Unit7 Temperature measurement
Temperature is the degree hotness of one body,substance,or medium compared with another.When measuring temperature,we usually compare this degree of hotness to a fixed zero as its reference point.The Celsius(which used to be called centigrade )scale uses a point of reference based on freezing-point of water (0℃) and the boiling piont of water(100℃) . Temperature measurement is important because,at different
temperatures,substances have different physical properties and behave in different ways .For example,the temperature of a substance will affect its electrical properies,whether it is soild,liquid,or a gas,and it will also affect its volume.Small changes in body temperature can show whether a person or animal is ill. There are many different types of thermometer.The main types of thermometer we shall look at are ones which measure temperature by means of: Liquid expansion. Metal expansion. Electrical resistance. Thermoelectricity. Heat radiation. Proximity of field lines:the force is stronger where the lines are closer together.

  7.1 Thermocouples
Thermocouples are probably the most widely used and least understood of all temperature measuring devices.Thermocouples provide a simple and efficient means of temperature measurement by generating a
1
voltage that is a function of temperature.All electrically conducting materials produce a thermal electromotive force(emf or voltage different) as a function of the temperature gradients within the material.This is called the Seebeck effect.The amount of the Seebeck effect depends on the chemical composition of the material used in the thermocouple.When two different materials are connected to create a T℃,a voltage is generated.This voltage is the different of the voltages generated by the two materials.In principle,a thermocouple can be made from almost any two metals.In practice,several thermocouple types have become de facto standards they possess desirable qualities,such as highly predictable output voltages and large voltage-to-temperature ratios.Some common thermocouple types areJ,K,T,E,N28,N14,S,R,and Fig.
  7.1depicts a type T thermocouple.In theory,the temperature can be inferred from such a voltage by consulting standard tables or using linearization algorithms.In practice,this voltage cannot be directly used,because the connection of the thermocouple wires to the measurement device constitutes a
thermocouple providing another thermal emf that must be compensated for;cold junction compensation can be used for this purpose.

  7.
  1.1Cold junction Compensation
Fig.
  7.2 shows a thermocouple that has been placed in series with a second thermocouple,which has been places in an ice bath.This ice bath is called the reference junction .And when properly constructed,an ice bath can produce an extremely accurate 0 ℃ temperature.This reference junction is required because all thermocouple emf tables,as published by NIST(Nation Institute of Standards and Technolony),are referenced to the emf output of a thermocouple which is held at a temperature of
  0.00 degrees Celsius.In Fig.
  7.2,the chromel/copper and alumel/copper thermocouples in the ice bath have a
  0.0V contribution to the voltage
2
measured at the meter.The voltage read is entirely from the chromel/alumel thermocouple.The copper wire connected to the copper terminals on the meter do not constitute a thermocouple because they are they the same metal connection and both terminals are the same temperature. Maintaining an ice bath and an additional reference thermocouple for every thermocouple probe is not practical in most systems.If we know the temperature at the point where we connect to our thermocouple wires,(the reference junction ),and if both connecting wires are at the same temperature,then the ice bath can be eliminated.In such a case,the opposing thermoelectric voltage generated by the nonzero temperature of the reference junction can simply be added to the thermocouple emf.This is cold junction compensation(CJC). CJC is an essential part accurate thermocouple readings.CJC must be implemented in any system that has no ice-point reference junction.The technique works best if the CJC device is close to the terminal blocks connect the external thermocouples,and if there are no temperature gradients in the region containing the CJC and teminals.

  7.
  1.2 Thermocouple Linearization
Thermocouple voltage is proportional to,but not linearly proportional to,the temperature at the thermocouple connection.There are several techniques for thermocouple linearization.Analog techniques can provide a voltage proportional to temperature from the thermocouple input.In addition,a voltage measurement can be made with an ADC and the temperature looked up in a table,as in the above example.To speed this process,the look-up table can be stored in computer memory and the search performed with an algorithm. Thermocouple linearization can also voltage curve.Fig.
  7.3 illustrates a portion of the NIST Polynomial
3
Coefficients table for some common thermocouple.

