Chapter 4 – Temperature and HeatFlux Measurement

2021-12-11cary_1Chapter 4 Temperature and Heat-Flux MeasurementTEMPERATURE is one of the most important of all physical quantities in industry. Its measurement plays a key part in industrial quality and process control, in the efficient use of energy and other resources, in condition monitoring and in health and safety. There are few material properties of technological or engineering importance which do not depend on temperature to a greater or lesser extent. 2021-12-11cary_2Chapter 4 Temperature and Heat-Flux MeasurementTemperature is to be used for describing the hotness and coldness of a object2021-12-11cary_38.1 Temperature Standards and Calibration A temperature scale provides for three essential aspects of temperature measurement:1. The definition of the size of the degree. 2. Fixed reference points for establishing known temperatures. 3. A means for interpolating between these fixed temperature points.2021-12-11cary_4Some history of temperature scales 随着温度测量技术的发展，温标也经历了一个逐渐发展，不断修改和完善的渐进过程。从早期建立的一些经验温标，发展为后来的理想热力学温标和绝对气体温标。到现今使用具有较高精度的国际实用温标，其间经历了几百年时间。 1.经验温标 根据某些物质体积膨胀与温度的关系，用实验方法或经验公式所确定的温标称为经验温标。2021-12-11cary_5Some history of temperature scales Concept from Claudius Galenus(1564-1642) (伽利略) 8 Mixtures of Ice/boiling water Latin temperatura (blending/mixing) At left Galileo s Florentine Thermograph Florentine ThermometerTemperature Concepts2021-12-11cary_6Florentine Thermometer2021-12-11cary_7Some history of temperature scales(1) Fahrenheit (华氏温标) 1650 Florentine style thermometers developed Started playing about 1707 Previous Florentine thermometers had arbitrary scaling Fahrenheit used fixed 2-3 point references to establish scale. Ice/Salt (0) Body temperature (96)2021-12-11cary_8Some history of temperature scales(1) Fahrenheit (华氏温标) In 1714 Gabriel Fahrenheit stunned the world by producing two thermometers that produced the same readings! Recognized the need for non-arbitrary temperature scales. Unlike Newton put that into practice. 2021-12-11cary_9(2) Celsius(摄氏温标) Astronomer Anders Celsius (1701-1744) Chose Ice/Boiling water for scale with 100 divisions During 16th Century about 20 different scales developed2021-12-11cary_102. Kelvin(理想热力学温标) Lord Kelvin (1824-1907) Ideal Gases Extrapolated gas behavior and Recognized zero point Same scaling as Celsius 2021-12-11cary_11Kelvin2021-12-11cary_12Thermodynamics defines a temperature scale that has an absolute reference, and defines an absolute zero for temperature. THERMODYNAMIC TEMPERATURE SCALE(热力学温标)2021-12-11cary_13The modern engineering definition of the temperature scale is provided by a standard called the International Temperature Scale of 1990 (ITS-90) .