MPS-2

DielectricWaterPotentialSensorOperator’sManual

Version1

Decagon Devices, Inc.2365NEHopkinsCourt

Pullman,WA99163Tel:(509)332-2756Fax:(509)332-5158www.decagon.com

Copyright©2011DecagonDevices,Inc.

AllRightsReserved

i

Contents1. Introduction ............................................ 1

Specifications.............................................................. 1ContactInformation................................................ 2WarrantyInformation.............................................. 3

2. About the MPS-2 ..................................... 43. Theory ...................................................... 5

WaterPotentialMeasurement................................ 5TemperatureMeasurement...................................... 7

4. Measurement Range And Accuracy ......... 8MeasurementRange................................................. 8MeasurementAccuracy............................................ 9Calibration................................................................10Hysteresis...................................................................10

5. Connecting to a Logger ...........................113.5mmStereoPlugWiring....................................12

6. Communication Non-Decagon Loggers .15SerialCommunication........................................... 15SDI-12Communication........................................ 15

7. Installing the Sensors ............................. 188. Campbell Scientific Programs ......................... 199. Handling and Care ................................. 20

MPS-2inFrozenSoils............................................20

10. Troubleshooting ................................... 22References .................................................. 23

MPS-2 Water Potential Sensor

IV

Appendix A ................................................ 24DeclarationofConformity....................................24

Index .......................................................... 26

Water Potential Sensor MPS-21.Introduction

1

1. Introduction

Thank you for choosing the Dielectric Water Potential Sensor,modelMPS-2formeasuringsoilwaterpotentialandtemperature.Thismanualisdesignedtohelpyouunderstandthesensor’sfeatures,andhowtousethisdevicesuccessfully.

Intheengineeringcommunity,thetermsoilsuctioniscommonlyusedinsteadofsoilwaterpotential.Soilwaterpotentialissimplythenegativeofsoilsuctionandwillbeusedthroughouttheremainderofthismanual.

SpecificationsWater Potential

Range: -5to-500kPa(pF1.71topF3.71)Resolution: 0.1kPaAccuracy: ±25%ofreadingfrom-5kPato-100kPa*

TemperatureRange: -40°Cto+50°CResolution: 0.1°CAccuracy: ±1°C

GeneralSurvival Environment: -40to60°C,0to100%RHOperating Temperature: 0to50°C (nowaterpotential measurementbelow0°C)Power Requirements:3.6-15VDC,0.03mAquiescent, 10mAmaxduring150msmeasurement

*SeeMeasurementRangeandAccuracychapterfordetailsonaccuracy.

MPS-2 Water Potential Sensor 1.Introduction

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Dimensions: 9.6cm(L)x3.5cm(W)x1.5cm(D)Sensor Diameter: 3.2cmDielectric Measurement Frequency: 70MHzMeasurement Time: 150ms(milliseconds)Output: RS232(TTL)with3.6voltlevelsor SDI-12communicationprotocolConnector Types: 3.5mm(stereo)plugorstripped& tinnedleadwires(3)Cable Length: 5mstandard;customlengthsavailable uponrequestData Logger Compatibility (not exclusive): Decagon:Em50,Em50R,Em50G(rev2.5+) Campbell Scientific:AnyloggerwithserialI/O includingCR10X,CR23X,anyCRBasictype logger(CR850,1000,3000,etc.) Other: Anydataacquisitionsystemcapableof 3.6-15VexcitationandserialorSDI-12 communicationHandheld Reader Compatability: ProCheck(rev1.29+)Software Compatibility: ECH2OUtility(rev1.62+) DataTrac3(rev3.3+)

Contact InformationTocontactDecagonforcustomersupportorquestions:

E-mail: support@decagon.com sales@decagon.comFax: (509)332-5158Telephone: 1-800-755-2751(USAandCanadaOnly) 1-509-332-2756International

Our Customer Support and Sales Representatives are availableMondaythruFriday,8am-5pmPacificTime.

Water Potential Sensor MPS-21.Introduction

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NOTE: With any correspondence please include your name, contact information, instrument serial number(s), and a description of your problem or question.

Warranty InformationThe Dielectric Water Potential Sensor has a 30-day satisfactionguaranteeandaone-yearwarranty.

