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1、university of tasmaniaschool of engineeringkne222 electronic engineeringtemperature measurement thermocouplesintroduction.a thermocouple is formed when two dissimilar metals are joined. if a second joint is made from the same materials, and one joint is heated with respect to the other, then a curre

2、nt will flow round the loop. alternatively, if the circuit is opened, then a small voltage will appear at these terminals, proportional to the difference in temperature between the junctions. this voltage is known as the seebeck voltage, after thomas seebeck who discovered the phenomenon in 1821. fi

3、gure 1. conceptual application of a thermocouple(ref: ”operational amplifier circuits theory and applications” e.j kennedy. hrw inc.) in order to use a thermocouple to measure temperature with one junction, a reference temperature must be established for the other. this can be done as illustrated in

4、 figure 1, where a 0 c ice bath is provided for the reference junction. however an ice bath is not really practical for everyday measurements. fortunately it is possible to introduce a correction voltage into the circuit so that the reference junction appears to be at zero degrees c. an example of t

5、his will be presented later.thermocouple characteristics.table 1 provides a list of commonly used thermocouple materials, together with the useful range of temperatures for each. while the seebeck voltage increases with the junction temperature difference, the relationship is unfortunately not entir

6、ely linear. the proportionality constant is known as the seebeck coefficient, , and has the units v/c. table 1 provides typical values at 25 c; it also gives some idea of how varies over the useable temperature range for each thermocouple. table 1. characteristics of common thermocouples(ref: “opera

7、tional amplifier circuits theory and applications” e.j kennedy. hrw inc.)figure 2 shows the variation of as a function of temperature for k and t type thermocouples. the k type device offers almost constant values over much of its useful range, and therefore is useable there without linearization. i

8、n contrast, the seebeck coefficient of the t type thermocouple varies considerably with temperature and as a result these devices must be corrected using a linearization algorithm in order to obtain accurate temperature measurements. figure 2. variation of the seebeck coefficient with temperature, t

9、 and k type thermocouples.(ref: ”operational amplifier circuits theory and applications” e.j kennedy. hrw inc.)finally, figure 3 shows the seebeck voltage as a function of temperature difference for the thermocouples listed in table 1. in each case the reference junction (sometimes called the cold j

10、unction), is held at zero c, thus all these curves pass through the origin. because they are slightly non-linear, it is common to apply a linearization algorithm to the seebeck voltage (e), in order to obtain the correct temperature (t). in its simplest form this algorithm assumes that the relations

11、hip between seebeck voltage and temperature is linear and an equation of the form is used. this assumption results in errors, particularly over a wide range of temperatures. for more accurate results, higher order polynomials are used.figure 3. seebeck voltage as a function of temperature difference

12、. (note: the reference junction temperature is 0 c.) (ref: ”operational amplifier circuits theory and applications” e.j kennedy. hrw inc.)a practical thermocouple circuit.the schematic shown in figure 5 is that of an electronically compensated thermocouple amplifier. in this case a j type thermocoup

13、le (iron-constantan) has been used. the two metallic junctions that make up this thermocouple, j1 and j2, are shown on the diagram. j1 is the sense junction, while j2 is the reference junction. ideally j2 should be held in an ice bath at 0c, as shown in figure 1, however this is generally inconvenie

14、nt. instead it is more usual to provide electronic reference junction compensation. this involves measuring the temperature of the reference junction and adding a compensating voltage to the thermocouple, so that the residual seebeck voltage matches that of a similar thermocouple with its reference

15、junction held at 0c.generation of a compensation voltagetable 2 shows the seebeck voltage as function of temperature for j type thermocouples, (referenced to 0c). from this we see that the thermocouple voltage will be 1.277mv when the sense junction is at room temperature, (25c or 298k). therefore t

16、his voltage must be subtracted from the thermocouple potential, when the reference junction is at 25c, in order to leave a potential proportional only to the sense junction temperature.if we can keep the reference junction firmly at 25c, then this fixed compensation voltage will suffice. however thi

17、s is generally impractical, and as ambient conditions change so will the reference junction temperature, and with it the required compensation voltage as well. on the other hand, if the compensation voltage is made to vary with temperature, in just the right way, then changes in ambient temperature