（1）重申国际实用温标单位仍为K，1 K等于水的三相点时温度值的 1/273.16；（2） 把水的三相点时温度值定义为0.01（摄氏度），同时相应把绝对零度修订为-273.15；这样国际摄氏温度T90和摄氏度t90: （）和国际实用温度 （K）关系为：ITS-90(国际温标国际温标)9090273.15tT2021-12-11cary_14Conversion formulae for temperature oF = (oC)(9/5) + 32 oC = (oF - 32)(5/9) K = oC + 273.152021-12-11cary_15This standard established 17 fixed points for temperature,and provides standard procedures and devices for interpolating between fixed points. The primary fixed points are shown in next page.ITS-90(国际温标国际温标)把整个温标分成4个温区，其相应的标准仪器如下；.0.655.0K，用3He和4He蒸汽温度计；.3.024.5561K，用3He和4He定容气体温度计；.13.803K961.78，用铂电阻温度计；.961.78以上，用光学或光电高温计。2021-12-11cary_16TABLE 8.1 Temperature Fixed Points as Defined by ITS-90 Temperature K CTriple point of hydrogen 13.8033 -259.3467 Liquid/vapor equilibrium for hydrogen at 25/76 atm 17 -256.15 Liquid/vapor equilibrium for hydrogen at 1atm 20.3 -252.87 Triple point of neon 24.5561 -248.5939 Triple point of oxygen 54.3584 -218.7916 Triple point of argon 83.8058 -189.3442 Triple point of water 273.16 0.012021-12-11cary_17Solid/liquid equilibrium for gallium at 1 atm 302.9146 29.7646 Solid/liquid equilibrium for tin at 1 atm 505.078 231.928 Solid/liquid equilibrium for zinc at 1 atm 692.677 419.527 Solid/liquid equilibrium for silver at 1 atm 1234.93 961.78 Solid/liquid equilibrium for gold at 1 atm 1337.33 1064.18 Solid/liquid equilibrium for copper at 1 atm 1357.77 1084.622021-12-11cary_182021-12-11cary_19Fixed Point Temperatures and InterpolationConsider the definition of the triple point of water as having a value of 0.01 for our temperature scale (Celsius scale). This provides for an arbitrary starting point.Fixed points are typically defined by phase-transition temperatures or the triple point of a pure substance. Such as the point at which pure water boils at one standard atm pressure. For our purposes ,assign this a value of 100.2021-12-11cary_20The size of the degree: Since we have two fixed points on our temperature scale:boiling point and freezing point.We can see that the degree is 1/100th of the temperature difference between the boiling point and freezing point.2021-12-11cary_21The calibration of a temperature measurement device entails not only the establishment of fixed points,but any temperature between fixed points.The process of establishing any temperature without a fixed point calibration is called interpolation.2021-12-11cary_22 标定方法标定方法 对温度计的标定，有标准值法和标准表法两种方法。 标准值法就是用适当的方法建立起一系列国际温标定义的固定温度点(恒温)作标准值，把被标定温度计(或传感器)依次置于这些标准温度值之下，记录下温度计的相应示值(或传感器的输出)，并根据国际温标规定的内插公式对温度计(传感器)的分度进行对比记录，从而完成对温度计的标定；被定后的温度计可作为标准温度计来测温度。 更为一般和常用的另一种标定方法是把被标定温度计(传感器)与已被标定好的更高一级精度的温度计(传感器)，紧靠在一起，共同置于可调节的恒温槽中，分别把槽温调节到所选择的若干温度点，比较和记录两者的读数，获得一系列对应差值，经多次升温，降温、重复测试，若这些差值稳定，则把记录下的这些差值作为被标定温度计的修正量，就成了对被标定温度计的标定。