Seller’s LiabilitySellerwarrantsnewequipmentofitsownmanufactureagainstde-fective workmanship and materials for a period of one year fromdateofreceiptofequipment(theresultsofordinarywearandtear,neglect,misuse,accidentandexcessivedeteriorationduetocorro-sionfromanycausearenottobeconsideredadefect);butSeller’sliabilityfordefectivepartsshallinnoeventexceedthefurnishingof replacement parts F.O.B. the factory where originally manu-factured. Material and equipment covered hereby which is notmanufacturedbySellershallbecoveredonlybythewarrantyofitsmanufacturer. Seller shall not be liable to Buyer for loss, damageor injuries to persons (including death), or to property or thingsofwhatsoeverkind(including,butnotwithoutlimitation,lossofanticipatedprofits),occasionedbyorarisingoutoftheinstallation,operation,use,misuse,nonuse,repair,orreplacementofsaidmate-rialandequipment,oroutoftheuseofanymethodorprocessforwhichthesamemaybeemployed.Theuseofthisequipmentcon-stitutesBuyer’sacceptanceofthetermssetforthinthiswarranty.Therearenounderstandings,representations,orwarrantiesofanykind,express,implied,statutoryorotherwise(including,butwith-out limitation, the implied warranties of merchantability and fit-nessforaparticularpurpose),notexpresslysetforthherein.

MPS-2 Water Potential Sensor 2.AbouttheMPS-2

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2. About the MPS-2

TheMPS-2measures thewaterpotentialandtemperatureof soilandotherporousmaterials.TheMPS-2hasa lowpowerrequire-mentwhichmakesitanidealsensorforpermanentburialinthesoilandcontinuousreadingwithadataloggerorperiodicreadingwithahandheldreader.

Vinyl cover �lled with polyurethane resin

Ceramic disks(static matrix)

Grounded stainlesssteel screens

Printed circuit board

Water Potential Sensor MPS-23.Theory

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3. Theory

Water Potential MeasurementTherearetwobasicparametersthatdescribethestateofwater insoil:oneissoilwatercontent,ortheamountofwaterperunitofsoil,andtheotherissoilwaterpotential,ortheenergystateofwaterinthesoil.Althoughwatercontentisusefulwhentryingtodescribethewaterbalanceofasoil,i.e.howmuchwaterismovingin,out,orbeingstored,waterpotentialisoftenpreferredoverwatercontentbecauseitshowshowwaterwillmoveinasoilorfromthesoiltotheplant.Inaddition,waterpotentialcanbeusedtodetermineplantavailabilityofwater,determinesoilstress,andscheduleirrigation,amongotherthings.All soilwaterpotentialmeasurementtechniquesmeasurethepo-tential energyofwater inequilibriumwithwater in the soil.TheSecond Law of Thermodynamics states that connected systemswithdifferingenergylevelswillmovetowardanequilibriumenergylevel.Thus,ifanobjectcomesintohydrauliccontactwiththesoil,thewaterpotentialoftheobjectwillcomeintoequilibriumwiththesoilwaterpotential.Examplesofinstrumentsthatmakeuseofthisprincipalaretensiometers,whichmeasurethepotentialenergyofaliquidwaterreservoirinequilibriumwiththesoilwater(liquidequilibration), and psychrometers/dewpoint hygrometers, whichmeasure the potential energy of water vapor in equilibrium withthesoilwater(vaporequilibration).Anothercategoryofwaterpotentialsensorsuseasolidmatrixequil-ibrationtechniquetomeasurethewaterpotentialofthesoil.Thistechniqueintroducesaknownmaterialwithastaticmatrixofporesintothesoilandallows it tocomeintohydraulicequilibriumac-

MPS-2 Water Potential Sensor 3.Theory

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cordingtotheSecondLawofThermodynamicsnotedabove.Be-causethetwoareinequilibrium,measuringthewaterpotentialofthesolidmatrixwillgivethewaterpotentialofthesoil.Historical-ly,instrumentshavemeasuredthethermalconductivityorelectri-calconductivityofthesolidmatrixtodetermineitswaterpotentialwithvaryingdegreesofsuccess.TheMPS-2usesthesameprinciple,butinsteadmeasuresthedielec-tricpermittivityofasolidmatrix-porousceramicdisks-todeter-mineitswaterpotential.Thedielectricpermittivityofair,thesolidceramic,andwaterare1,5,and80respectively.So,thedielectricpermittivityoftheporousceramicdisksishighlydependentontheamountofwaterthatispresentintheporespacesoftheceramic.Thus,bymeasuringthedielectricpermittivityoftheceramicdisks,awiderangeofwatercontentscanberesolved.

