First, the basic principle of thermocouple temperature measurement
A thermocouple is formed by connecting two conductors or semiconductors A and B of different materials to form a closed loop. As shown in figure 1. The temperature T end is called the temperature sensing end, and the temperature t0 end is called the reference end or the cold end. When there is a temperature difference between two persistent points T and t0 of conductors A and B, an electromotive force EAB(t, t0) is generated in the loop, thus forming a current in the loop, which is called "thermoelectric effect". This electromotive force is called thermoelectric potential, and thermocouple works by using this effect. The magnitude of thermoelectric potential is related to the difference between t and t0. while
EAB(t，t0)=EAB(t)-EAB(t0)
Where eab (t, t0)- thermoelectric potential of thermocouple;
Eab (t)-the thermoelectric potential at the working end when the temperature is t;
EAB (t0)-Thermoelectric potential at the cold end when the temperature is t0.
As can be seen from the above formula! When the temperature of the measured medium at the working end changes, the thermoelectric potential also changes. Therefore, as long as the known EAB(t, t0) is measured, EAB(t) can be obtained, and the thermoelectric potential can be sent to a display instrument for indication or recording, or sent to a microcomputer for processing, so that the temperature t value at the measuring end can be obtained.
In order to truly understand the application of thermocouple, we have to mention several important characteristics of thermocouple circuit:
Law of mass: a closed loop composed of a homogeneous material, no matter how the temperature is distributed along the length of the material, there is no thermoelectric potential in the loop. This law requires that the two materials that make up the thermocouple must be homogeneous, otherwise extra potential will be generated due to the temperature gradient along the length direction of the thermocouple, which will introduce errors due to the inhomogeneity of thermocouple materials.
Law of Intermediate Conductor: A third (or multiple) homogeneous material is inserted into the thermocouple circuit, so long as the temperature of the connecting points at both ends of the inserted material is the same, the inserted third material will not affect the thermoelectric potential of the original circuit. This law shows that as long as the instrument for measuring thermoelectric potential is at a stable ambient temperature, it can be pulled into the thermocouple circuit. At the same time, it also shows that the junction of thermocouple can be not only welded, but also connected with uniform isothermal conductor.
Intermediate temperature law: the thermoelectric potential EAB (t, to) of a thermocouple loop composed of two different materials is equal to the algebraic sum of the corresponding thermoelectric potentials EAB(t, tn) and EAB (tn, to) when the junction temperatures are (t, tn) and (TN, to), where TN is the intermediate temperature. This law shows that when the reference temperature of thermocouple is not 0℃, as long as the thermoelectric potential EAB(t, to) can be measured and to is known, the measured temperature T can still be obtained from the thermocouple graduation table.
Law of connecting conductor: In the thermocouple loop, if the electrode materials A and B of the thermocouple are connected to the connecting wires A 1 and B 1 respectively (as shown in the following figure), and the temperatures of the relevant contacts are T, tn and To, the total thermoelectric potential of the loop is equal to the thermoelectric potential EAB (T, tn) at both ends of the thermocouple and the connecting wire A 1 at the temperature of T and tn.
The law of intermediate temperature and the law of connecting conductor are the theoretical basis for the application of compensation line to industrial thermocouple temperature measurement.
Second, the causes and solutions of various errors
2. The influence of1thermocouple's unstable thermoelectric characteristics
2. 1. 1 Effects of pollution and stress and their elimination methods
In the production process of thermocouple, the surface of uniform wire will always be contaminated after multiple necking and stretching. At the same time, from the internal structure of uniform wire, there are inevitably stresses and lattice inhomogeneities. The stress introduced by quenching or cold working can be basically eliminated by annealing, and the error caused by unqualified annealing can reach several tenths to several degrees. It is related to the measured temperature and the temperature gradient on the thermocouple electrode. Even low-cost metal thermocouple wires are usually delivered in an "annealed" state. If the low-cost metal thermocouple needs to be annealed at high temperature, the annealing temperature should be higher than the upper limit of its service temperature and the insertion depth should be greater than the actual service depth. Precious metal thermocouples must be carefully cleaned (pickling and sodium tetraborate cleaning) and annealed to remove the pollution and stress of thermocouples.
2. 1.2 Influence of inhomogeneity
Generally speaking, if a thermocouple is made of a uniform conductor, its thermoelectric potential is only related to the temperature at both ends. If the material of the hot electrode is not uniform and the hot electrode is in a temperature gradient field, the thermocouple will generate an additional thermoelectric potential, that is, an "uneven potential". Its size depends on the temperature gradient distribution along the length of the hot electrode, the uneven form and degree of the material and the position of the hot electrode in the temperature field The main reasons of uneven hot electrode are: uneven distribution of impurities, composition segregation, local metal volatilization, oxidation or selective oxidation of a metal element on the surface of hot electrode, thermal diffusion at the measuring end at high temperature, pollution and corrosion of thermocouple in harmful atmosphere. In the physical state, there are uneven stress distribution and uneven electrode structure.
In industrial use, the additional error caused by uneven potential is sometimes as high as 30℃, which will seriously affect the stability and interchangeability of thermocouple. The main solution is to test them and use thermocouples only within the allowable error range.