2021-12-11cary_23接触式与非接触式测温特点比较 方 式 接 触 式 非 接 触 式 测量 条件 感温元件要与被测对象良好接触；感温元件的加入几乎不改变对象的温度；被测温度不超过感温元件能承受的上限温度;被测对象不对感温元件产生腐蚀 需准确知道被测对象表面发射率；被测对象的辐射能充分照射到检测元件上 测量 范围 特别适合1200以下、热容大、无腐蚀性 对象的连续在线测温，对高于l 300以上 的温度测量较困难 原理上测量范围可以从超低温到极高温，但1000以下，测量误差大，能测运动物 体和热容小的物体温度 精 度 工业用表通常为1.0、0.5、0.2及0.1级，实验室用表可达0.01级 通常为1.0、1.5、2.5级 响应 速度慢，通常为几十秒到几分钟 快，通常为23秒钟 其它 特点 整个测温系统结构简单、体积小、可靠、维 护方便、价格低廉，仪表读数直接反映被 测物体实际温度；可方便地组成多路集中 测量与控制系统 整个测温系统结构复杂、体积大、调整麻烦、价格昂贵；仪表读数通常只反映被测物体表现温度(需进一步转换)；不易组成测温、控温一体化的温度控制装置 2021-12-11cary_24各类温度检测方法构成的测温仪表的大体测温范围表2021-12-11cary_258.2 THERMAL-EXPANSION METHODSLiquid-in-glass thermometers (玻璃液体温度计)2021-12-11cary_26The manner in which a thermometer is calibrated needs to correspond to how it used. Under normal circumstances, you can get accuracy from 0.2 to 2C.1. Liquid-in-glass thermometersThermometry based on thermal expansion （热膨胀温度计）2021-12-11cary_27 Temperature measurements using liquid-in-glass thermometer can provide accuracies to under very carefully controlled conditions;0.01 Chowever,such extraneous variables as pressure and changes in bulb volume overtime can introduce significant errors in scale calibration.2021-12-11cary_28 玻璃液体温度计简称玻璃温度计，是一种直读式仪表。 水银是玻璃温度计最常用的液体，其凝固点为-38.9、测温上限为538。对于较低温度测量，可以用其它有机液体(如酒精下限为-62，甲苯下限为-90，而戊烷则可达-20l)。 玻璃温度计具有结构简单，制作容易，价格低廉，测温范围较广，安装使用方便，现场直接读数，一般无需能源，易破损，测温值难自动远传记录等特点。2021-12-11cary_29Liquid-in-glass thermometernPROSnVery inexpensivenGive a reading quicklynSimple, so can maintain accuracy for long periods of timenUseful in calibrating other temperature measurement devicesnCONSnNeed to make sure liquid is continuous in column and the glass envelope is free of cracksnHuman perception of readings results in inaccuracies2021-12-11cary_30 压力温度计是根据一定质量的液体、气体、蒸汽在体积不变的条件下其压力与温度呈确定函数关系的原理实现其测温功能的。压力温度计的典型结构示意图如图所示。2021-12-11cary_31 它由充有感温介质的感温包、传递压力元件(毛细管)及压力敏感元件齿轮或杠杆传动机构、指针和读数盘组成。 测温时将其温包置入被测介质中，温包内的感温介质(为气体或液体或蒸发液体)因被测温度的高低而导致其体积膨胀或收缩造成压力的增减，压力的变化经毛细管传给弹簧管使其产生变形，进而通过传动机构带动指针偏转，指示出相应的温度。 这类压力温度计其毛细管细而长(规格为160m)它的作用主要是传递压力，长度愈长，则使温度计响应愈慢，在长度相等条件下，管愈细，则准确度愈高。 压力温度计和玻璃温度计相比，具有强度大、不易破损、读数方便，但准确度较低、耐腐蚀性较差等特点。压力温度计测温范围下限能达-100以下，上限最高可达600，常用于汽车、拖拉机、内燃机、汽轮机的油、水系统的温度测量。2021-12-11cary_323. Bimetallic ThermometersIf you take two metals with different thermal expansion coefficients and bond them together, they will bend in one direction if the temperature rises above the temperature at which the boding was done and in the other if it gets less. 2021-12-11cary_33Bimetallic ThermometersFIGURE 8.3 Expansion thermometry: Bimetallic strip.Metal BBonded attemperaturedAt temperatureMetal AMetal AMetal B221()()ABTTTCC1TcrSpiralSpiralHelix2021-12-11cary_34)(3212TTtBAThe physical basis for the relationship between the radius of curvature and temperature is given as:2021-12-11cary_35Bimetallic Example2021-12-11cary_36Invar is often used as one of the metals,since its 81.7 10/Cm mCThe bimetallic sensor is used in many temperature control systems,and is theprimary element in most dial thermometers.The uncertainties is 1For steels : 210-5 to2010-5m/m-2021-12-11cary_37目前实际采用的双金属材料及测温范围：100以下，通常采用黄铜与34镍钢；150以下，通常采用黄铜与因瓦合金；250以上，通常采用蒙乃尔高强度耐蚀镍合金与3442镍钢。 