Common fault analysis and solution of thermocouple

Analysis and solution of common faults of thermocouples The price of thermal resistance and thermocouple depends on the model.

You need to specify your model, length, relevant specifications and quantity For example, the price of 350 long platinum resistance of Pt100 230 is about 60 yuan. If it's a fast thermocouple, the price of KS KB thermocouple is generally 4-5 yuan. The price of kW is about 2 yuan. The price of ordinary Ktype thermocouple is about 80 yuan. If the price of anti riot thermocouple is about 200 yuan, the price of integrated thermocouple is about 200 yuanThe price of one meter long wear-resistant thermocouple and 300 long wear-resistant head is about 400 yuan and the price of one meter long platinum rhodium thermocouple. At present, the price of thermocouple is about 2000-2500 yuan.

Due to the price change of raw materials, the price of the above thermocouple should go with the market, Thermal resistance measurement - 100-550 degrees, thermocouple measurement 0-1300, generally below 500 degrees with thermal resistance, more than 500 degrees with thermal resistance

The correct use of k type thermocouples can not only accurately get the temperature value, ensure the product is qualified, but also save the material consumption of thermocouples, save money and ensure the product quality.If the installation is not correct, errors such as thermal conductivity and time lag will be produced, which are the main errors in the use of thermocouples. 　　1.Error caused by improper installation The thermocouple shall not be installed too close to the door and heating place. The insertion depth shall be at least 8-10 times of the diameter of the protective tube. The installation position and insertion depth cannot reflect the real temperature of the furnace.The gap between the thermowell and the wall of the thermocouple is not filled with insulation material, which causes hot overflow or cold air intrusion in the furnace. Therefore, the gap between the thermowell and the hole of the furnace wall shall be blocked with insulation material such as refractory mud or asbestos rope, so as to avoid the influence of cold and hot air convection on the accuracy of temperature measurement.The installation of thermocouples shall avoid strong magnetic field and electric field as much as possible, and the thermocouple and power cable shall not be

installed in the same conduit to avoid interference and error.Thermocouples can not be installed in the area where the measured medium has little flow. When measuring the gas temperature in the tube with thermocouples, the thermocouples must be installed against the flow direction and fully contacted with the gas http://www.tksiliconerubbercable.com.

2.Error caused by insulation variation For example, too much dirt or salt slag on thermocouple protection tube and stay wire plate results in poor insulation between thermal dipole and furnace wall, which is more serious under high temperature. This will not only cause loss of thermal potential, but also introduce interference, resulting in errors sometimes up to Baidu.

3.Error introduced by thermal inertia Because of the thermal inertia of the thermocouple, the indicating value of the instrument lags behind the change of the measured temperature, especially in the rapid measurement.Therefore, thermocouples with thin thermoelectric pole and small diameter of protective tube should be used as far as possible.When the temperature measurement environment permits, the protective pipe can even be taken away.Because of the measurement lag, the amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation.The larger the measurement lag is, the smaller the amplitude of thermocouple fluctuation is, and the greater the difference with the actual furnace temperature is.When a thermocouple with a large time constant is used to measure or control the temperature, although the temperature displayed by the instrument fluctuates very little, the actual furnace temperature may fluctuate a lot.In order to measure temperature accurately, thermocouple with small time constant should be selected.The time constant is inversely proportional to the heat transfer coefficient and directly proportional to the diameter of the hot end of the thermocouple, the density of the material and the specific heat. To reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to minimize the size of the hot end.In use, the protective casing with thin wall and small inner diameter is usually made of materials with good thermal conductivity.In the more precise temperature measurement, the bare wire thermocouple without protective sleeve is used, but the thermocouple is easy to be damaged, so it should be corrected and replaced in time http://www.tiankangcable.com.

4.Thermal resistance error

In case of high temperature, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance will increase and block the heat conduction. At this time, the temperature indication is lower than the true value of the measured temperature.Therefore, the outside of the thermocouple protection tube should be kept clean to reduce the error.

Preface

In the existing temperature measurement system, the most commonly used temperature sensor thermocouple, because of its simple structure, is often mistaken as "two wires of thermocouple, connect it to finish", in fact, this is not the case.Although the structure of thermocouple is simple, there are still various problems in use, such as improper installation or use method, which will cause large measurement error. Even the certified thermocouple will be out of tolerance due to improper operation and unqualified in use. In the reductive atmosphere such as carburizing, if not paid attention, K-type thermocouple will also be out of tolerance due to selective oxidation.

In order to improve the measurement accuracy, reduce the measurement error and extend the service life of the thermocouple, the user is required to have not only instrument operation skills, but also physical, chemical and material knowledge.Based on many years of practice and related materials, the author introduces in detail the measurement errors of thermocouples and points for attention.

