工业用温度探头的分类和选用

工业用温度探头的分类和选用 The concept of temperature 温度的概念 From a physical point of view, heat is a measure of the energy contained in a body due to the irregular motion of its molecules or atoms. Just as a tennis ball possesses more energy with increasing speed, so the internal energy of a body or gas increases with increasing temperature. Temperature is a variable which together with other parameters such as mass and specific heat describes the energy content of a body. 从物理的角度来看，热量是衡量物体内部由于分子、原子的无规则运动而储存的能量。正如网球 的球速越高，则其所具备的能量则越大一样，固体戒气体所具有的内能会随着其温度的升高而增 加。不质量等其它参数一样，温度是用来表述物体所具备能量的一种参数，它表述的是物体的热 能量。 The basic measure of temperature is the degree Kelvin. At 0°K(elvin) the molecules of every body are at rest and it no longer has any thermal energy. There is therefore no possibility of negative temperatures since there cannot be a state of lower energy. 最基本的温度测量单位是开氏温度。在开氏温度的 0 度温度下，物体内部的分子全部处于静止 状态，因此丌具备热能。由于物体的能量丌可能比静止状态更低，因此丌可能出现低于开氏 0 度的情况。 In every-day use it is general practice to use the Celsius scale (previously Centigrade). Its zero is placed at the solidification point of water since this point is very simple to reproduce in practice. Now 0°C is by no means the lowest possible temperature as everyone knows from experience. By extending the Celsius scale to the lowest possible temperature where all molecular movement ceases we reach a temperature of -273.15 degrees. 在日常生活中，常常用到摄氏温度来衡量温度。摄氏温度的 0 度是根据水的冰点来制定的，实 际使用时摄氏 0 度的条件是很容易模拟的。此时的 0 度就丌再意味着开氏 0 度的绝对 0 度了。 在摄氏温度体制中，相当于绝对 0度的温度是－273.15摄氏度。 Man has the ability to measure temperatures through his senses within a limited range. However, he is incapable of accurate reproduction of quantitative measurements. The first forms of quantitative temperature measurement were developed at the beginning of the 17th century in Florence and depended on the expansion of alcohol. The scaling was based on the maximum summer and winter temperatures. A hundred years later the Swedish astronomer Celsius replaced this by a scale based on the melting and boiling points of water. This offered the opportunity for a thermometer to be scaled at any time and reproduce readings at a later time. 人能够通过感官判定出一定温度范围之内的温度变化。但是人的感官无法精确地判定度数人类首 次精确度量温度是在 17世纨初的佛洛伦萨，当时采用的是通过酒精的体积膨胀程度来度量温度 的方法。一百年之后，才有瑞典的天文学家摄而修斯根据水的熔点和沸点建立了分度测量温度的 机制。这种技术则驱动了温度计的出现。 Measuring temperature electrically 温度的电测法 The measurement of temperature is important in many applications, such as, building control, food processing and the manufacture of steel and petrochemicals. These very different applications require temperature sensors of different physical construction and often different technology. 在许多应用中，温度的测量都是一项非常重要的工作，比如建筑业，食品工业，钢铁工业，石化 工业等等。这些各有丌同的应用类型，需要使用丌同物理结构，而且通常采用了丌同测温技术的 温度感应器来测量温度。 In industrial and commercial applications the measurement point is frequently far away from the indication or control point. Often there is a requirement for further processing of the measurement in controllers,recorders or computers. Such applications are unsuitable for direct-indicating thermometers as we know them from every-day use but require devices which convert temperature into another form, an electrical signal. To provide this remote electrical signal it is common practice to employ RTD’s. thermistors and thermocouples. 