双金属温度计不仅可用于测量温度，而且还可方便地用作简单温度控制装置(尤其是开关的“通断”控制)。2021-12-11cary_38 双金属温度计的感温双金属元件的形状有平面螺旋型和直线螺旋型两大类，其测温范围大致为-80600，精度等级通常为1.5级左右。 其既可测温又可作恒温控制，可彻底解决水银玻璃温度计和水银压力温度计易破损造成泄汞危害的问题。所以在测温和控温精度不高的场合，双金属温度计应用范围不断扩大。 双金属片常制成螺旋管状来提高灵敏度。双金属温度计抗振性好，读数方便，但精度不太高，只能用做一般的工业用仪表。2021-12-11cary_39Bimetallic Thermometer 2021-12-11cary_40Bimetallic Thermometer Example2021-12-11cary_418.3 Thermoelectric sensors(热电式热电式) Thermocouples (热电偶热电偶)Seebeck effect (塞贝克效应)If two wires of dissimilar metals are joined at both ends and one end is heated, current will flow.If the circuit is broken, there will be an open circuit voltage across the wires.Voltage is a function of temperature and metal types.For small Ts, the relationship with temperature is linearFor larger Ts, non-linearities may occur.VT VT 2021-12-11cary_42 Thermocouples (热电偶热电偶)A thermocouple consists of two electrical conductors that are made of dissimilar metals and have at least one electrical connectionjunction . The output of a thermocouple circuit is a voltage.nThermocouple sensor at junctionnVoltage generated by the Seebeck effectnReally the sum of the Peltier effect (junction) and the nThompson effect (temperature gradient)2021-12-11cary_43 Thermocouples (热电偶热电偶)em f1Material BMaterial A1T12T2Material BIf T1 and T2 are not equal,a finite open-circuit electric potential, emf1 electromotive force(热电势), will be measured.2021-12-11cary_44The characteristic behavior of these free electrons results in a useful relationship between temperature and emf.There are two kinds of emf in the closed circuit of thermocouple(热电偶闭合回路中产生的热电势由两种电势组成：温差电势（又称汤姆逊电势）和接触电势（又称珀尔帖电势）)。1. The Peltier effect （珀尔帖电势）2. The Thomson effect （汤姆逊电势）2021-12-11cary_451. Peltier Effect (接触电势)em f2(External)Current IMaterial AHeattransferMaterial BConsider the two conductors having a common Junction , shown in the following figure:2021-12-11cary_46For any portion of either of the conductors,the energy removal rate required to maintain a constant temperature is I2R, R is the resistance to a current flow. The difference in I2R and the amount of energy generated by the current flowing through the junction is due to the Peltier effect.The Peltier heat is the quantity of heat in addition to the quantity I2R that must be removed from the junction to maintain the junction at a constant temperature.2021-12-11cary_47This amount of energy is proportional to the current flowing through the junction. The proportionality constant is the Peltier coefficient ,and the heat transferA Brequired to maintain a constant temperature is:A BQIBt1At11ABU-U)(teBt2At22ABU-U)(teBAABNNekTTEln)(2021-12-11cary_48BAABNNekTTEln)(kk玻耳兹曼常数，玻耳兹曼常数，ee电子电荷量，电子电荷量，TT接触处的温度接触处的温度NANA，NBNB分别为导体分别为导体A A和和B B的自由电子密度。的自由电子密度。