Water content and water potential are related by a relationshipuniquetoagivenmaterial,calledthemoisturecharacteristiccurve.TheceramicusedwiththeMPS-2hasawideporesizedistributionandisconsistentbetweendisks.So,ifthewatercontentofthece-ramic ismeasuredaccurately,alongwithameasurementofactualwaterpotential,thenacalibrationcurveisgeneratedthatwillgivea standardcalibrationfortheMPS-2 intermsofwaterpotential.ThiscalibrationisnotdependentonthetypeofsoilintowhichtheMPS-2isinstalled.

Thetotalsoilwaterpotential(Yt)ismadeupoffourwaterpoten-tialcomponents:

Yt=Yp+Yg+Yo+Ym

wherethesubscriptsp,g,o,andmarepressure,gravitational,os-motic,andmatricrespectively.Ofthesefourcomponents,onlyYo

Water Potential Sensor MPS-23.Theory

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andYmaresignificantandoftenmeasuredinsoil.Yoarisesfromdissolvedsaltsinthesoil,andonlybecomesimportantifasemiper-meablebarrierispresentthatpreventsionicmovement(e.g.plantroots,cellmembranes).Ymarisesfromtheattractionofwatertothesoilparticles,andisthemostimportantcomponentofwaterpoten-tialinallbutthemostsaltaffectedsoils.TheMPS-1onlymeasuresthematricpotentialofthesoil(Ym).Inhighlysaltaffectedsoils,itmaybenecessarytoquantifyYoindependentlyifmeasuresofsoilwaterpotentialrelatingtobiologicalactivityarebeingconducted.

Temperature MeasurementTheMPS-2usesasurface-mountedthermistortotaketemperaturereadings. It is located underneath the sensor overmold, and willreadthetemperatureofthesensorsurface.TheMPS-2willoutputtemperaturein°CunlessotherwisestatedinyourpreferencesfileineithertheDataTrac3orECH2OUtilityprograms.Itisimportanttonotethatiftheblackplasticovermoldofthesensorisindirectsunshine, thetemperaturemeasurementmayreadhigh.Exposureoftheovermoldtosolarradiationwillalsodrasticallydecreasethelifeexpectancyofthesensor.Wedonotrecommendthatthesensorbeinstalledwiththeovermoldinthesun.

MPS-2 Water Potential Sensor 4.MeasurementRangeAndAccuracy

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4. Measurement Range And Accuracy

Measurement RangeAs described in the Theory chapter, the MPS-2 sensor mea-suresthewatercontentofporousceramicdisksandconvertsthemeasured water content to water potential using the moisturecharacteristiccurveof theceramic. It is important to the sen-sor design that the ceramics drain over a wide water potentialrange.Thewaterpotentialatwhichaporedrains(theairentrypotentialorbubblepressure)isdefinedbythesizeoftheporesotheidealceramicwouldhaveporesthatrangefromverysmalltorelativelylarge.

The MPS-2 ceramic has been specially designed to approachthisideal. Despiteseveraldecadesofdevelopment,theMPS-2ceramicstillhasatotalporevolumethatisweightedtowardthelarger pores that drain at wetter water potentials. This limitstheeffectivemeasurementrangeoftheMPS-2totherangeof-5kPa(theairentrypotentialofthelargestporesintheceramic)to-500kPa,withsomelimitedsensitivitydrierthan-500kPa(itcangivemeaningfulqualitativeinformationtoairdry).Howev-er,iftheoutputfromthesensorisdrierthan-500kPa,itshouldnotbeusedforquantitativeanalysisbecauseofpotentiallylargeerrors in the magnitude of water potential. Also note that asthesensordriespast-500kPa,themeasuredwaterpotentialbe-comesincreasinglynoisyduetoverysmallchangesinmeasuredwatercontentoftheceramictranslatingintoverylargechangesinwaterpotential.Thisphenomenonismostpronouncedwhenthesensorisairdry.Itisexpectedthatthemeasuredwaterpo-

Water Potential Sensor MPS-24.MeasurementRangeAndAccuracy

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tential of an air dry sensor can randomly oscillate throughouttherangeof-30,000kPato-100,000kPa.