  7.2 Additional Concerns
Care must be taken when using thermocouples to meaperature.Sources of minor error can add up to highly inaccurate reading.Additional concerns include.

  7.
  2.1Thermocouple Assembly
Thermocouples are assembled via twisting wires together,soldering,or welding;if done improperly,all of these techniques can introduce errors in the temperature measurement.

  7.
  2.2Twisting Wires Together
Thermocouple junctions should not be formed by twisting the wires together.This will produce a very poor thermocouple junction with large errors.

  7.
  2.3 Soldering
For low temperature work the thermocouple wires can be joined by soldering,however,soldered junctions limit the maximum temperature that can be measured(ususlly less than 200 degrees Celsius).Soldering thermocouple wires introduces a third metal.This should not introduce any appreciable error as long as both sides of the junction are the same temperature.

  7.
  2.4 Welding
Welding is the preferred method of connecting junctions.When welding thermocouple wires together,care must be taken to prevent any of the characteristics of the wire from changing as a result of the welding
4
process.These concerns complicated by the different composition of the wires being joined.Commercially manufatured thermocouple are typically welded using a capacitance-discharge technique that ensure uniformity.

  7.
  2.5 Decalibration of Thermocouple Wire Decalibration is another serious fault condition. Decalibration is particularly troublesome because it can result in an erroneous temperature reading that appears to be correct. Decalibration occurs when the physical makeup of the thermocouple wire is altered in such a way that the wire no longer meets NIST specifications. This can occur for a variety of reasons including, temperature extremes, coldworking of the metal, stress placed on the cable during installation, vibration, or temperature gradients.
  7.
  2.6 Insulation Resistance Failure Temperature extremes can also introduce error because the insulation resistance of the thermocouple will often decrease exponentially as the temperature increases. This can lead to two types of errors: leakage resistance with an open thermocouple and leakage resistance with small thermocouple wire. Fig.
  7.4 illustrates insulation resistance failure.
  7.
  2.7 Leakage Resistance with an Open Thermocouple In high temperature applications, the insulation resistance can degrade to the point where the leakage resistance RL will complete the circuit and give an erroneous reading.
  7.
  2.8 Leakage Resistance with Small Thermocouple Wire In high temperature applications using small thermocouple wire,
5
the insulation RL can degrade to the point where a virtual junction T 1 is created and the circuit output voltage will be proportional to T 1 instead of T
  2. Furthermore, high temperatures can cause impurities and chemicals within the thermocouple wire insulation to diffuse into the thermocouple metal, changing the characteristics of the thermocouple wire. This causes the temperature-voltage dependence to deviate from published values. When thermocouples are used at high temperatures, the atmospheric effects can be minimized by choosing the proper protective insulation. Due to the range of thermocouple choices, thermocouple quality is an issue. Select the thermocouple that meets your application criteria (i. e. , high or low temperature range, proper grounding, etc. )
6
第七章 温度测量
温度是身体,物质或媒介与另一物体相比的热度.测量温度时,我们通常将 这个温度与它固定的参考点-零度做一个比较.摄氏(通常被称为摄氏度)所使 用的参考点范围是基于水的冰点(0℃)和沸点(100℃) . 温度测量很重要,因为在不同温度下,物质有不同的物理性质和不同的表现 方式.例如,一种物质的温度会影响其电力,无论它是固体,液体或气体,而且 它也将影响该物质的容积. 身体温度的细微变化可以显示出一个人或者动物是否 得病. 有很多很多不同类型的温度计.对温度计的主要类型,我们将看看这是那些 通过以下方式测量温度: 液体膨胀. 金属扩张. 电阻. 热电. 热辐射 邻近的磁力线:强大的力量是其中线紧密联系在一起.