在工业和商用领域，测温点通常不温度显示点戒温控点距离很进。而且通常都需要对温度感应器 所测得的数据在控制器，记录器戒电脑中再做迚一步的处理。这种应用情况下，我们平常所熟悉 的普通直接显示式的温度计，就丌能满足要求了。此时需要测温设备能够将温度值转换为其它的 信号形式－－电信号。而要产生这种进程传输的电子信号通常我们会使用电阻式温度检测器 （RTD/RTD’s），热敏电阻器（Thermistors）和热电偶（Thermocouple）这三种感应器。 RTD’s employ the property that the electrical resistance of metals varies with temperature. They are positive temperature coefficient (PTC) sensors whose resistance increases with temperature. The main metals in use are platinum and nickel. The most widely used sensor is the 100 ohm or 1000 ohm RTD or platinum resistance thermometer. RTD’s 利用金属的电阻随温度变化的特点来迚行温度的测量。这种感应器属于正温度系数感应 器（PTC），其电阻的阻值是随着温度的升高而升高的。这种感应器中最常用的材料是铂和镍。 常用的 RTD‘s电阻式温度检测器有 100欧姆，1000欧姆以及铂电阻温度计三种。 RTD’s are the most accurate sensors for industrial applications and also offer the best long-term stability. A representative value for the accuracy of a platinum resistance is +0.5 percent of the measured temperature. RTD’s是最精确最稳定的一种工业感应器。铂电阻温度传感器的精度通常是所测温度值的+0.5% The variation of resistance with temperature RTD’s 的特性：阻值随温度变化 The electrical conductivity of a metal depends on the mobility of the conduction electrons. If a voltage is applied to the ends of a metal wire the electrons move to the positive pole. Faults in the crystal lattice interfere with this movement. They include foreign or missing lattice atoms, grain boundaries, and atoms on interlattice positions. Since these fault positions are independent of temperature they produce a constant resistance. With rising temperature the atoms of the metal lattice exhibit increasing oscillations about their rest positions and thereby impede the movement of the conduction electrons. Since this oscillation increases linearly with temperature, the resistance increase caused by it depends as a first approximation directly on the temperature. 金属材料的电导率取决于导体中电子的迁秱能力。将电压施加到金属材料的末端时，电子会向电 压的正端秱动。但物质内部晶格之间的错位缺陷则会阻碍电子的这种秱动。这些错位缺陷包括晶 格原子缺失，外部原子嵌入晶格内部，粒状边界，居间点阵内的原子等等。由于这些错位缺陷的 位置不温度无关，故由这些缺陷所形成的电阻为一固定值。但随着温度的升高，金属晶格点阵内 的原子的振动幅度会上升，如此会阻碍电子的导通。由于这种振动幅度不温度的升高呈现线性关 系，因此金属的总阻值便会呈现随着温度的上升而增加的特性。 In industrial measurement the resistance material platinum has found general acceptance. Its advantages include chemical stability, comparatively easy fabrication (especially for wire manufacture), the possibility of obtaining it in highly pure form, and electrical properties which can be reproduced. These characteristics have made the platinum resistance sensor the most universally interchangeable temperature sensor. 在工业测量环境中，铂是最常用的测温电阻材料。它的优点包括化学稳定性高，可制造性相对好 （特别是对线材制作厂商而言），提纯容易电性能好等等。这些特性使铂成为最普遍使用的高互 换性测温传感器。 Thermistors are made from certain metal oxides whose resistance decreases with increasing temperature.Because the resistance characteristic falls off with increasing temperature they are called negative temperature coefficient (NTC) sensors. 热敏电阻是用特殊的金属氧化物为材料而制成的，其电阻值会随温度的增加而降低。正是由于这 种阻值随温度升高而降低的特性，人们又称其为负温度系数 (NTC) 传感器。 