18、can be compensated for automatically. specifically, if the compensation voltage has a temperature coefficient equal to the seebeck coefficient for the thermocouple in question, at say 25c, then ambient changes around this temperature will automatically be compensated for.let the input voltage to the

19、 differential amplifier, ic2 in figure 5, be vi(ts) where ts is the temperature of the sense junction (j1). (ic2 is an instrumentation amplifier* provided simply to amplify the net thermocouple voltage to more useable levels.) this voltage will equal the difference between the junction potentials (

20、and ), plus the compensation voltage, vc(ta). remember that the reference junction, (j2) is at ambient temperature, and that the compensation potential must therefore be a function of the ambient temperature as well. thus we can write: (1)so (2)therefore the compensation voltage must equal the diffe

21、rence between the ambient reference junction voltage and the reference voltage at 0c. equation (2) shows that if ta=0 c then . equation (1) shows that when ts = ta = 25 c then vc(25) = vi(25) = 1.277mv since this is the seebeck voltage generated when the junction difference is 25c. finally by differ

22、entiating equation (2) we see that the compensation voltage temperature coefficient must equal that of the thermocouple, (). figure 4 summarizes this information.figure 4. compensation voltage vs temperature* an instrumentation amplifier is a special form of differential amplifier, designed for larg

23、e voltage gains and having a high common mode rejection ratio.the circuit in figure 5 generates this compensation voltage and applies it together with the thermocouple voltage, to the differential amplifier, ic2. the isothermal block, shown dotted, contains both the thermocouple terminals as well an

24、d the reference junction, j2. it also contains an ad590 integrated circuit. this device generates a temperature dependent dc current with a sensitivity of 1a/k. thus at room temperature, (298k) this current equals 298a. it is assumed that everything within the isothermal block remains at ambient tem

25、perature. in practice this may mean that these components are simply mounted on the same circuit board in close proximity to each other, or preferably they may be thermally bonded to a common ground plane or heatsink.figure 5. electronic reference junction compensation amplifier.(ref: ”operational a

26、mplifier circuits theory and applications” e.j kennedy. hrw inc.)from figure 5 the input voltage to the differential amplifier vi, is equal to, where is the voltage dropped across r1. is itself a function of two components, one from the temperature dependent current source ic1, and the other from th

27、e voltage reference, vref. it is easiest to consider these sources independently, by applying the principle of superposition. figure 6 shows the circuit with the voltage source removed and replaced by its internal impedance, (zero ohms). in this case v1 represents the voltage developed across r1 by

28、the current i1, thus . note that the component v1 is in the same direction as vr1.figure 6. superposition: i1 acting alonefigure 7, on the other hand, shows the circuit with the current source removed and replaced with its internal impedance (an open circuit). note that the component v2 now opposes

29、v1.figure 7. superposition: vref acting alonefrom potential divider theory . by summing these components we can write an expression for the compensation voltage, vr1, as follows: (3)since i1 and vref are fixed, it is necessary to find values for r1 and r2. from figure 4 we know that: (4)and (5)by so

30、lving (4) and (5) values for r1 and r2 can be found. (you should be able to show that r1 = 51.66 ohms and that r2 = 4540 ohms). by using a similar process, r1 and r2 values can be found for any of the thermocouples listed in table 1.finally a word about the isothermal block. notice that in figure 5

31、that the thermocouple terminals also appear on the isothermal block. this is no accident. these terminals are both constantan alloy, and here they join to the copper conductors if the amplifier circuit. it might be argued that these junctions form a second thermocouple, one that might introduce an u

32、nwanted seebeck voltage into our circuit. this is indeed a valid argument; however no disturbance to our circuit will occur provided the new thermocouple junctions are held at the same temperature. the isothermal block achieves this requirement as well!linearization algorithmsin some applications th

33、ermocouples are used without reference junction compensation or linearization. these applications generally involve sense temperatures many times greater than ambient, and therefore the resulting reference junction temperature error is small. for example, k type thermocouples are frequently used to