  7.1 热电偶
热电偶可能是使用最广泛和最容易理解测量温度设备.热电偶提供一组电 压,温度的变化对温度测量一种简单而有效的手段.所有导电材料产生的热电动 势(电动势或电压不同)作为内温度梯度功能材料.这就是所谓的塞贝克效应. 的塞贝克效应数额取决于在热电偶所用材料的化学成分. 当两种不同的材料连接 创建一个 T℃,电压产生.这个电压是由两种不同材料所产生的电压不同.原 则上,热电偶可以是任何两个金属.在实践中,一些热电偶类型,已成为事实上 的标准,他们具有高度可预测的,如输出电压和大的电压温度比,一些常见 的热电偶类型是 J,K,T,E,N28,N14,S,R 和图
  7.1 所示 T 型热电偶.从理论 上讲,可以推断温度从这样的标准电压的咨询表及使用线性化算法.在实践中, 这个电压不能直接使用,因为热电偶导线连接到测量设备构成热电偶
热电动势要另外提供,必须得到补偿;冷端补偿,可用于这一目的.
7

  7.
  1.1 冷端补偿

  7.2 表明热电偶与第二个热电偶相串联, 同时相接与有冰水混合物的浴器 中.这冰浴器称为参考交界处.要妥为建造,冰浴器可以产生非常精确的 0℃的 温度.该参考交界处,是必需的,因为所有的热电偶电动势表,如 NIST 的出版(国家标准与技术研究所) ,被引用的电动势输出热电偶是举行了摄氏

  0.00 度.在图
  7.2 中,在铬镍/铜和镍基热电偶合金/冰浴中的铜热电偶有一个
以在计测量的电压
  0.0 V 的贡献读取的电压是完全从镍铬合金/镍基热电偶合金 热电偶. 铜线连接的仪表铜终端不构成热电偶, 因为他们是他们相同的金属连接, 这两个码头是相同的温度. 保持冰浴器和每个热电偶探头更多的参考热电偶是不实际的大多数系统. 如 果我们知道在我们的地步连接到我们的热电偶线温度, (参考交界处) ,如果连接 线都在同一温度,那么冰浴可以消除.在这种情况下,反对热电电压的参考结非 零温产生可以简单地添加到热电偶电动势. 冷端补偿的主要部分是一个准确的热电偶读数. 冷端补偿必须全面贯彻于任 何系统没有冰结点参考.这一技术最好冷端补偿设备接近端子连接外部热电偶, 如果有在包含冷端补偿和终端地区没有温度梯度.

  7.
  1.2 热电偶线性化
热电偶电压成正比,但不是线性比例关系,在连接的热电偶温度.有热电偶 线性化的几种技术.模拟技术可以提供从热电偶输入电压正比于温度,此外,电 压测量可与 ADC 和温度看着表时,在上述的例子.为了加快这一进程,查找表 可以存储在计算机内存和算法进行搜索.热电偶线性化也可以电压曲线.图
  7.3 说明了一些共同的热电偶的 NIST 的多项式系数表部分.

  7.2 特别关注 特别关注
必须小心使用热电偶测量温度.轻微误差来源可以添加最多高度准确测量. 特别关注还包括.

  7.
  2.1 热电偶装配 热电偶装配
热电偶是通过扭转电线组装在一起,焊接,或焊接;如果操作不恰当,所有 这些技术可以引进,温度测量误差.
8

  7.
  2.2 捻线丝一起
热电偶路口不应该形成扭曲在一起的电线.这将产生一个大错误很差热电偶.

  7.
  2.3 焊接
焊接是连接路口的首选方法.当热电偶丝焊接在一起,必须小心,以防止由 于焊接过程的结果改变了线的特色. 由不同成分复杂的电线, 这些问题正在加入. 商业制造热电偶通常使用一个电容焊接放电技术,确保统一性.

  7.
  2.5 标定降级热电偶丝
标定降级是另一个严重故障条件.标定降级特别麻烦,因为它会导致一个错 误的温度读数显示是正确的. 标定降级时发生的热电偶丝体质是这样一种方式改 变这一线路不再符合 NIST 的规范.这可能是由包括各种原因, 极端温度,在金属冷加工,强调放置在安装过程中电缆,振动或温度梯度.

  7.
  2.6 绝缘电阻失效
极端的温度还可以引入错误, 因为热电偶的绝缘电
 

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