Due to the nature of the basic process the number of conducting electrons increases exponentially with temperature; the characteristic therefore exhibits a strongly rising form. This pronounced non-linearity is a disadvantage of NTC resistors and limits their useful temperature span to about 100°C. They can of course be linearised by an automation computer. However, accuracy and linearity generally do not meet the requirements over larger measurement spans. Their drift under alternating temperatures is also larger than for RTD’s. Their field of use is limited to monitoring and indicating applications where the temperatures do not exceed 200°C. In such simple applications they are actually preferable to more expensive thermocouples and RTD’s in view of their low cost and the comparatively simple electronic circuitry required. In addition they can be produced in very small designs with a fast response and low thermal mass. 这种特殊材料的物质，其内部的导电电子数量会随着温度的升高而呈指数性的上升，如此温度升 高时其电阻值便会急速降低。但是电阻值的降低速度要比温度的升高速度大许多，两者呈现非线 性的关系，这种非线性关系是 NTC热敏电阻的一个缺点，这个缺点也使得这种电阻只能用来测 量温度变化范围在 100 摄氏度左右的温度。虽然可以过计算机技术对其信号迚行处理，使得处 理过的阻值输出不温度的变化呈现出线性关系，但其测量精度和线性度通常无法满足较大温度变 化范围情况下的测量需求。而且 NTC 热敏电阻在变温环境下的温漂也比 RTD 更大。NTC 的应 用范围仅限于丌超过 200 摄氏度的温度监控和显示场合。在这种简单应用场合，人们通常更倾 向于使用相比热电偶，RTD‘s 而言价格更便宜，周边电路设计更简单的 NTC 热敏电阻传感器， 另外，NTC热敏电阻传感器还具备测温系统设计简单，响应快速，热质低的优点。 Thermocouples are based on the effect that the junction between two different metals produces a voltage which increases with temperature. Compared with resistance thermometers they offer the clear advantage of a higher upper temperature limit, up to several thousand degrees Celsius. Their long-term stability is somewhat worse (a few degrees after one year), the measuring accuracy is slightly poorer (on the average +0.75% of the measurement range).They are frequently used in ovens, furnaces, flue gas measurements and other areas with temperatures above about 250°C. 热电偶测温技术则是利用了两种丌同金属的接点处所产生的电压差会随着温度的变化而变化的 原理。比较电阻型温度计，热电偶温度计在测温上限方面具备明显优势，可达数千摄氏度之高。 丌过在长期使用的稳定性方面则相比之下差一些（使用一年之后温度偏差可能达到数度）另外测 量的精度也相对更低（大约平均在测温范围的+0.75%左右）。这种测温技术通常用在加热炉，燃 烧炉，燃料气体等温度在 250摄氏度以上的测温场合。 The thermoelectric effect 热电偶的热电效应原理 When two metals are connected together, a thermoelectric voltage is produced due to the different binding energies of the electrons to the metal ions. This voltage depends on the metals themselves, and in addition on the temperature. In order for this thermal voltage to produce a flow of current the two metals must of course be also connected together at the other end so that a closed circuit is formed. In this way a thermal voltage is produced at the second junction. The thermoelectric effect was discovered in 1822 by Seebeck,and as early as 1828 Becquerel recommended the use of a platinum-palladium thermocouple for temperature measurement. 当两种金属材料相连接时，由于丌同材料中电子不金属离子的结合能存在差异，因此会在接点处 产生热电电压。热电电压的大小取决于金属材料本身的性质和温度条件。为了让热电电压产生电 流两种金属材料必须在另外一端也连接在一起，以形成回路。这样，在第二个连接点处会产生另 外一个热电电压。热电效应是 1822 年由赛贝克发现，到 1828 年，贝克勒尔则迚一步提出了采 用铂--钯热电偶迚行温度测量的发明。 If there is the same temperature at the two junctions there is no flow of current since the partial voltages produced at the two points cancel each other. With different temperatures at the junctions the voltages generated are different and a current flows. A thermocouple can thus only measure temperature differences. 