product manual11.10

H-4385

Resistivity Meter

this is a blank page corrector

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: hmc@humboldtmfg.com

WarrantyHumboldtMfg.Co.warrantsitsproductstobefreefromdefectsinmaterialorworkmanship.TheexclusiveremedyforthiswarrantyisHumboldtMfg.Co.,factoryreplacementofanypartorpartsofsuchproduct,forthewarrantyofthisproductpleaserefertoHumboldtMfg.Co.catalogonTermsandConditionsofSale.Thepurchaserisresponsibleforthetransportationcharges.HumboldtMfg.Co.shallnotberesponsibleunderthiswarrantyifthegoodshavebeenimproperlymaintained,installed,operatedorthegoodshavebeenalteredormodifiedsoastoadverselyaffecttheoperation,useperformanceordurabilityorsoastochangetheirintendeduse.TheHumboldtMfg.Co.liabilityunderthewarrantycontainedinthisclauseislimitedtotherepairorreplacementofdefectivegoodsandmakinggood,defectiveworkmanship.