TheairentrypotentialofthelargestporesintheMPS-2ceram-icisabout-5kPa.However,theMPS-2ceramicmusthaveac-cesstoairforthelargeporestobegindrainingandtheresponseofthesensortochange. If thesoilwheretheMPS-2isplacedhasanairentrypotentialmorenegative(drier)than-5kPa,airwon’tbeabletoreachthesensoruntiltheairentrypotentialofthesoilisreached.So,inthisscenario,thewetendrangeoftheMPS-2willbelimitedbytheairentrypotentialofthesoilratherthantheceramicdisksthemselves.Thisisgenerallyonlyanis-suewhenusingtheMPS-2sensorinpoorlystructuredsoilswithhighclaycontent.

Measurement AccuracyThe MPS-2 ceramic has excellent properties to produce good ac-curacyinthewetrange. Thisresultsinaccuracyof±25%ofthemeasuredwaterpotentialintherangeof-5kPato-100kPa.Astheceramicdriestoward-500kPa,thetotalporevolumethatdrainsatagivenwaterpotentialdecreases substantially,creating lowerandlowersensitivityastheceramicdries.Thislackofsensitivitycreateslargeruncertaintyinthemeasurementaccuracy.Ourtestsindicatetheaccuracyat-300kPahasdecreasedtoapproximately±35%ofthemeasuredwaterpotential,andat-500kPatheaccuracyisap-proximately±50%.Notethatsomeofthevariabilityinthesensorcalibrationdataatwaterpotentialsdrierthan-100kPaislikelyduetotheuseofpressureplatestocollecttestdata.Despitethewell-documentedproblemswiththepressureplatemethod(Campbell,1988;Geeet.al,2002;BittelliandFury,2009;FrydmanandBaker,2009),wehavenotyetfoundamoreconvenientmethodtocheckaccuracydrierthan-100kPa.

MPS-2 Water Potential Sensor 4.MeasurementRangeAndAccuracy

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CalibrationEachMPS-2 sensor is calibratedair-dryandvacuumsaturatedatDecagon.Thistwo-pointcalibrationresultsintheaccuracystatedabove.Betteraccuracycanbeachievedthroughusercali-brationofthesensor.However,ifyouplantocalibratethesen-sorsusingthepressureplatemethod,pleasenotetheproblemsthatareintrinsictothatmethodasdescribedabove.

TheMPS-2calibrationisnotaffectedbysoiltypebecauseitonlymeasures the water potential of the ceramic disks in equilibriumwiththesoil.TheMPS-2sensorwillmeasureaccuratelyinanysoiltypeandotherporousmediaaslongasitisinstalledcorrectlywithadequatehydrauliccontact(toensuretimelywaterpotentialequi-libriumbetweenthesensorandthemediumofinterest).

HysteresisTheamountofwaterthatasoilholdsatagivenwaterpotentialwillbegreaterifthematerialisdriedtothatwaterpotentialthanifthematerialiswetuptothatwaterpotential;aphenomenonknownashysteresis. BecausetheMPS-2essentiallymakesadielectricmea-surementofwatercontentandconvertsthattowaterpotential,theMPS-2measurementswillhavesomehysteresis.Inmostsituations,soilundergoesbriefperiodsofwetup(precipitationor irrigationevents) followedby longerdrydownperiodswherewaterpoten-tialmeasurementsaremostuseful.ThecalibrationoftheMPS-2isperformedonthedryinglegofthehysteresisloop,sothemeasure-mentswillbemostaccurateasthesoildries.MPS-2measurementsas thesoilwetsupwillbeslightlydrier(morenegativewaterpo-tential)thanthetruewaterpotentialofthesoil.Ourwettinganddryingtestsshowthemagnitudeoftheerrorisabout10kPainthe-20kPato-100kParange.

Water Potential Sensor MPS-25.ConnectingtoaLogger

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5. Connecting to a Logger

The MPS-2 sensor was designed to be used with Decagon’sEm50,Em50R,Em50GdataloggersortheProCheckhandheldreader. The standard sensor (with 3.5 mm stereo connector)quicklyconnects toand is easilyconfiguredwithinaDecagonloggerorselectedinProCheck.