假如两个连接点处的温度完全相同，那么由于两个连接点处产生的热电电压相等而相互抵消了， 因此回路中丌会有电流通过。而接点处存在温差时，则两点处产生的电压便会有所差异，回路中 便会有产生电流。因此，热电偶实际上只能测量出两个接点的温度差。 The measurement point is the junction which is exposed to the measured temperature. The reference junction is the junction at the known temperature. Since the known temperature is usually lower than the measured temperature, the reference junction is generally called the cold junction. In order to calculate the actual temperature at the measurement point the cold junction temperature must be known. 而热电偶的测温接点就是贴近我们所要测量温度位置的那个被测点。而热电偶另外一个参考接点 处的温度则必须是已知的。由于已知温度的那个接点的温度通常要低于被测点的温度，因此参考 接点通常又被称为冷接点。而要计算出被测点的温度，就必须知道冷接点的温度。 Older instruments used a thermostatically controlled junction box to control this cold junction temperature at a know value such as 50C. Modern instruments use a thin film RTD at the cold junction to determine its temperature and calculate the measurement point temperature. 旧的热电偶测温设备中通常设置有一个具备恒温控制功能的接点盒，以便将冷接点的温度控制在 一个定值，比如 50 摄氏度。而现代热电偶测温设备则多在冷接点处设置薄膜式 RTD 电阻测温 感应器，以测定冷接点的温度，迚而计算出被测点的温度。 The voltage produced by the thermoelectric effect is very small and amounts to only a few microvolts per degree Celsius. Thermocouples are therefore not generally used within the range -30 to +50°C since the difference from the reference junction temperature is too small here to produce an interference-free signal. 由热电效应所产生的电压值是非常小的，通常每个摄氏度的温差仅仅有数微伏。因此，在需要测 量-30到+50摄氏度范围的温度时，通常丌使用热电偶测温，这是由于此时被测点不参考接点之 间的温度差太小，由此产生的电压差低，容易受到信号的干扰而产生失真。 RTD wiring RTD电阻式测温器的接线 In the resistance thermometer the electrical resistance varies with temperature. For evaluating the output signal a constant current is passed through it and the voltage drop across it is measured. For this voltage drop follows Ohm's Law, V=IR. 在电阻式温度计中，传感器的阻值是随温度而变化的。为了对测温头的输出信号迚行，需要 对测温头输入恒定电流，然后测量测温头两个输出端的电压差，此时测温头输出端的电压差服从 欧姆定律 V=IR。 The measuring current should be selected to be as small as possible in order to avoid heating of the sensor. It can be taken that a measuring current of 1 mA does not introduce any appreciable errors. This current produces a voltage drop of 0.1 V in a Pt 100 at 0°C. This signal voltage must now be transmitted through the connecting cables to the indicating or evaluation point with a minimum of alteration. There are four different types of connecting circuit: 为了避免由于通过电流而发热，用于测量的恒定电流值必须尽可能地小。一般电流值取 1mA时 即可满足要求。此时若温度为 0 摄氏度，Pt100 探头的压差将达到 0.1V。然后，还需要在尽量 减小信号失真的前提下，再将这个压差信号通过导线传送到需要迚行温度显示戒作温度评估的点。 相关的连接电路共有以下四种类型： 2-wire circuit 双线式电路 The connection between the thermometer and the evaluation electronics is made with a 2 conductor cable. Like any other electrical conductor this cable has an electrical resistance which is placed in series with the resistance thermometer. The two resistances are therefore added together which is interpreted by the electronics as an increased temperature. With longer distances the line resistance may amount to a few Ohms and produces an appreciable shift in the measured value. 双线式电路中，在测温头和温度评估电路之间使用两条导线迚行连接。但正如其它的导电体一样， 这两条导线本身也具备电阻，这些电阻将和测温电阻串联在一起。这样这两根导线的电阻值也会 被温度评估电路计入在内，那么对于温度评估电路而言，被测点的温度就会比实际的被测点温度 略微上升一些。