热电偶及补偿方法

Thermocouple

A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots, where a temperature differential is experienced by the different conductors (or semiconductors). It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of temperature sensor for measurement and control,[1] and can also convert a temperature gradient into electricity.In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius(°C) can be difficult to achieve.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges.Thermocouples are usually standardized against a reference

temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

补偿导线的作用：

1）、利于热电偶参考端温度的修正和集中控制。

测量仪表一般安装在远离热源和环境温度较稳定的地方，而普通热电偶的接线盒（参考端安装的地方）距被测对象很近。必须用导线把热电偶参考端与仪表连接。若用普通铜导线连接则热电偶参考端温度较高而且不稳定，给测量带来误差。若将热电偶延长使热电偶参考端远离热源，理论上是可以的，但会造成热电极材料的浪费。补偿导线就是为解决这个矛盾而产生的。它的特点是在参考端温度可能的变化范围内（0-100℃或0-150℃）由补偿导线两极组成的热电偶其热电特性与所配热电偶的热电特性相同，即：

Ea'b'（tn，0）=Eab（tn，0）

根据中间温度定律可知，热电偶回路中加入补偿导线后，其热电势仅与测量端温度t和补偿导线与仪表连接处t0有关，而与参考端温度tn的变化无关，就象把热电极延长到仪表处，所以补偿导线的作用只是延长了热电偶，它并不能消除参考端温度不为0℃时的影响。因此还必须利用诸如0℃恒温器法、计算法、仪表自动补偿法等方法将热电偶参考端温度修正到0℃。

Thermopile

A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel.

Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient.

Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the infrared thermometers widely used by medical professionals to measure body temperature. They are also used widely in heat flux sensors (such as the Moll thermopile and Eppley pyrheliometer)[1][2][3]and gas burner safety controls. The output of a thermopile is usually in the range of tens or hundreds of millivolts.[4]As well as increasing the signal level, the device may be used to provide spatial temperature averaging.

Thermopile, composed of multiple thermocouples in series. If both the right and left junctions are the same temperature, voltages cancel out to zero. However if one side is heated and other side cooled, resulting total output voltage is equal to the sum of junction voltage differentials.

冷端补偿计算方法

1、初学者容易走的思路是：知道了测量回路的实际热电势E（T，T0）后，其中T为被测点温度，T0为冷端温度，查热电偶的分度表得知T-T0的值；再根据冷端补偿回路的热电势Ea（T0,0）,其中T0为冷端温度值，0是0摄氏度，查分度表，得到T0的温度值，然后再用T-T0的值和T0的值相加即为被测点的温度。这个思路可以概括为“先查表，后求和”。

2、初学之之所以走进这个思路，是因为没有彻底理解“中间温度定律”。

3、中间温度定律是指：热电偶回路中两接点（温度为T、T0）间的热电势，等于热电偶两节点在温度为T、Tn时的热电势与在温度为Tn、T0时的热电势的代数和。其中Tn称中间温度。

4

4、中间温度定律的应用：

当冷端温度不为0摄氏度时，不能利用已知回路实际热电势E（t,t0)直接查表求取热端温度值。更不能利用已知回路实际热电势E（t,t0)直接查表求取的温度值，再加上冷端温度值确定热端被测温度值，而需要按中间温度定律进行修正。5 5、热电偶测温，冷端补偿计算方法，一句话概括：“先求和，再查表得温度”。

理论上测量是以冷端在零度为标准测量的,然而，通常测量时仪表是处于室温之下的，由于冷端不为零度，造成热电势差减小，使测量不准，出现错误。所做的补偿措施就是冷端温度补偿 .

热电偶测量温度时要求其冷端（测量端为热端，通过引线与测量电路连接的端称为冷端）的温度保持不变，其热电势大小才与测量温度呈一定的比例关系。若测量时，冷端的（环境）温度变化，将影响严重测量的准确性。在冷端采取一定措施补偿由于冷端温度变化造成的影响称为热电偶的冷端补偿。

热电偶的冷端补偿通常采用在冷端串联一个由热电阻构成的电桥。电桥的三个桥臂为标准电阻，另外有一个桥臂由（铜）热电阻构成。当冷端温度升高，热电偶产生的热电势将减小，而此时串联电桥中的热电阻阻值也将变化并使电桥两端的电压也升高。如果参数选择得好且接线正确，电桥产生的电压正好与热电势随温度变化而变化的量相等，整个热电偶测量回路的总输出电压（电势）正好真实反映了所测量的温度值。这就是热电偶的冷端补偿原理。