The MPS-2 sensor incorporates several features that also make itanexcellentsensorforusewiththirdpartyloggers.Thesensormaybepurchasedwithstrippedandtinnedwires(pigtail)forterminalconnections.

Extending sensor cablesThe MPS-2 sensor comes standard with a 5 meter cable. Ourtestingindicatesthatsensorcablescanbeextendedupto75me-ters (250 ft). Sensors may be purchased with custom cablelengthsforanadditionalper-meterfee.Thisoptioneliminatesthe need for splicing the cable. If you do need to extend thecable,pleasebesuretoadequatelywaterproofthecable splicesas inadequately waterproofed cable splices are a major failurepoint.Visitwww.decagon.comfordetailedsuggestionsonwa-terproofingcablesplices.

Connecting to Em50/Em50R/Em50G loggerTheMPS-2hasbeendesignedtoworkspecificallywiththeEm50datalogger.Simplyplugthe3.5mm“stereoplug”connectordirectlyintooneofthefivesensorports.ThenextstepistoconfigureyourloggerportfortheMPS-2andsetthemeasurementinterval,thismaybedoneusingeitherECH2OUtilityorDataTrac3(seerespec-

MPS-2 Water Potential Sensor 5.ConnectingtoaLogger

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tivemanuals).PleasecheckyoursoftwareandfirmwareversionstoensureitwillsupporttheMPS-2.Toupdatetothelatestversion,pleasevisitDecagon’ssoftwaredownloadsite:

www.decagon.com/support

Thefollowingsoftware/firmwaresupportstheMPS-2sensor: ECH2OUtilityversion1.62orgreater DataTrac3version3.3orgreater Em50’sversion2.5orgreater ProCheckreadersversion1.29orgreater

Todownloaddata fromaDecagon logger toyourcomputer,youwillneedtousetheECH2OUtilityorDataTrac3.

3.5mm Stereo Plug WiringDigital out

Ground Excitation

Connecting to a Non-Decagon LoggerMPS-2 sensor may be purchased for use with non-Decagon dataloggers.Thesesensorstypicallycomepre-configuredwithstrippedandtinned(pigtail)leadwiresforusewithscrewterminals.Refertoyourdistinct loggermanualfordetailsonwiring.Ourintegra-tor’s guide gives detailed instructions on connecting the MPS-2sensor to non-Decagon loggers. Please visit www.decagon.com/

Water Potential Sensor MPS-25.ConnectingtoaLogger

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supportforthecompleteintegrator’sguide.

Pigtail End Wiring

Digital out (Red)

Ground (Bare)

Excitation (White)

Sensor cable

Connectthewirestothedataloggerasshown,withthesupplywire(white)connectedtotheexcitation,thedigitaloutwire(red)toadigitalinput,thebaregroundwiretogroundasillustratedbelow.

L G

SupplyDigital

out Ground

Datalogger

DigitalIn

3.6-15Vdc

NOTE:Theacceptablerangeofexcitationvoltagesisfrom3.6to15VDC.IfyouwishtoreadtheMPS-2withtheCampbellScien-tificDataLoggers,youwillneedtopowerthesensorsoffofthe12Vorswitched12Vport.

IfyourMPS-2isequippedwiththestandard3.5mmplug,andyouwishtoconnectittoanon-Decagondatalogger,youhavetwoop-tions.First,youcanclipofftheplugonthesensorcable,stripandtinthewires,andwireitdirectlyintothedatalogger.Thishasthe

MPS-2 Water Potential Sensor 5.ConnectingtoaLogger

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advantageofcreatingadirectconnectionwithnochanceofthesen-sorbecomingun-plugged;however,itthencannotbeeasilyusedinthefuturewithaDecagonreadoutunitordata logger.TheotheroptionistoobtainanadaptercablefromDecagon.The3-wiresen-soradaptercablehasaconnector for thesensor jackononeend,and threewireson theotherend forconnection toadata logger(thistypeofwireisoftenreferredtoasa“pigtailadapter”).Boththestrippedandtinnedadaptercablewireshavethesameterminationas seenabove; thewhitewire isexcitation, red isoutput,andthebarewireisground.