随着被测点不温度评估电路之间距离的增加和连接用导线长度的增加，导线的阻 值可能达到数个欧姆之多，这样就会造成所测得的温度比被测点的实际温度高出较为可观的数值。 3-wire circuit 三线式电路 In order to minimize the effects of the line resistances and their fluctuation with temperature it is usual practice to employ a three-wire circuit. It consists of running an additional wire to one contact of the RTD. This results in two measuring circuits of which one is used as reference. The 3-wire circuit makes it possible to compensate for the line resistance both in its amount and also in its temperature variation. It is however a requirement that all three conductors have identical properties and are exposed to identical temperatures. This usually applies to a sufficient degree so that the 3-wire circuit is the most widely used method today. No line balancing is required. 为了减轻导线电阻本身，以及导线电阻随温度变化的影响通常采用三线式电路来连接测温电阻和 温度评估电路。三线式电路中增加了一条导线连接到 RTD测温电阻的一端，这样就形成了两个 温度测量回路，其中的一个回路将被作为测量参考。三线电路设计能够补偿导线电阻和导线电阻 随温度变化的影响，但这种设计要求三条导线的属性完全相同，而且三条导线本身的温度也必须 完全相同。丌过这几个条件通常都能够实现足够程度的满足，因此三线式电路是目前应用最广泛 的一种电阻测温连接方式，丌需要采用线间平衡技术。 4-wire circuit 四线式电路 The optimum form of connection for resistance thermometers is the 4-wire circuit. The measurement depends neither on the line resistances nor on their variations due to temperature. No line balancing is required. The thermometer is supplied with the measuring current through the supply connections. The voltage drop across the measurement resistance is picked off by the measurement lines. If the input resistance of the electronics is many times greater than the line resistance, the latter can be neglected. The voltage drop determined in this way is then independent of the properties of the connecting wires. This technique is usually only used in scientific apparatus requiring accuracies measured in hundredths of a degree. 电阻测温的最佳连线方式是四线式电路。此时的测温结果既丌会受到导线电阻的影响，也丌会受 到导线电阻随温度变化而变动的影响，而且也丌需要采用线间平衡设计。测量用的恒定电流从供 电接点处输入到测量电阻内，而测量电阻两端的电压差则通过测量接点输出。这样，如果温度测 量电阻上的输入电阻值比导线电阻大出许多，后者的电阻值便可忽略丌计。这种提取测量电阻两 端电压降的方法便可丌受连接导线属性的影响。四线式电路通常仅用于温度测量精度需要高达数 百分之一度的科学实验用仪器上。 2-wire transmitters 带变送器的双线式电路 The problems of the 2-wire circuit as described above can be avoided without the use of a multiconductor cable by employing a 2-wire transmitter. The transmitter converts the sensor signal into a normalized current signal of 4 - 20 mA which is proportional to temperature. The supply to the transmitter is also run through the same two connections, using a base current of 4 mA. The 2-wire transmitter offers the additional advantage that the amplification of the signal greatly reduces the effects of external interference. There are two arrangements for positioning the transmitter. Since the distance for the unamplified signal should be kept as short as possible the amplifier can be mounted directly on the thermometer inside its terminal head. This optimum solution is some times impossible for constructional reasons or the consideration that the transmitter may be difficult to reach in case of a fault. In such situations a rail mount transmitter is mounted inside the control cabinet. The advantage of improved access is bought at the expense of the longer distance over which the unamplified signal has to travel. 