance)betweentheelectrodeundertestandthe“PotentialElectrode”,withrespecttoafixedpositionforthe“CurrentElectrode”.
Acharacteristic“resistanceversusdistance”plotisillustratedinFigure4below.
Figure3
Thistypeofplotassumesthatthe“CurrentElectrode”(seeFigure3)ispositionedfarenoughawayfromtheelectrodeundertestsothata“plateau”regionisobtainedintheplot.A100footseparationistypicallyrecommended.However,itissuggestedthataresistanceversusdistancecurvebegeneratedforanygivensituationinordertoverifytheexistenceofaplateau-typeregionwhichwillallowanaccuratedeterminationoftheelectrode-to-earthresistance.
Assumingthataplateau-typeregionisobtained,theresistance-to-earthvaluefortheelectrodeundertestwillbetheresistancevalueontheplotcorrespondingtotheplateauregionasillustratedinFigure4.
Ageneralrule-of-thumb,whichassumesthatthe“CurrentElectrode”ispositionedsufficientlyfarawayfromtheelectrodeundertest,isthatifthe“PotentialElectrode”ispositionedatadistancefromthetestelectrodeof0.62xD,whereDisthedistancebetweentheelectrodeundertestandthe“CurrentElectrode”,theresistancereadingobservedwillcorrespondtotheresistance-to-earthvaluefortheelectrodeundertest.
ThisparticularconfigurationisillustratedinFigure3.Forexample,ifthedistancebetweentheelectrodeundertestandthe“CurrentElectrode”is100feet,the“PotentialElectrode”shouldbepositioned62feetfromtheelectrodeundertest.However,again,itissuggestedthatresistanceversusdistanceplotsbegeneratedineachinstance.
2-Electrode Application
The2-ElectrodeApplicationcanbeusedtomeasuretheresistancebetweentwoburiedelectrodes, such as two ground rods, or two anodes, for example.
Inthiscase,thetwoelectrodesarethetwoburiedcomponents.Figure5belowillustratesthemeasurementconfiguration.
Figure4
AsindicatedinFigure5,inthisconfiguration,jumperwiresareconnectedbetweenterminalsC1andP1andbetweenterminalsC2andP2.Inaddition,testleadsconnectoneoftheelectrodestoterminalC1andtheotherelectrodetoterminalC2.
Anotherconfigurationoption,whichisrecommendedforusewhenmeasuringsmallresistances,istoconnectthepotentialterminals(P1andP2)directlytotheelectrodeswithseparatetestleads,thuseliminatingthejumperwires.SuchaconfigurationeliminatesanyvoltagedropinthetestleadsduetocurrentflowintheleadswhichwouldappearinserieswiththevoltagedropoccurringbetweentheelectrodesinthecaseoftheconfigurationshowninFigure5.
TheresistancereadingdeterminedbytheH-4385willbeadirectmeasurementofresistancecomprising2components;theresistance-to-earthcontributionofeachelectrode (which itself comprises 3 components (see the “3-Electrode Application” sectionabove)andtheearth’sresistancebetweenthetwoelectrodes.
Maintenance & Calibration
Otherthanchangingthebatteries,theH-4385requiresnomaintenance.Whenthe“LowBattery”indicatorlightsup(andremainsON)duringabalancingprocedure,thebatteriesshouldbereplaced.Theproceduretochangethe8C-sizealkalinebatteriesintheunitisasfollows:
1) Removethe4screwsonthefrontpanelandliftthepaneloutoftheplasticcase.
2) Turnthepanelupsidedownandplaceonaclean(anddry)surface3) Undothefourretainingclipsandremovethetwosetsof4oldbatteriesfrom
their holder tubes.
Figure5
4) Insert4newC-sizealkalinebatteriesintoeachholdertubeandreinstallthetubesmakingsurethatthebatterypolarityiscorrect(aslabeled)ineachcase.
5) Re-install the retaining clips.6) Re-install the panel into its plastic case and re-install the 4 screws on the
front side of the panel.With respect to calibration, it is recommended that the H-4385 be re-calibrated on an annual basis.
www.humboldtmfg.comHUMBOLDT
Testing Equipment for Construction Materials
Humboldt Mfg. Co.875 Tollgate RoadElgin, Illinois 60123 U.S.A.
U.S.A.TollFree:1.800.544.7220Voice:1.708.456.6300Fax:1.708.456.0137
Email: [email protected]
WarrantyHumboldtMfg.Co.warrantsitsproductstobefreefromdefectsinmaterialorworkmanship.TheexclusiveremedyforthiswarrantyisHumboldtMfg.Co.,factoryreplacementofanypartorpartsofsuchproduct,forthewarrantyofthisproductpleaserefertoHumboldtMfg.Co.catalogonTermsandConditionsofSale.Thepurchaserisresponsibleforthetransportationcharges.HumboldtMfg.Co.shallnotberesponsibleunderthiswarrantyifthegoodshavebeenimproperlymaintained,installed,operatedorthegoodshavebeenalteredormodifiedsoastoadverselyaffecttheoperation,useperformanceordurabilityorsoastochangetheirintendeduse.TheHumboldtMfg.Co.liabilityunderthewarrantycontainedinthisclauseislimitedtotherepairorreplacementofdefectivegoodsandmakinggood,defectiveworkmanship.