Water Potential Sensor MPS-26.CommunicationNonDecagonLoggers

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6. Communication Non Deca-gon Loggers

TheMPS-2sensorcancommunicateusingtwodifferentmethods,Serial (TTL) and SDI-12. In this chapter we will briefly discussthespecificsofeachofthesecommunicationmethods.Pleasevisitwww.decagon.com/support for the complete integrator’s guide,whichgivesmoredetailedexplanationsandinstructions.

Serial CommunicationWhenexcitationvoltage isapplied, theMPS-2makesameasure-ment.Withinabout40msofexcitationtwomeasurementvaluesaretransmittedtothedataloggerasaserialstreamofASCIIchar-acters.Theserialoutis1200baudasynchronouswith8databits,noparity,andonestopbit.Thevoltagelevelsare0-3.6VandthelogiclevelsareTTL(activelow).Thepowermustberemovedandreap-pliedforanewsetofvaluestobetransmitted.

The ASCII stream contains 2 numbers separated by spaces. Thestream is terminated with the carriage return character. The firstnumberiswaterpotentialoutputinkilopascals.Thesecondnum-beristhetemperatureindegreesCelsius.

SDI-12 CommunicationThe MPS-2 sensor can also communicate using the SDI-12 pro-tocol, a three-wire interfacewhereall sensorsarepowered(whitewire),grounded(barewire),andcommunicate(redwire)onsharedwires (for more info, go to www.sdi-12.org). Below is a brief de-scriptionofSDI-12forcommunication.IfyouplanonusingSDI-

MPS-2 Water Potential Sensor 6.CommunicationNonDecagonLoggers

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12forcommunicationwiththeMPS-2,pleaseseeourintegrator’sguideatwww.decagon.com/supportfordetailedinstructions.

Sensor BusThereareseveralbenefitsanddrawbacksregardingtheSDI-12pro-tocol.Onebenefitisthatupto62sensorscanbeconnectedtothesame12Vsupplyandcommunicationportonthedatalogger.Thissimplifieswiringbecausenomultiplexerisnecessary.Thedrawbacktousingmultiplesensorsononebusisthataproblemwithasinglesensor can bring down the entire array (through a short circuit,etc.).Toavoidthisproblem,werecommendtheusermakeanin-dependentjunctionboxwithwireharnesseswhereallsensorwiresarewiredtowirelugssosensorscanbedisconnectedifaproblemarises.Asinglethree-wirebundlecanberunfromthejunctionboxtothedatalogger.

AddressTheSDI-12protocolrequiresthatallsensorshaveauniqueaddress.MPS-2sensorscomefromthefactorywithanSDI-12addressof0. To add more than one SDI-12 sensor to a bus, the sensor ad-dressmustbechanged.Addressoptionsinclude{0…9,A…Z,a…z}.ThebestandeasiestwaytochangeanaddressistouseDecagon’sProCheck(iftheoptionisnotavailableonyourProCheck,pleaseupgradetothelatestversionoffirmware).SDI-12addressingcanbeaccessedinthe“CONFIG”menubyselecting“SDI-12Address”.Addressesmaythenbechangedbysimplypressingtheupordownarrowsuntilyouseethedesiredaddressandpushing“Enter”.

Water Potential Sensor MPS-26.CommunicationNonDecagonLoggers

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PowerThesensorcanbepoweredusinganyvoltagefrom3.6to15VDC,but12Visoptimal.AlthoughSDI-12protocolallowsthesensorsto be continuously powered, you may connect the power (whitewire) to a switching source. This can help reduce power use (al-thoughtheMPS-2sensorsuseverylittlepower).

ReadingSDI-12communicationallowsmanyparameters tobecommuni-catedatonce.Thisallowsyoutoseeinformationsuchasthesensormodel, SDI-12 version, temp, etc. Reading the MPS-2 sensor inSDI-12moderequiresfunctioncalls.

Thewaterpotentialinkilopascalsisthefirstnumberoutputbythesensor.ThesecondnumberistemperatureinCelsius.The SDI-12 communication protocol is supported in CampbellScientificdataloggersliketheCR10X,CR200,CR1000,CR3000,etc.DirectSDI-12communicationissupportedinthe“TerminalEmulator”modeunderthe“Tools”menuonthe“Connect”screen.DetailedinformationonsettingtheaddressusingCSIdataloggerscanbefoundonourwebsiteathttp://www.decagon.com/support.