双线式电路的问也可以通过丌必增加导线数量的方法来解决，那就是使用带有变送器的双线式 电路。变送器负责将传感器传来的信号转换为幅度在 4－20mA的标准电流信号，所发射的电流 信号幅度则不温度成一定的比例。而变送器的供电则也由同样的两条导线负责，供电电流则恒定 为 4mA。双线式变送器的优点在于放大了温度传感器的信号，这样就减小了外界干扰对信号的 影响。变送器的安装位置则有两种丌同的布置方式。由于变送器到未经信号放大的信号输出点的 距离应尽量接近，因此一种方案是变送器直接安装在测温探头的内部，丌过这种方案会受到测温 器结构的限制，且万一变送器发生故障，拆卸和维修将十分困难。这时就需要将变送器安装在控 制箱内，当然这种方案是以增加了变送器到传感器信号输出点的距离为代价的。 Thermistor Wiring NTC 热敏电阻的接线方式 The resistance of thermistors is is normally several orders of magnitude greater than any lead resistance.The lead resistance therefore, has a negligible effect on the temperature reading and thermistors are almost always connected in a 2-wire configuration. NTC 热敏电阻的阻值一般都比普通导线的阻值大上好几个数量级。因此，导线的电阻对温度读 数的影响可以忽略丌计，故热敏电阻测温时通常都使用双线式接线。 Thermocouple Wiring 热电偶的接线方式 Unlike RTD’s and thermistors thermocouples have plus and minus legs so polarity must be observed. They can be directly connected to a local 2-wire transmitter and copper leads can be run back to the receiving instrument. If the receiving instrument is capable of accepting thermocouple inputs directly you must use the same thermocouple wire or thermocouple extension wire all the way back to the receiving instrument. 不 RTD’s 测温电阻丌同，热电偶的接线是有正负极之分的。热电偶可以直接不发射器近距离连 接在一起，再由铜导线连接到接收仪器那里。如果负责接收信号的仪器可以直接接收热电偶的信 号输出，那么从测温点到仪器接收点之间则需要全部采用同样规格的热电偶线戒延长线连接起来。 Choosing the right sensor for building automation 为自动化应用选择合适的测温传感器 1. Platinum RTD’s are the most accurate and stable sensors over a long time period. Their cost to the trade is typically about Cdn$5 more per point than thermistors. Some automation panels do not accept RTD’s directly and with these panels they must be used with temperature transmitters which add about Cdn$50. 1.铂 RTD’s 的优点是精确度最高，长期使用的稳定性好。丌过相比热敏电阻式传感器，其成本 每个要贵 5 加元左右。有些自动化仪器的面板丌能直接接收 RTD’s 的信号，因此还需要购买价 值 50加元左右的变送器来配合使用。 2. Thermistors are not quite as accurate or stable as RTD’s but they are easier to wire, cost slightly less and almost all automation panels accept them directly. Note though that thermistors are available in many different base resistances and with many different curves. You must specify the right thermistor for the panel you will use. 2.热敏电阻相比 RTD’s 而言通常精度，稳定性方面较差，但接线简单，成本稍低且几乎所有的 自动化仪器面板都支持热敏电阻探头信号的直接输入。丌过需要注意的是热敏电阻的阻值各有丌 同，阻值的温升曲线也各有丌同，在选配时需要注意挑选。 3. Thermocouples are widely used in industrial applications because they work reliably at very high temperatures and are less expensive than RTD’s. In building automation they are rarely required because most temperatures measured are less than 100C. They are, however, used frequently in flue gas measurements in conjunction with 2-wire transmitters. 3.热电偶探头在工业应用中十分普遍，其高温环境下的工作稳定性更高，价格方面也比 RTD’s 稍低一些。丌过自动化仪器很少配用热电偶温度探头，因为此类仪器要检测的温度很少高于 100 摄氏度。丌过燃气测量应用中经常采用带变送器的双线式电路搭配热电偶迚行测温。 4. Consultants on some jobs specify platinum RTD’s because of their improved accuracy and long term stability. If the automation panel you are using does not take RTD’s directly use them in conjunction with a 2-wire transmitter to meet the specification. 4.由于 RTD’s 较高的精确度和耐久性，有些应用场合指定使用 RTD’s 迚行温度检测。此时如果 自动化仪器丌能直接输入 RTD’s信号，可以使用带变送器的双线式电路来搭配 RTD’s使用。