自动化专业英语(第三版)教学课件l

1,Lesson2.4TemperatureMeasurement,Topicscoveredinthisunitareasfollows:Theprinciplesoftemperaturemeasurement(温度测量)Theprincipleofthermocouple(热电偶)Thewidevariety(种类)oftemperaturemeasuringdevices,1,2,1.Principlesoftemperaturemeasurement,Temperaturemeasurementisveryimportantinspheresoflifeandespeciallysointheprocessindustries.However,itposesparticularproblems,sincetemperaturemeasurementcannotberelatedtoafundamentalstandardoftemperatureinthesamewaythatthemeasurementofotherquantitiescanberelatedtotheprimarystandardsofmass,lengthandtime.,2,3,1.Principlesoftemperaturemeasurement,Thisisarootcauseofthefundamentaldifficultiesthatexistinestablishinganabsolutestandardfortemperatureintheformofarelationshipbetweenitandothermeasurablequantitiesforwhichaprimarystandardunitexists.Intheabsenceofsucharelationship,itisnecessarytoestablishfixed,reproduciblereferencepointsfortemperatureintheformoffreezingandboilingpointsofsubstances,wherethetransitionbetweensolid,liquidandgaseousstatesissharplydefined.,3,建立起温度的绝对标准，使其与其它可测的标准单位量联系起来，这是温度测量的基本困难之一。,由于缺乏这种关系，有必要以物质冰点和沸点形式建立固定的、可复制的参考点，物质状态特指在固体、液体和气态状态之间变化的瞬间。,4,1.Principlesoftemperaturemeasurement,TheInternationalPracticalTemperatureScale(IPTS)usesthisphilosophyanddesignessixprimaryfixedpointsforreferencetemperaturesintermsof:Thetriplepointofequilibriumhydrogen-259.34CTheboilingpointofoxygen-182.962CTheboilingpointofwater100.0CThefreezingpointofzinc419.58CThefreezingpointofsilver961.93CThefreezingpointofgold1064.43C,4,国际实用温标使用这种思想并设计六个主要固定点作为参考温度：平衡氢的三态点：-259.34C，氧的沸点：-182.962C水的沸点：100.0C，锌的冰点：419.58C银的冰点：961.93C，金的冰点：1064.43C。,5,1.Principlesoftemperaturemeasurement,Thefreezingpointsofcertainothermetalsarealsousedassecondaryfixedpointstoprovideadditionalreferencepointsduringcalibrationprocedures.Instrumentstomeasuretemperaturecanbedividedintoseparateclassesaccordingtothephysicalprincipleonwhichtheyoperate.Themainprinciplesusedare:,5,某种其它金属的冰点也可以用作第二个固定点，从而在标定过程中提供额外的参考点。,根据温度测量装置操作的物理原理，它们可以被分为不同的类型。使用的主要原理是：,6,1.Principlesoftemperaturemeasurement,ThethermoelectriceffectResistancechangeSensitivityofsemiconductordeviceRadiativeheatemissionThermographyThermalexpansionResonantfrequencychangeSensitivityoffiberopticdevicesAcousticthermometryColourchangeChangeofstateofmaterial,6,热电势效应电阻变化半导体装置的灵敏度辐射散热热像仪热膨胀（系数）谐振频率变换光纤器件的灵敏度声测温技术颜色变化材料状态变化,7,2.Thermocouples,Thermoelectriceffectsensorsrelyonthephysicalprinciplethat,whenanytwodifferentmetalsareconnectedtogether,ane.m.f.,whichisafunctionofthetemperature,isgeneratedatthejunctionbetweenthemetals.Thegeneralformofthisrelationshipis:e=a1T+a2T2+a3T3+anTnWhereeisthee.m.f.generatedandTistheabsolutetemperature.,7,热电势效应传感器基于的物理原理是，当任意两个不同的金属连接在一起时，将在金属的接头处产生感应电动势，该电动势是温度的函数。,这个关系的一般形式是：e=a1T+a2T2+a3T3+anTn其中，e是感应电动势，T是绝对温度。,8,2.Thermocouples,Thisisclearlynon-linear,whichisinconvenientformeasurementapplications.Fortunately,forcertainpairsofmaterials,thetermsinvolvingsquaredandhigherpowersofT(a2T2,a3T3,etc.)areapproximatelyzeroandthee.m.f.-temperaturerelationshipisapproximatelylinearaccordingto:ea1T,8,它很明显是非线性的，对于测量应用是不方便的。