MPS-2 Water Potential Sensor 7.InstallingtheSensors

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7. Installing the Sensors

Becauseitmeasureswaterpotential,theMPS-2isnotassensitivetoairgapsorsoildisturbanceaswatercontentsensors.Itdoes,how-ever, need goodhydraulic contact with the surrounding soil. Thepreferred method for installing the sensor is to take some nativesoil,wetit,andpackitinaballaroundtheentireMPS-2,makingsurethatthemoistsoilisincontactwillallsurfacesoftheceramic.Thesensorandmoistsoilarethenpackedintothesoilatthedesireddepth.

Insandysoils,thesoilmaynotadheretothesensorevenwhenwet.Inthiscasethesensorcanbepackedintosoilatthebottomofaholedugtothedesiredinstallationdepth.Again,careshouldbetakentopackthesandysoilaroundthesensorwithgoodcontacttoallceramicsurfaces.

Afterinstallingthesensorandmoistsoil,theholethatwasexcavat-edtoburythesensoratdepthshouldbeback-filledwithcaretakentore-packthesoilbacktoitsnativebulkdensity.Itisbesttoleaveatleastsixinchesofsensorcablebeneaththesoilbeforebringingthecabletothesurface.Thecableshouldneverbebentinatightradiusasitleavesthesensorbody.Atleastfourinchesofcableshouldexitthesensorbodyinastraightlinebeforebendingthecable.

Water Potential Sensor MPS-28.CampbellScientificPrograms

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8. Campbell Scientific Programs

Becausethesensorsusesdigitalratherthananalogcommunication,theyrequirespecialconsiderationswhenconnectingtoaCampbellScientificdatalogger.Pleasevisitourwebsiteathttp://www.deca-gon.com/supporttoviewsampleCampbellScientificprograms

MPS-2 Water Potential Sensor 9.HandlingandCare

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9. Handling and Care

The MPS-2 sensor measures the water potential of two engi-neered ceramic disks sandwiched between stainless steel screensand the MPS-2 circuit board. The ceramic disks are somewhatbrittle, and can chip or crack if abused. The metal screens affordthediskssomeamountofprotection,butsharptraumaonthediskedgesormassiveimpact(suchasdroppingthesensorontoahardsurface) can cause the ceramic to break. One or two small chipsontheedgeofthediskwillnotaffectthesensoraccuracysignifi-cantly. However, a cracked ceramic will create a loss of accuracy.FortheMPS-2toaccuratelymeasurewaterpotential,theceramicdisksmustreadily takeupwater. If theceramic isexposedtooilsor other hydrophobic substances, then the ability of the disks totake up water from the soil can be compromised leading to slowequilibration times and/or loss of accuracy. It is recommendedthat exposure of the ceramic material to skin oils be minimizedas much as is conveniently possible, and it is highly recommend-ed that the disks not be handled with greasy hands, or other-wise exposed to synthetic oils or other hydrophobic compounds.

MPS-2 in Frozen SoilsThe MPS-2 measures the dielectric permittivity of two ceramicdisks to measure their water content and then derive their waterpotential.Thedielectricpermittivityofwaterintheceramicdisksis 80 compared to a dielectric permittivity of ~5 for the ceramicmaterial,and1forair.Whenwaterfreezestoice,thedielectricper-mittivitydropsto5atthefrequencyoftheMPS-2measurementmeaningthattheMPS-2cannolongeraccuratelymeasurethewa-terintheceramic.Underfrozensoilconditions,theMPS-2willnot

Water Potential Sensor MPS-29.HandlingandCare

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accuratelymeasurethewaterpotentialofsoil.However,underfro-zensoilconditions,thewaterpotentialofthesoilcanbemeasuredsimply by measuring the soil temperature accurately (KoopmansandMiller,1966).Foreach1°Cdecreaseintemperaturebelow0°C,thewaterpotentialinthesoildecreasesby~1200kPa.SpaansandBaker(1996)showedthatthisrelationshipisvalidinfieldsoilsforwaterpotentialsbelowabout-50kPa.RigoroustestingindicatesthattheMPS-2ceramicdisksareunaf-fectedbyrepeatedfreeze-thawcycles.Severalsensorswereequili-brated in saturated soil, and then subjected to numerous freeze-thawcyclesinatemperaturecontrolchamber.Thefreezingrateofthe soilcontainerswasat leastanorderofmagnitude faster thancouldbeachievedinfieldsoilundernaturalconditions.Atseveralpointsduringthetest,andattheendofthetest,theceramicdiskswereevaluatedfordamageduetorepeatedrapidfreezingwithporespacesfullofwater.Noneoftheceramicdisksshowedanysignsofphysical damage, and none of the sensors showed any significantchangeinoutputduetothefreezingtests.