2. 1.3 Influence of thermocouple instability
Instability refers to the change of the dividing value of thermocouple with the use time and conditions. In most cases, it may be the main reason for inaccuracy. The factors affecting the instability are pollution, high-temperature volatilization of hot electrodes, redox, embrittlement, radiation and so on. If the change of calibration value is slow and uniform, it is often supervised or periodically verified according to the actual use, which can reduce the error caused by instability.
2.2 the influence of reference end temperature and its correction method
Thermoelectromotive force of thermocouple is related to the material of hot electrode and the temperature of working end. The graduation table of thermocouple and the temperature display instrument calibrated according to the graduation table are all based on the condition that the temperature at the reference end of thermocouple is equal to 0℃. In practical use, the cold end temperature (reference end) of thermocouple is not only 0℃, but also changes frequently, and the temperature value measured by thermometer will produce great error. In this case, we usually adopt the following methods to correct it.
2.2. 1 thermoelectric potential correction method
According to the law of intermediate temperature, when the reference temperature is tn, the thermoelectric potential EAB (t, tn) = EAB (t, t0)-EAB (TN, t0). Therefore, with the temperature sensor at room temperature, we only need to measure the temperature tn at the reference end, and then find out the thermoelectric potential E(tn, t0) at the corresponding temperature from the index table of the corresponding thermocouple, and then add this thermoelectric potential to the measured E(t, tn) algebra. The result is that when the temperature at the reference end of the thermocouple is 0 degrees, the thermoelectric potential E(t, t0) at the corresponding temperature at the measuring end will finally be from the index table. Today, with the increasing application of computers, software processing can be used, especially in multi-point measurement system or high temperature measurement and control, which can solve the problem of temperature change at the reference end well. As long as tn is accurately measured at any time, the temperature at the measuring end can be accurately obtained. At the same time, the graduation table of the corresponding thermocouple is fully applied, which corrects the nonlinear error and is suitable for all kinds of thermocouples.
2.2.2 Starting point method for instrument adjustment
Because the indicated value of the instrument is that EAB(tn, t0) corresponds to thermoelectric potential, if the zero position of the instrument pointer is set to tn when the measuring line is open, a potential EAB(tn, t0) is added to the instrument in advance. When measuring the temperature with a closed measuring line, the thermoelectric potential EAB(tn, t0) input by thermocouple is superimposed with EAB(t, tn), and the sum of them is. Therefore, it is very simple to adjust the starting point of the direct reading instrument.
Compensating conductor
Extend the reference end of the thermocouple to a constant temperature with a compensation line, and then correct it. In essence, when the reference temperature is not 0℃, it cannot eliminate the influence. Therefore, the temperature at the connection between the compensation line and the instrument should be corrected to 0℃ in combination with other correction methods. At this time, the reference terminal has become a new reference terminal, and the temperature remains unchanged or changes little. At this time, the thermoelectric potential generated by thermocouple is not affected by the temperature change of the original reference end. EAB (T, T 10) is the thermoelectric potential measured when the new reference terminal temperature T 10 (not equal to℃) is constant, and TAB( T, T 10) is the reference terminal temperature T0 = 0℃.
When using the compensation line, we should not only pay attention to the polarity of the compensation line, but also pay special attention not to misuse it. At the same time, pay attention to keep the temperature at both ends of the joint between the compensation line and the thermocouple equal, and the temperature is between 0- 100℃ (or 0- 150℃), otherwise the measurement error will occur.
Reference temperature compensator
The compensator is an unbalanced bridge, and the resistances of the three legs of the bridge are made of manganese copper wires with small temperature coefficient of resistance. Its resistance basically does not change with the change of temperature, and makes r 1 = R2 = R3 = 1ω. The other arm resistance Rt is made of copper with large temperature coefficient of resistance, and Rt = r 1 = 1ω at 20℃. At this time, the bridge is balanced and there is no voltage output. When the temperature of the bridge changes, the resistance of Rt also changes, so there is an unbalanced voltage output to offset the thermoelectric potential error caused by the temperature change of the reference terminal, thus obtaining compensation. (Note: China also takes 0℃ as the equilibrium point temperature.) When the temperature reaches 40℃ (i.e. the calculation point temperature), the output voltage of the bridge just compensates for the thermoelectric potential change caused by the deviation of the reference temperature of the thermocouple from the equilibrium point temperature.
For electronic potentiometers, the measuring bridge itself has the function of automatic temperature compensation, and it is not necessary to adjust the temperature starting point of the instrument when using. Except for the equilibrium point and calculation point, only approximate compensation can be obtained when obtaining the temperature values of other reference terminals, so the processing method of using the cold end compensator as the reference terminal temperature will bring some additional errors.