ABTT 0BAABNNekTTEln)(002021-12-11cary_492. Thomson Effect (温差电势)Consider the conductor shown in FIGURE, which is subject to a longitudinal temperature gradient,and also subject to a potential difference,such that there is a flow of current and heat in the conductor.T 1T 2q 1q 2iV o lta g e su p p lyq 1 E n e rg y flo w d u e to te m p e ra tu re g ra d ie n tq 2 H e a t tra n sfe r to m a in ta in co n sta n t te m p e ra tu re2021-12-11cary_50To maintain a constant temperature in the conductor ,a quantity of energy different than the Joule heat, ,must be removed from the conductor .2I RThis energy is expressed in terms of the Thomson coefficient, as12()QI TT2021-12-11cary_51(T)C-f(T)f(T-f(T)T(T,E00AB)T(T,e-)T(T,e)(Te-(T)e)T(T,E0B0A0ABAB0AB 实践证明，在热电偶回路中起主要作用的是接触电动势，温差电动势只占极小部分，可以忽略不计，故可以写成 )(Te-(T)e)T(T,E0ABAB0AB T为热端;T0称为自由端（也称参考端）或冷端。冷端T0固定：)f(T-f(T)02021-12-11cary_52Thermocouple Laws1. Law of Homogeneous Materials （均质导体定律）均质导体定律）A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone,regardless of how it might vary in cross section.TT0AA0)T(T,E0A根据这个定律，可以检验两个热电极材料成分是否相同(称为同名极检验法)，也可以检查热电极材料的均匀性。2021-12-11cary_53If you break your thermocouple and add something of another material, it will have no effect as long as both ends of the new material are at the same temperature.00(,)(,)ABCABET TET T2. Law of Intermediate Materials(中间导体定律中间导体定律)热电偶的这种性质在实用上有着重要的意义，它使我们可以方便地在回路中直接接入各种类型的显示仪表或调节器，也可以将热电偶的两端不焊接而直接插入液态金属中或直接焊在金属表面进行温度测量。2021-12-11cary_543.Law of Intermediate Temperatures(中间温度定律中间温度定律)If you get emf1 when the two temperatures are T1 and T2, and you get emf2 when you have T2 and T3, you will get emf1 + emf2 when the temperatures are T1 and T3.)T,(TE)T,(TE)T,(TE32AB21AB31AB中间温度定律为补偿导线的使用提供了理论依据。它表明：若热电偶的热电极被导体延长，只要接入的导体组成热电偶的热电特性与被延长的热电偶的热电特性相同，且它们之间连接的两点温度相同，则总回路的热电动势与连接点温度无关，只与延长以后的热电偶两端的温度有关2021-12-11cary_5500( , )( , )( , ).( ,0)( ,25)(25,0)ABABnABnABABABET TET TET Teg ETETEby measuringFrom Table2021-12-11cary_56 Law of Standard poles(标准电极定律定律标准电极定律定律)If the emf of metals A and C is EAC and that of metals B and C is EBC , then the emf of metals A and B is EAC +ECB.)t(t,E)t(t,E)t(t,E0CB0AC0AB例如：热端为100，冷端为0时，镍铬合金与纯铂组成的热电偶的热电动势为2.95mV，而考铜与纯铂组成的热电偶的热电动势为-4.0mV，则镍铬和考铜组合而成的热电偶所产生的热电动势应为2.95mV-(-4.0mV)=6.95mV2021-12-11cary_5700( , )( , )( , ).( ,0)( ,25)(25,0)ABABnABnABABABET TET TET Teg ETETEby measuringFrom Table2021-12-11cary_58Making Thermocouple BeadsnSoldering, silver-soldering, butt or spot or beaded gas welding, crimping, and twisting are all OK. nThe third metal introduced doesnt effect results as long as the temperature everywhere in the bead is the same.nWelding should be done carefully so as to not degrade the metals.nIf you consistently will need a lot of thermocouples, you can buy a thermocouple welder; you stick the two ends into a hole, hit a button, and the welding is done.2021-12-11cary_59Thermocouple junction types(熱電偶的封裝圖)Bare wire beadedGrounded junctionInsulated