,幸运地是，对于一些材料，T的平方和更高次幂近似等于零，电动势-温度的关系近似为线性，如下所示：ea1T,9,2.Thermocouples,Wiresofsuchpairsofmaterialsareconnectedtogetheratoneend,andinthisformareknownasthermocouples.Thermocouplesareaveryimportantclassofdeviceastheyprovidethemostcommonlyusedmethodofmeasuringtemperaturesinindustry.,9,这对材料的线一头连接起来，这种形式被认为是热电偶。,热电偶是一种很重要的装置，因为它们为工业温度测量提供了最为普遍的方法。,10,2.Thermocouples,Atypicalthermocouple,madefromonechromelwireandoneconstantanwire,isshowninFigure2.4.1(a).Foranalysispurposes,itisusefultorepresentthethermocouplebyitsequivalentelectricalcircuit,showninFigure2.4.1(b).,10,典型的热电偶由镍铬合金和康铜线制成，如图2.4.1(a)所示。,出于分析的目的，使用等效电路代替热电偶，如图2.4.1(b)所示。,11,2.Thermocouples,Thee.m.f.generatedatthepointwherethedifferentwiresareconnectedtogetherisrepresentedbyavoltagesource,E1,andthepointisknownasthehotjunction.ThetemperatureofthehotjunctioniscustomarilyshownasThonthediagram.Thee.m.f.generatedatthehotjunctionismeasuredattheopenendsofthethermocouple,whichisknownasthereferencejunction.,11,相连接处不同的导线所产生的感应电动势可以用电压源E1表示，该点为热结点。,热结点的温度在图中习惯性的表示为Th。,12,2.Thermocouples,Thermocouplewiresaremanufacturedtoclosetolerancesandtendtobeexpensive.Theiruseisthuslimitedtotheprobeitself.Thermocoupleextensionwires,compatiblewithT/Cwires,areusedasthelinkbetweentheT/Candthemeasuringdeviceortransducer.,12,热电偶导线的制造具有高精度公差，并且昂贵。因此，它们的使用仅限于电极。,热电偶延伸导线，和热电偶导线兼容，用于热电偶和测量装置或变送器之间的连接。,13,2.Thermocouples,TheevolutionofmodernelectronicshascreatedtransducersmallenoughtofitinsidetheT/Cbox(orhead).Themajoradvantageofthisarrangementistheavoidanceoflong-distancetransmissionofaverylowT/Cvoltagesignal.Suchalowvoltagesignalispronetoelectricalnoise(asopposedtothe420mAsignal).,13,现代电子技术的发展已经能够制造足够小的变送器，可以将其安装在热电偶盒子（接头）内部。,这种配置的主要优势是可以避免热电偶低压信号的长距离传输。,这种低压信号倾向于是电噪声（相对于420mA的信号）。,14,2.Thermocouples,ApplicationNotesThethreebasictypesofT/Cconstructionare:Ceramicbeaded,Insulated(plastic,glass,orceramicfiber);theyaregenerallyextruded(Figure2.4.2),14,现代T/C的三种基本结构形式：陶瓷珠状（热电偶）绝缘热电偶（塑料、玻璃、陶瓷纤维）；它们通常可以被挤压（图2.4.2）,15,2.Thermocouples,Metal-sheathedmineral-insulated(MSMI);thesheathisgenerallystainlesssteelorInconel,andmineralinsulationisgenerallymagnesiumoxideoraluminumoxide.SheathedmaterialcangivetheT/Cexcellentprotectionfromoutsidechemicalandmechanicaleffect.However,sheathedT/Cs,sincetheyareaone-piececonstruction,aremoredifficulttostripandterminatethanothertypes.Thejunctionshouldbeweldedandinsulated.,15,带有金属护套与矿物质绝缘热电偶；护套通常是不锈钢的或铬镍铁合金的，矿物质绝缘体通常是氧化镁或者是氧化铝。,护套材料可以保护热电偶，防止外部带来的化学（腐蚀）和机械（损坏）。,然而，由于护套材料是一种片状结构，这种结构形式使它们比其他类型更加难以解除并终止。接触端应被焊接并绝缘。,16,2.Thermocouples,Thermocouplescanbeconstructedtobeprotectedorexposed.Ifprotected,theycanbegroundedorungrounded.Whengroundedtheygiveafasterresponse(sincegoodtemperaturetransferisobtained);however,theyaresusceptibletoelectricalnoise(duetostrayelectricalsignalpickup).,16,热电偶可以被设计成被保护或易接触的结构。,如果被保护的，它们可以接地或者不接地。,当接地时，它们可以得到快速响应（因为可以实现良好的温度传递）；,然而，它们对电噪声是敏感的（由于离散的电信号拾取器）。,17,2.Thermocouples,WhenT/Csareungrounded,theyareslowertorespondbutareelctricallyisolated.Inaddition,aT/Cmaybespring-loadedinthethermowellsothatthetipandwellsurfaceremainincontacttoensuregoodheattransfer.Ifexposed,thefastestresponse