2.3 Influence of heat transfer and thermocouple installation
Because thermocouple temperature measurement belongs to contact measurement, when the thermocouple is inserted into the measured medium, it will absorb heat from the measured medium to raise its temperature, and at the same time radiate heat to a low temperature place through thermal radiation and thermal conduction. When the heat lost outside the measuring end is equal to the heat absorbed from the airflow, the dynamic balance will be reached. At this time, the thermocouple reaches the stable indication, but it does not represent the real temperature of the airflow, because the heat lost in the environment of the measuring end is compensated by the heating of the airflow, that is, the heat exchange between the measuring end and the airflow. The stronger the heat transfer between the measuring end and the environment, the greater the temperature deviation between the measuring end and the airflow temperature.
2.3. 1 thermal radiation error
The thermal radiation error is caused by the radiation heat exchange between the measuring end of thermocouple and the environment, and it is the result that the convective heat exchange between thermocouple and airflow can not reach the thermal balance. The way to reduce radiation error is to strengthen convection heat transfer and weaken radiation heat transfer. Specific methods are:
Minimize the temperature difference between the pipe wall and the measuring end, that is, lay insulation layer on the pipe wall;
Shielding the working end of the thermocouple;
Improve the heat release coefficient of the fluid, that is, increase the flow rate, strengthen the disturbance, reduce the uniform wire diameter or make the hot electrode cross-flow with the gas flow.
Thermal conductivity error
When measuring the temperature of high-temperature airflow, due to the temperature gradient along the length of thermocouple, the measuring end will inevitably conduct heat along the hot electrode, which makes the indicated temperature deviate from the actual temperature. The greater the difference of heat conduction, the greater the corresponding error, so all factors that can aggravate convection and weaken heat conduction can be used to reduce the heat conduction error. Specific methods are:
Increase l/d;
Change the vertical installation of thermocouple to inclined installation or elbow installation, and pay attention to make the end of thermocouple face the direction of airflow and be in the position of maximum flow;
Choose thermocouple and materials with low thermal conductivity of the pillar.
2.4 Measuring system leakage impact
Poor insulation is the main cause of current leakage, which has a great influence on the accuracy of thermocouple, which can distort the measured thermoelectric potential, distort the instrument display and even fail to work normally. There are many errors caused by electric leakage, such as the poor insulation resistance of hot electrode insulating porcelain tube, which bypasses the hot current. If the electrical measuring equipment leaks electricity, it can also bypass the working current and make the measurement error. Because the potentiometer for measuring thermoelectric potential has low resistance, the requirements for insulation resistance are not high. The leakage that affects the thermoelectric potential measurement mainly comes from the high temperature of the tested system, because the insulation resistance of the insulation materials of the thermocouple protection tube and the hot electrode will decrease with the increase of temperature. This is what we usually call the "shunt error" of armored thermocouple. Generally, grounding or other shielding methods are adopted. We usually increase the diameter of shunt error of armored thermocouple; Increase the thickness of insulation layer; Shorten the length of the heating belt; Reduce the resistance of thermocouple and other methods to reduce the error.
2.5 Dynamic response error
After the thermocouple is inserted into the measured medium, due to its thermal inertia, it can not immediately indicate the temperature of the measured airflow, and only when the suction and heat release at the measuring end reach a dynamic balance can it achieve a stable indication. In the whole unstable process from inserting thermocouple to indicating stability, there is a deviation between the instantaneous indication of thermocouple and the indication of stability. At this time, in addition to various steady-state errors, there is also a deviation introduced by thermal inertia of thermocouple, that is, dynamic response error. The methods to overcome this error are: first, determine the dynamic response error and correct it; The second is to reduce the dynamic response error to the allowable range. At this time, it can be considered that t measurement = T gas.
2.6 Influence of short-range ordered structure change (K state)
When K-type thermocouple is used in the range of 250-600℃, its microstructure changes, forming a short-range ordered structure, which will affect the thermoelectric potential and produce errors. This is the so-called K-state. This is a unique lattice change of Ni-Cr alloy. When the WCr is in the range of 5%-30%, the atomic lattice changes from order to disorder. The errors caused by these factors vary with chromium content and temperature. Generally, after short-time heat treatment above 800℃, its thermoelectric characteristics can be restored. Because of the existence of K-state, the verification sequence of K-type thermocouple is clearly stipulated in the verification regulation: from low temperature to high temperature, the verification is carried out point by point. Moreover, at the verification point of 400℃, not only the heat transfer effect is not good, it is difficult to reach thermal balance, but also it is within the maximum range of K-state error. So be careful when judging whether this is qualified or not. The short-range ordered structure change of Ni-Cr alloy exists not only in K-type but also in E-type thermocouple anode. But as a variable, E-type thermocouple is only 2/3 of K-type thermocouple. In short, K-state is related to temperature and time. When the temperature distribution or thermocouple position changes, its deviation will also change greatly. Therefore, it is difficult to make an accurate evaluation of the deviation.
Three. abstract
By summarizing the principle and error source of thermocouple, we have a systematic understanding of thermocouple temperature measurement error and draw some conclusions. The instability and inhomogeneity of thermocouple, the temperature change of reference end, heat conduction and improper installation and use will all cause measurement errors, some of which are due to errors in the manufacturing process, or errors in the measuring system and instrument itself, and some are artificial. This part can be avoided as long as you are careful and have a certain understanding of the characteristics of thermocouples.