junction2021-12-11cary_602021-12-11cary_61熱電堆圖)Thermopile is a term used to describe a multiple-junction thermocouple circuit that is designed to amplify the output of the circuit.2021-12-11cary_62Designations(型号)2021-12-11cary_632021-12-11cary_64(分度表)2021-12-11cary_65Thermocouple OutputETJKRSN2021-12-11cary_66(分度表)2021-12-11cary_672021-12-11cary_682021-12-11cary_69Thermocouple Calibration2021-12-11cary_70Typical Thermocouples 2021-12-11cary_71Thermocouple pros and consPROSEasy to useRelatively cheapsimpleCONSVoltage measurement must be made with no current flowConnections result in additional junctionsVoltage depends on composition of metals in the wires2021-12-11cary_72Selecting a thermocouplenSensitivitynLinearity of outputnStability and corrosion resistancenTemperature rangenCost2021-12-11cary_73nType R and S are expensive and resistant to corrosion and high temperatures, but arent very sensitivenType T are inexpensive and very sensitive but arent very temperature and corrosion resistantnType K are most popular being moderately priced, and reasonably temperature and corrosion resistant 2021-12-11cary_74热电偶的选用除了考虑被测对象的温度范围外，还需考虑热电偶使用环境的气氛。 通常被测对象的温度范围在-200300时可优选T型热电偶，因为它在贱金属热电偶中精度最高，或选E型热电偶，它是贱金属热电偶中热电势最大、灵敏度最高的热电偶； 当上限温度1000，可优先选K型热电偶，其特点为使用温度范围宽(上限最高可达1300)、高温性能较稳定，价格较满足该温区的其它热电偶低； 当上限温度1300，可选N型或K型； 当测温范围为10001400时，可选S或R型热电偶； 当测温范围为14001800时，应选B型热电偶； 当测温上限大于1800，应考虑选用还属非国际标准的钨锦系列热电偶(其最高上限温度可达2800，但超过2300其准确度要下降；要注意保护，因为钨极易氧化，必须用惰性或干燥氢气把热电偶与外界空气严格隔绝。不能用于含碳气氛)或非金属耐高温热电偶(国内还未商品化)。2021-12-11cary_75 在氧化气氛下，且被测温度上限小于1300，应优先选用抗氧化能力强的贱金属N型或K型； 当测温上限高于1300，应选S、R或B型贵金属热电偶。 在真空或还原性气氛下，当上限温度低于950时，应优先选用J型热电偶(它不仅可在还原气氛下工作，也可在氧化气氛中使用)，高于此限，或选钨铼系列热电偶，或非贵金属系列热电偶，或选采取特别的隔绝保护措施的其它标准热电偶。2021-12-11cary_76Reference-Junction Consideration1.Use of isothermal block for connection ensures formation of reference junction (冷端恒温法冷端恒温法)A、 Ice-bath reference-junction (0恒温器) 将热电偶的冷端置于温度为0的恒温器内(如冰水混合物)，使冷端温度处于0。这种装置通常用于实验室或精密的温度测量。 2021-12-11cary_77Thermocouples cant measure a single temperature, but can only tell us the difference in temperature between two points. If we can put one of those points at a known temperature, we are set. 2021-12-11cary_78Reference-Junction ConsiderationnMany thermocouples dont have one copper wire. Shown below is a “Type J” thermocouple.nIf the two terminals arent at the same temperature, this also creates an error.2021-12-11cary_79B、 Isothermal block reference-junction (其他恒温器) 将热电偶的冷端置于各种恒温器内，使之保持温度恒定，避免由于环境温度的波动而引入误差。这类恒温器可以是盛有变压器油的容器，利用变压器油的热惰性恒温；也可以是电加热的恒温器。这类恒温器的温度不为0，故最后还需对热电偶进行冷端温度修正。