MPS-2 Water Potential Sensor 10.Troubleshooting

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10. Troubleshooting

IfyouencounterproblemswiththeMPS-2sensor,theymostlikelywillmanifestthemselvesintheformofincorrectorerroneousread-ings.BeforecontactingDecagonaboutthesensor,dothefollowing:

Data Logger

1.Checktomakesuretheconnectionstothedataloggerarebothcorrectandsecure.

2.Ensurethatyourdatalogger’sbatteriesarenotdeadorweak-ened.

3.ChecktheconfigurationofyourdataloggerinECH2OUtil-ityorDataTractomakesureyouhaveselectedMPS-2.

4.Ensurethatyouareusingthemostuptodatesoftwareandfirmware.

Sensors

1.Ensurethatyoursensorsareinstalledaccordingtothe“Installa-tion”sectionofthismanual.

2.Checksensorcablesfornicksorcutsthatcouldcauseamal-function.

3.Checktheceramicdisctoensurethatitisnotdamagedorcontaminated.

Water Potential Sensor MPS-2References

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References

Bittelli, M. And M. Flury (2009) Errors in Water Reten-tion Curves Determined with Pressure Plates. SoilScience Society of America Journal 73: 1453-1460

Campbell, G. S. (1988) Soil water potential measure-ment: an overview. Irrigation Science 9:265-273.

Frydman,S.andR.Baker(2009)TheoreticalSoil-WaterCharacteristicCurvesBasedonAdsorption,Caivitation,andaDoublePoroci-tyModel.InternationalJournalofGeomechanics9(6):250-257.

Gee, G. W., A. L. Ward, Z. F. Zhang, G. S. Campbell and J.Mathison (2002) The influence of hydraulic nonequilibri-um on pressure plate data. Vadose Zone Journal 1:172-178.

Koopmans,R.W.R.,andR.D.Miller1966.Soilfreezingandsoilwatercharacteristiccurves,SoilSci.Soc.Am.Proc.,30,680–685,1966.

Spaans, E. J. A., and J. M. Baker 1996. The soil freezing char-acteristic: Its measurement and similarity to the soil mois-ture characteristic, Soil Sci. Soc. Am. J., 60, 13– 19, 1996.

MPS-2 Water Potential Sensor AppendixA

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Appendix A

Declaration of Conformity

Application of Council Directive: 89/336/EE6

Standards to which conformity is declared:EN61326 :1998EN51022:1998

Manufacturer’s Name:

DecagonDevices,Inc.2365NEHopkinsCourtPullman,WA99163USA

Type of Equipment:DielectricWaterPotentialSensor Model Number:MPS-2Year of First Manufacture:2011

This is tocertify that thedielectricwaterpotential sensor,manu-facturedbyDecagonDevices,Inc.,acorporationbasedinPullman,Washington,USAmeetsorexceedsthestandardsforCEcompli-anceaspertheCouncilDirectivesnotedabove.AllinstrumentsarebuiltatthefactoryatDecagonandpertinenttestingdocumenta-tionisfreelyavailableforverification.

Water Potential Sensor MPS-2AppendixA

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MPS-2 Water Potential Sensor AppendixA

26

IndexAAbouttheMPS-14Accuracy9

CCalibration10Care20Conformity24ContactInformation1

DDielectricpermittivity6

EE-mail2Energyofwater5

FFrozenSoils20

HHandling20Hydrauliccontact18Hysteresis10

IInstallingtheSensors18

Water Potential Sensor MPS-2AppendixA

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LLiability3

MManufacturer24ModelNumber24

RRange8References21

SSandysoils18Soilwatercontent5Soilwaterpotential5

TTelephone2Tensiometers5

WWarranty3Waterpotentialcomponents6