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倒车雷达英文文献

AREVERSINGTEMPERATURE-DIFFERENCEMEASUREMENTSYSTEMFORBOWENRATIODETERMINATION

J.H.McCAUGHEY

DepartmentofGeography,Queen’sUnirersity,Kingston,Ontario,Canada,K7L3N6

（Received10October,1980）

Atatract.ThedesignandperformanceofareversingtemperaturedifferencemeasurementsystemarereporteclThissystememploysfive-junctioncopper-constantanthermopilcsforthemeasurementofA'andATp,whilealinearizedthermistorisusedtomeasureTp.Fieldperformancehasbeencheckedagainstaprecisionlysimeteraswellasagainstasecondtemperaturedifferencemeasurementsysteminwhichdiodeareusedfortemperaturemeasurement.Inbothcases,theagreementbetweenthesystemsissatisfactoryforthemeasurementofhourlyvaluesoftheBowenratio.

1.Introduction

TheexperimentaldeterminationoftheBowenratiobythepsychrometricmethodrequirescarefulmeasurementofthegradientsoftemperatureandhumidityaboveasurface.Whereverthesegradientsbecomesmal（forexampleoveranaerodynamicallyroughsurfacesuchasforest,thenverycarefulandstringentmeasurementtechniquesarerequiredinordertoachievesatisfactoryresults.AnowcommonlyusedmethodtodeterminetheBowenratioistomeasuredirectlythedifferencesofdry-bulbtemperature（AT）andwet-bulbtemperature（ATp）oversomeheightinterval（Az）,andtointerchangethepsychrometersonaregularbasis.ThisapproachwasproposedbyTanner（1960）,andhasbeenoperationalizedbySargeantandTanner（1967）,BlackandMcNaughton（1971）,andSpittlehouseandBlack（1980）.Therationaleofthismethodisthat,ifsystematicerrorispresentinthemeasurementandiseitherconstantorslowlychangingtheninterchangingthepsychrometersbetweenlevelsshouldeliminateorgreatlyreducutheamountoferrorinthemeasurementofthemeantemperaturedifferencesandintheBowenratio（McNeilandShuttleworth,1975JThemethodwillnoteliminateasystematicerrorwhichisheightdependent,e.g.,differentradiationalheatingerrorsatdifferentlevels（SpittlehouseandBlack,1980）.

TheBowenratio（JI）,inthecaseofdirecttemperaturemeasurementsbetween

twolevels,isgivenbyFuchsandTanner（1970）as

wheresisthesloJuofthesaturationvapourpressurecurveatT,,,yisthepsychro-metricconstant,6Tpisthewet-bulbtemperaturedifferenceoverAz,Al"isthedry-bulbtemperaturedifferenceoverAz,andTpisthemeanwet-bulbtemperatureoftheairlayer.TheBowenratioinassociationwiththesurfaceenergybalanceequationgives

Boundary-Layer3feieorofogr21（1981）47-55.0006-8314/81/021I-4J047S0I.35.

Copyright©1981byD.ReidelPublishingCo..Dordrecht,Holland.andBoston.II.5.Z.

48

and

J.H.MCCAUGHEY

（3）

whereQgisthelatentheatflux,Qpisthesensibleheatflux,Q*isthenetradiationflux,andcthesoilheat0ux.

Inthesystemdescribedinthispaper,ATand6Tparemeasureddirectlywith

five-junctioncopper-constantanthermopilesandTp“nmeasuredwithathermistor.Thepsychrometersareinterchangcdregularlybetweenthetwolevelsofmeas-urement.ThesystemistermedtheRTDMS—reversingtemperature-differencemeasurementsystem—throughouttheremainderofthispaper.Thechoiceofthermopilesassensorswasmadebecause

（1）theyaresimpletoconstruct,

（2）theyaredirectreadingdeviceswhichrequirenoauxilliarycircuitry,（3）theircalibrationisstable,and（4）highaccuracyisobtainable.FuchsandTanner（1970）haveprovidedananalysisoftheerrorsin

（1）andsuggestthat,forsatisfactoryestimatesoflatentheattransferusing

（2）,aprecisionof0.01’CisnecessaryforATand

ATjandTpshouldbemeasuredto0.1O.TheRTDMSachievesthisprecision

forTzforthetemperaturedifferencesaprecisionof0.018’Cisapplicable（seeEquation4}

2.ConstructionoftheSystem

Thethermopileswereconstructedfrom30a.w.g.copper-constantanthermocouplewire.Foreachsensor,eachgroupoffivethermojunctionswasmountedinsideathin-walled,stainlesssteeltube（30cmlong,4.7mmO.D.,and0.25mmwallthickness）,andtheyweresealedatthetipofthetubeinpolyesterresin.Thetipofthetubehadbeenmachinedto4.4mmO.D.whichgaveawallthicknessof

0.10mmThewiresbetweentheendsofthethermopileweresleevedinplastictubingtominimizeabrasioninthefield.

Thethermopilewerecalibratedagainstaplatinumresistancethermometer,

andthecalibrationequation,whichappliestoeachsensor,is

6T———0.009=4.913-AT,

r'—0.999,s—0.018（‘CJ（4）

whereATistheelectricaloutput（mV）ofthesensor.AllthermopilesweretestedforsymmetryaspartofthecalibrationIfasensordeviatedbymorethan0.02’Cfromsymmetry,itwasrejectedforfielduse.Also,thetimeconstantofeachthermopilewasdetermined,andtheaveragevaluefoundwasoneminute.