34、measure cylinder head temperatures on internal combustion engines. these are usually applied in the form of a thermocouple washer placed between the spark plug and the cylinder head. aircraft exhaust gas temperatures are another such example. here a thermocouple is placed inside the header pipe not

35、far from the exhaust valve. (the information obtained helps the pilot correctly lean the fuel air mixture in the engine.) in both these cases the thermocouple usually directly drives a temperature indicator, without the need for any additional electronics. in these applications the seebeck voltage i

36、s assumed to be linear with temperature, and while this is not quite true, the resulting error is accepted.in other applications however sufficiently accurate results can only be achieved by providing both reference junction compensation and voltage linearization. the latter can be achieved by the u

37、se of a polynomial of the form:where t is the sense temperature, e is the compensated seebeck voltage, expressed in microvolts, and a1 a4 are empirical constants for the thermocouple concerned. table 3 includes a list of these for j and k type thermocouples, for the temperature ranges specified. thi

38、s polynomial will generally be evaluated by the software of the scada system to which the thermocouple is interfaced.thermoelectric-to-temperatureconversion, type j*a0 = 0.0a1 = 1.8843850x10-2a2 = 1.2029733x10-6a3 = -2.5278593x10-10a4 = -2.5849263x10-14(-200c to 0c with error range -0.4c to 0.5c)a0

39、= 0.0a1 = 1.9323799x10-2a3 = 3.7084018x10-12a4 = -5.1031937x10-17(0c to 760c with error range -0.9c to 0.7c)*nist monograph 125, table a6.2.3thermoelectric-to-temperatureconversion, type k*a0 = 0.0a1 = 1.2329875x10-2a2 = -1.4434305x10-5a3 = -4.2824995x10-9a4 = -4.2028679x10-13(-270c to 0c with error

40、 range -11c to 8c)a0 = 0.0a1 = 2.5132785x10-2a2 = -6.0883423x10-8a3 = 5.5358209x10-13a4 = 9.3720918x10-18(0c to 1370c with error range -2.4c to 1.2c)*nist monograph 125, table a6.2.3table 3. linearization coefficients for type j and k thermocouples(ref: national institute of standards and technology

41、 usa. (nist) andsoftware linearization of a thermocouple rex klopfenstein: king industries inc, usa)table 2. seebeck voltages for j type thermocouples, (referenced to 0c).kne222电子工程温度测量 - 热电偶塔斯马尼亚大学工程学院摘要当两种不同的金属连接起来，形成一个热电偶。如果第二接头从同一材料制成的，以及一个接头相对于其他加热，则电流将流轮循环。或者，如果电路断开，则一个小的电压将出现在这些端子，正比于路口之间的温度差

42、。该电压被称为塞贝克电压，托马斯塞贝克后谁在1821年发现的现象。为了使用热电偶来测量与一个结点温度，基准温度必须为其它建立。这可以如在图1，其中提供了基准结一个0c冰浴中所示来进行。然而，一个冰浴不适合日常测量确实可行。幸运的是，可以引入一个校正电压进电路，使得参考结似乎是在这个零摄氏度的例子将在后面介绍。热电偶特性表1提供了常用的热电偶材料清单，温度为每个有用范围在一起。而塞贝克电压与结温度差增加，关系是不幸不完全是线性的。比例常数被称为塞贝克系数，并且具有单位v/。表1列出了在25c典型的值;这也给的超过可用的温度范围为每个热电偶是如何变化的一些想法。表1.常见的热电偶的特点（参考：“运

43、算放大器电路理论与应用”e.j肯尼迪hrw公司。）图2示出作为温度在k和t型热电偶的功能的变化。的k型设备在其大部分有用且具有提供几乎恒定的值，因此，是不线性可用那里。与此相反，所述t型热电偶的塞贝克系数与温度变化相当大，因此这些装置必须在为了获得准确的温度测量使用线性算法来校正。图2.变化塞贝克系数随温度，t和k型热电偶。（参考：“运算放大器电路理论与应用”e.j肯尼迪hrw公司。）最后，图3示出了塞贝克电压作为温差为表1中列出在各种情况下的基准结（有时也被称为冷端）的热电偶的功能，在零c被保持，因此，所有这些曲线通过原点。因为它们是略微非线性的，是很常见的一个线性算法应用于塞贝克电压（e）