nThe block is an electrical insulator but good heat conductor. This way the voltages for J3 and J4 cancel out. Thermocouple data acquisition set-ups include these isothermal blocks.nIf we eliminate the ice-bath, then the isothermal block temperature is our reference temperature1blockVTT2021-12-11cary_80Thermocouple tablesType E: chromelconstantanType J: ironconstantanType K: chromelalumelType N: nicrosilnisilType S: platinum/10% rhodiumplatinumType T: copperconstantan2021-12-11cary_812021-12-11cary_822021-12-11cary_83ExampleSay we hook a J type thermocouple to a volt meter and read 0.507 mV. An independent temperature measurement at the connection to the volt meter tells us that the temperature there is 20C. What is the temperature at the thermocouple junction?EMF Table is relative to 0C (notice that the voltage at that temperature is zero). At 20C, the voltage from the table is 1.019 mV. So our voltage relative to 0C is the measured voltage plus the 20 value:1.019 + 0.507 = 1.526.Going back to the table, this corresponds to 29.79C.2021-12-11cary_84Extension Leads(补偿导线法补偿导线法)2021-12-11cary_85Extension Leads (补偿导线法补偿导线法) 热电偶由于受到材料价格的限制不可能做得很长，而要使其冷端不受测热电偶由于受到材料价格的限制不可能做得很长，而要使其冷端不受测温对象的温度影响，必须使冷端远离温度对象，采用补偿导线就可以做温对象的温度影响，必须使冷端远离温度对象，采用补偿导线就可以做到这一点。所谓补偿导线，实际上是一对材料化学成分不同的导线，在到这一点。所谓补偿导线，实际上是一对材料化学成分不同的导线，在0150温度范围内与配接的热电偶有一致的热电特性，但价格相对要温度范围内与配接的热电偶有一致的热电特性，但价格相对要便宜。若我们利用补偿导线，将热电偶的冷端延伸到温度恒定的场所便宜。若我们利用补偿导线，将热电偶的冷端延伸到温度恒定的场所(如如仪表室仪表室)，其实质是相当于将热电极延长。根据中间温度定律，只要热电，其实质是相当于将热电极延长。根据中间温度定律，只要热电偶和补偿导线的二个接点温度一致，是不会影响热电动势输出的。下面偶和补偿导线的二个接点温度一致，是不会影响热电动势输出的。下面举例说明补偿导线的作用。举例说明补偿导线的作用。2021-12-11cary_86 采用镍铬-镍硅热电偶测量炉温。热端温度为800，冷端温度为50。为了进行炉温的调节及显示，必须将热电偶产生的热电动势信号送到仪表室，仪表室的环境温度恒为20。 首先由镍铬-镍硅热电偶分度表查出它在冷端温度为0，热端温度为800时的热电动势为E(800，0)33.275mV；热端温度为50时的热电动势为E(50，0)2.023mV；热端温度为20时的热电动势为E(20，0)=0.798mV。 如果热电偶与仪表之间直接用铜导线连接，根据中间导体定律，输入仪表的热电动势为：E(800，50)=E(800，0)-E(50，0)=(33.277-2.022)mV=31.255mV(相当于751)。 如果热电偶与仪表之间用补偿导线连接，相当于将热电偶延伸到仪表室，输入仪表的热电动势为：E(800，20)=E(800，0)-E(20，0)=(33.277-0.798)mV=32.479mV(相当于781)。与炉内的真实温度相差分别为：751-800=-49781-800=-19 可见，补偿导线的作用是很明显的。 补偿导线的类型表可在热电偶使用有关书查到.2021-12-11cary_87n 所谓补偿导线就是用热电性质与热电偶相近的材料制成导线用它将热电偶的参比端延长到需要的地方，而且不会对热电偶回路引入超出允许的附加测温误差。n 随着热电偶的标准化，补偿导线也形成了标准系列。国际电工委员会也制定了国际标准，适合于标准化热电偶使用。2021-12-11cary_88Software CompensationnHow can we find the temperature of the block? Use a thermister or RTD. nOnce the temperature is known, the voltage associated with that temperature can be subtracted off.nThen why use thermocouples at all?nThermocouples are cheaper, smaller, more flexible and rugged, and operate over a wider temperature range.nMost data acquisition systems have software compensation built in. To use Labview,youll need to know if you have a thermister or RTD.2021-12-11cary_89Hardware CompensationnWith hardware compensation, the temperature of the isothermal block again is measured, and then a battery is used to cancel out the voltage of the reference junction. nThis is also called an “electronic ice point reference”. With this reference, you can use a normal voltmeter instead of a thermocouple reader. You need a separate ice-point reference for every type of thermocouple.2021-12-11cary_90计算修正法计算修正法电桥补偿法电桥补偿法2021-12-11