Inthefield,thedry-andwet-bulbthermopilesweremountedinsidedoubleradiationshieldsandwereheldrigidlyinplacebyanacrylicplug（FigureIB）.Thewet-bulbwasmounted5cmbehindthedry-bulb.ThebasicdesignofthishousingisduetoLourenceandPruitt（1969）withmodificationsadaptedfrom

AREVERSINGTEMPERATURE•DIFFEREI"ICEMEASUPEMENTSYSTEM49

Fig.1.Detailsofreversingtemperaturedifferencemeasurementarray.（A）Fieldplacement;

（B）Radiationshielding;（C）Aspirationsystem;（D）Circuitdiagram.

Allen（1972}InoneofthehousinggalinearizedthermistorwhichmeasuredTpwasinserte‹1Theradiationshieldsweremadeofpolyurethanetubingwrappedontheoutsidewithaluminizedtape.Thetapehasaveryhighreflectivityandformsaneffectivebarriertosolarradiation（FuchsandTanner,1965）.Theinnersurfaceoftheinnershieldwaspaintedwithflatblackpaint,andtheinnersurfaceoftheoutershieldwascoveredinaJuminizedtape.Thesensorswereaspiratedbysmallfans（Rotron‘Nugget’NTH4（whichweremountedclosetothepivotpointofthesensorarm（FigurelC）.Theaspirationratepastthesensorswas3.4ins’.Thiswasdeterminedbyplacingahot-wireanemometer（Lambrecht641N）orthogonaltotheairflowattheentrancetotheinnershiel‹1Giventhesizeofthesensortips,thisventilationrateensuredvirtually100%wet-bulbdepression（Wylie,1962）.Also,thetheoreticalvalueofthepsychrometerconstant6.60›‹I0‘*（I+0.00115Tp）’C',wasapplicable（Tanner,1972）.

Inordertoensurethatthedoubleradiationshieldingworkstomaximum

efficiency,thetemperatureoftheinnershieldshouldbeequaltoairtemperature.

Itisimportantthereforethatairisallowedtopassbetweenthetwoshields.Thiswasachievedbycuttingventilationportsintheoutershieldtoallownaturalventilationbetweentheshields.Toenhancethisfreeairmotion,thewholesensorassemblyswivelledsothattheintakesofthehousingsfacedintotheprevailingwind.Thiswasaccomplishedbymountingtheassemblyonarotationbearing（FigureIA）,andattachingavaneattheendoftheshaftwhichheldthesensorarmTheassemblyrotatedthrough350‘andwasrestrainedbyastopconstructedfromathickrubberstripstretchedacrossaT-shapedsupportTheassemblyrotatedifthewindspeedwasabove0.25ins'

Distilledwaterwasconductedfromindividualreservoirstoeachwet-bulbbyawickmadefromcottonshoelace.Beforeuse,eachwickwasboiledinamildsolutionofdetergentandbleachtoremoveanydirtwhichmightbepresent.Thewickswerechangedregularlybyanoperatorwearingplasticgloves.Eachwet-bulbreservoir,mountedonthesideoftheacrylicplughadacapacityof

50mlarid,dependinguponthesaturationdeficitoftheair,hadtoberefilledevery1to3days.Tencentimetresofwick,sleevedoverthewet-bulb,wereezposedintheairstreaminsidetheradiationshieldtoensurethattherewasnoheatcon-ductiontothesensortipbythedistilledwater（LourenccandPruitt,1969）.Thereservoirswereconstructedofclearplastictubingandwerewrappedinaluminizedtapeinordertomi.nimizeradiationalheating.

Thesensorarmwasturnedthrough180‘bya4r.p.m.Hurstreversingmotor（modelDB）.Themotorwasmountednexttotheshaftwhichheldthesensorarm.Agearwheel,7.6cmindiameter,wasattachedtotheendofthemotor’sshaft,andanotheronewasmountedonthesensorarmatitspivotpointThesetwowheelswerelinkedtogetherbyachain.

Thereversingcyclewascontrolledbyanelectronictimerwhichactedasa

‘down-counter’（FigureID）.Thetimecouldiesetanywherefrom1to99minbytheuseofthumbscrews.Thecountwasinitiatedbyamanualswitchclosure,andthetimeremainingbefprereversalwasshownbyanLEDdisplay.Ifforanyreasontherewereaninterruptionintheexperiment,thetimercouldberesetbymeansofaresetswitch;thisinstantresetcapabilityvirtuallyeliminatedanypossibilityoftimingorco-ordinationproblemsbetweentheRTDMSandtherecordingsystem.Atpresent,thetimerrunsonACpowerandconsumesabout2tDW.MostofthepowerisusedintheLEDdisplay.AtotallyDCversionwhichwilloperatefroma12VbatteryisbeingdevelopedandwillhaveexactlythesamecountingcircuitastheACversionbutamuchsmallerpowerconsumption.ItisestimatedthataregularcarbatterywilloperatetheDCtimerforseveralweeksonasin8lecharge.Thecircuitforeithertimerisavailableonrequest.

Duringoperation,thetimercounted