44、中，为了获得正确的温度（t）。在其最简单的形式，该算法假定塞贝克电压和温度之间的关系是线性的，所使用的形式的等式。这个假设的结果中的错误，特别是在宽的温度范围内进行。为了更准确的结果，使用更高阶的多项式。图3.塞贝克电压作为温差的函数。（注：参比端温度为0c）。（参考：“运算放大器电路理论与应用”e.j肯尼迪hrw公司。）实用热电偶电路在图5所示的示意图是一个电子补偿热电偶放大器。在这种情况下，第j型热电偶（铁 - 康铜）已被使用。这两个金属接头构成这个热电偶，j1和j2，显示在图上。 j1是感交界处，而j2为基准结。理想地j2应在0冰浴中进行，如图1中，然而，这通常是不方便的。相反，它是更常

45、见的提供电子冷端补偿。这涉及到测量参考结的温度并增加了补偿电压热电偶，使残留的塞贝克电压匹配以在0举行的参比端类似的热电偶。补偿电压的产生表2示出了塞贝克电压作为温度面向j型热电偶，（参考0）的功能。从这里我们看到，热电偶电压将1.277mv当感结在室温下，（25或298k）。因此，此电压必须与热电偶潜力中减去，当参考端温度为25，为了留下一个潜在的比例只感结温。如果我们能在25牢牢参考结点，那么这个固定的补偿电压就足够了。然而，这通常是不切实际的，并作为环境条件改变，因此将参比端温度，并与它所需的补偿电压为好。另一方面，如果补偿电压由具有温度而变化，在刚以正确的方式，然后在环境温度的变化可以

46、被用于自动补偿。具体地说，如果补偿电压具有温度系数等于塞贝克系数为有问题的热电偶，在说25，随后在这一温度环境的变化将自动补偿。让输入电压到差动放大器，ic2在图5中，是六（ts），其中ts是感结（j 1）的温度。 （ic2是提供简单地扩增净热电偶电压更多可用水平的仪表放大器*）。该电压将等于结电位（和），加上补偿电压，vc（ta）的之间的差异。请注意，参考结，（j2）是在环境温度下，并且该补偿潜在因此必须是环境温度的函数，以及。因此，我们可以这样写： so 因此，补偿电压必须等于环境基准结电压和在0的参考电压之间的差。等式（2）表明，如果钽=0c然后。公式（1）表明，当ts =钽=25c然后

47、vc的（25）=六（25）=1.277mv，因为这是当结差是25产生的塞贝克电压。最后通过微分等式（2），我们看到，补偿电压的温度系数必须等于热电偶，（）。图4概述了此信息。图4.补偿电压与温度的关系*仪表放大器的差分放大器的一种特殊形式，专门为大的电压增益和具有高共模抑制比图5中的电路产生该补偿电压和热电偶电压一起应用它，向差动放大器，ic2。等温块虚线所示，包含热电偶终端以及与参比端，j2。它还包含一个ad590集成电路。这个装置产生具有1a/k的灵敏度的温度依赖性直流电流。由此在室温下，（298k）此电流等于298a。假定在等温块内一切保持在环境温度。在实践中，这可能意味着这些部件被简单地安装在同一电路板上靠近彼此，或优选它们可以热结合到一个公共接地平面或散热器。图5.电子冷端补偿放大器。（参考：“运算放大器电路理论与应用”e.j肯尼迪hrw公司。）从图5的输入电压到差动放大器六，等于，其中所述电压在r1两端丢弃。本身是两个组件，一个来自受温度影响电流源ic1的一个功能，并且从参考电压，vref另一个。这是最简单的独立考虑这些资源，通过应用叠加原理。图6示出了具有除去由它的内部阻抗，（零欧姆）代替电压源的电路。在这种情况下，v1表示由电流i1 r1两端产生的电压，从而。注意，分量v1在相同的方向vr1。图6.叠加：i1单