电容培训

2019/9/17Endress+HauserChina周晓1电容物位测量此选择是您的明智决定2019/9/17Endress+HauserChina周晓2电容器基础Eachelectricalchargedmaterialissurroundedbyanelectricalfield.Betweendifferentchargedmaterialselectricalfieldlinesfrompositivetonegativearecreated.Twounequalchargedmetalplatesbuildacapacitor.2019/9/17Endress+HauserChina周晓3电容器的基础Theprincipleofthecapacitancelevelmeasurementisbasedonthechangeincapacitancethroughthechangeofthelevel.Thecapacitanceofacapacitordependson3factors:Distancebetweentheelectrodes(d)Dimensionsoftheelectrodes(A)Dielectricbetweentheelectrodes(er)3dACree0e0:electricalfieldconstant:8,854*1012C/(Vm)er:permitivity,dielectricconstantA2019/9/17Endress+HauserChina周晓4r不同介电常数的材料固体DolomiteFerricoxideGrainBrokenglassCoffeeSlakedlimeQuicklimeCoallumpsCoaldustFeedFlourNylongranulatesSilicapowderRiceSand,dryWashing-powderer7,8...89,32,2...3,61,2...54,653,34,52,32,3...34,51,823,53...51,1...1,3液体AlcoholPetrolChloroformDesmophen5100Desmodur44VDeuteriumLiquidgas-Chlorine-Air-140°C-Butane-PropaneGlycerinFueloilLatexPetroleumWaterer32...35,52,278,121,31,81,51,21,213,22,1162,181Air=12019/9/17Endress+HauserChina周晓5电容器基础Inlevelmeasurementusuallyonlyonepartofthecapacitorisfilledwithmaterial.Thetotalcapacitanceiscalculatedfromthesumofthetwopartialcapacitances.ThechangeofcapacitanceDCdependsonlyontheheighthandthedielectricconstanterofthematerial.C=CMAT+CAIR2019/9/17Endress+HauserChina周晓6电容器基础Becauseofthechangeinpolarityatanalternatingvoltage,electricallosseswillbecreated.TheselossesaredescribedbyanohmicresistorXCparalleltothecapacitanceC.CRCXC1C:capacitance[F]:2··f[1/s]f:measuringfrequency[Hz]XC:capacitiveblindresistance2019/9/17Endress+HauserChina周晓7电容器基础Normally,cylindricaltanksareusedinthefield.Then,thecapacitorisformedfromtheprobeandthetankwall.LDderCapacitiveroddDLCr/ln20eeTankwall2019/9/17Endress+HauserChina周晓8InordertoconvertthechangeofcapacitanceintoanelectricalsignalthecapacitorisconnectedtoanACvoltage.测量原理Capacitancechangescausedbythelevelresultinachangeofthemeasuringcurrent.ACvoltagesourceMeasuredcurrentCapacitiverod------------------------------------------------------------------------------------------------+--+-+-+2019/9/17Endress+HauserChina周晓9KLJAchangeofthedielectricconstanthasnoinfluenceonthecapacitancebecausethefilledpartofthevesselwallapparentlymovestotheprobe.Themeasuringcurrentdoesnotonlydependonthedielectricconstant,butalsoontheelectricalconductivityofthemedia.电导率ConductingmediaLowconductingmediaNonconductingmediaThedielectricconstanthastobestable.Ifitchanges,e.g.byhumiditythelevelcannotbedeterminedreliableanymore.2019/9/17Endress+HauserChina周晓10最小探头长度Foralimitdetection(Nivotester)theminimalchangeofthecapacitanceDCminshouldbe5...10pF.Forcontinuousmeasurement(Silometer)theminimalchangeofthecapacitanceDCminshouldbe10pF(FEC12).NonconductivemediaLmin=DCminCs•(er-1)Lmin=DCminCsConductivemediaer=dielectricconstant:oil=2chlorine=1.8Cs=capacityoftheproberodatadistanceof250mmfromvessel:13pF/mprobewithgroundtube:55pF/mconductivemedia:400pF/mDCmin=minimalchangeofcapacityLmin2019/9/17Endress+HauserChina周晓11最小探头长度(导电介质,如：酸)Lmin===1,3cmDCminCs5pF400pF/mLmin===2,5cmDCminCs10pF400pF/mLimitdetectioninacids:Continuousmeasurementinacids:Lmin2019/9/17Endress+HauserChina周晓12最小探头长度(非导电介质,如：油)Withgroundtube:Withoutgroundtube:Limitdetectioninoil:Lmin===9cmDCminCs•(er-1)5pF55pF/m•(2-1)Lmin===38,5cmDCminCs•(er-1)5pF13pF/m•(2-1)Lmine2019/9/17Endress+HauserChina周晓13最小探头长度(非导电介质,如：油)Withgroundtube:Withoutgroundtube:Continuousmeasurement:Lmin===18cmDCminCs•(er-1)10pF55pF/m•(2-1)Lmin===77cmDCminCs•(er-1)10pF13pF/m•(2-1)mineL2019/9/17Endress+HauserChina周晓14Rodandropeprobe实际的解决方案Full-orpartiallyinsulatedprobesProbeswithgroundtube2019/9/17Endress+HauserChina周晓15实际的解决方案Screening(inactivelength)Activebuild-upcompensationTemperaturespacer2019/9/17Endress+HauserChina周晓16Advantagesoftheropeprobe:-shippingissimpler-thereisusuallylimitedspaceforinstallation-mechanicalstabilityoflongrodprobescannotbeguaranteedRodProbes100....4000mmRopeprobes420....26.000mm实际的解决方案杆式和缆式的探头2019/9/17Endress+HauserChina周晓17实际的解决方案温度的考虑及全绝缘和半绝缘探头Athighoperatingtemperaturesinthevesselstheelectronicisprotectedbyatemperaturespacer.ConductivemediaNonconductivemediaFullyinsulatedprobePartiallyinsulatedprobeFullyinsulatedprobePartially/fullyinsulatedprobeforaggressivemedia2019/9/17Endress+HauserChina周晓18-Increaseinsensitivityformediawithlowdielectricconstants(factor5)-Referencepotentialinnonmetallicvessels-Increasedlateralstabilityofprobesusedintankswithagitators-Lineariselevelindicationinhorizontalcylindricalandball-shapedtanks实际的解决方案带接地管的探头Advantagesofthegroundtube:Thegroundtubebuildsthesecondelectrodeoftheprobe2019/9/17Endress+HauserChina周晓19实际的解决方案带屏蔽管的探头Advantagesofscreenedprobes:Thesepreventerrorscausedbytheeffectofcondensationormaterialbuild-uponthevesselwallneartheprocessconnection.2019/9/17Endress+HauserChina周晓20-Usedifaconductivelayercoverstheprobe-Highlycriticalapplicationscanbesolvedwiththeactivebuild-upcompensation.实际的解决方案带抗粘附的探头Advantagesofactivebuild-u