II. Main influencing factors of measurement error

1.

Effect of insertion depth

(1) Selection of temperature measuring points The installation position of thermocouple, that is, the selection temperature measuring point, is the most important.The location of temperature measurement point must be typical and representative for production process, otherwise it will lose the significance of measurement control.

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(2) Insertion depth When the thermocouple is inserted into the measured site, heat flow will be generated along the length direction of the sensor.When the ambient temperature is low, there will be heat loss, resulting in the temperature difference between the thermocouple and the object to be measured, resulting in temperature measurement error.In a word, the error caused by heat conduction is related to the insertion depth.The insertion depth is related to the material of the protection pipe.Because of its good thermal conductivity, the insertion depth of metal protective tube should be deeper (about 15-20 times of the diameter), and the thermal insulation of ceramic material should be better, which can be inserted shallower (about 10-15 times of the diameter).For engineering temperature measurement, the insertion depth is also related to the static or flowing state of the measurement object. For example, the measurement of the temperature of flowing liquid or high-speed air flow will not be subject to the above restrictions, and the insertion can be shallower. The specific value shall be determined by the experiment.

2.

Impact of response time

The basic principle of contact temperature measurement is to achieve thermal balance between the temperature measuring element and the measured object.Therefore, it is necessary to keep the temperature measurement for a certain period of time in order to achieve the heat balance between the two.The holding time is related to the thermal response time of the temperature measuring element.However, the thermal response time mainly depends on the sensor structure and measurement conditions.For gas medium, especially static gas, it should be kept for at least 30 minutes to reach equilibrium; for liquid, it should be kept for at least 5 minutes.

The response time of the sensor is required to be in millisecond level when the whole process is only 1s for the measured place with constant temperature change, especially for the instantaneous change process.Therefore, the common temperature sensor not only can't keep up with the temperature change speed of the measured object, but also can't reach the thermal balance and produce the measurement error.It's better to choose a sensor with fast response.In addition to the influence of the protective tube, the diameter of the measuring end of the thermocouple is also the main factor, that is, the finer the coupling wire, the smaller the diameter of the measuring end, and the

shorter the thermal response time.The thermal response error of the temperature measuring element can be determined by the following formula. Δθ=Δθ0exp(-t/t) （1） In style Δ θ - error caused by temperature measuring element at time t, K or ℃ Error caused by temperature measuring element at Δ θ 0 - "t = 0", K or ℃ T -- measurement time, s τ - time constant, s ε - base of natural logarithm (2.718) Therefore, when t = τ, Δθ = Δθ 0 / E That is 0.368, If t = 2 τ, then Δθ = Δθ 0 / E2 That is 0.135.

When the temperature of the measured object rises or falls at a certain speed α (K / s or ℃ / s), after enough time, the response error can be expressed as follows: Δθ∞=-ατ (2) In style Δ θ∞ - error caused by temperature measuring element after enough time It can be seen from equation (2) that the response error is directly proportional to the time constant (τ).In order to improve the efficiency of verification, many enterprises use automatic verification device to verify thermocouple, but the device is not perfect.It is found in the heat treatment workshop of the second steam transmission plant that if the constant temperature time at 400 ℃ is not enough and the heat balance cannot be achieved, it is easy to misjudge.

3.

Effects of thermal radiation

The thermocouple inserted into the furnace for temperature measurement will be heated by the heat radiation from the high temperature object.It is assumed that the gas in the furnace is transparent, and when the temperature difference between the thermocouple and the furnace wall is large, the temperature measurement error will be generated due to the energy exchange.

In unit time, the radiant energy exchanged by the two is p, which can be expressed as follows: P=σε(Tw4-Tt4) (3) In style σ - Stefan Boltz constant ε - emissivity TT -- temperature of thermocouple, K Tw - temperature of furnace wall, K In unit time, the energy of heat exchange between thermocouple and surrounding gas (temperature T) through convection and heat conduction is p ' P'=αA(T-Tt) (4) Where α - thermal conductivity A - surface area of thermocouple In normal state, P = P ', the error is: Tt-T=σε(Tt4-Tw4)/ αA (5) For unit area, the error is Tt-T=σε(Tt4-Tw4)/ α (6) Therefore, in order to reduce the thermal radiation error, the heat conduction should be increased and the furnace wall temperature TW should be as

close to the thermocouple temperature TT as possible.In addition, attention shall be paid during installation: the thermocouple installation position shall avoid the thermal radiation from the solid as far as possible, so that it cannot radiate to the thermocouple surface; the thermocouple is better to be equipped with a thermal radiation shield.

4.

Effect of increasing thermal impedance

For the thermocouple used at high temperature, if the measured medium is in gas state, the dust deposited on the surface of the protection tube will be burned and melted on the surface, which will increase the thermal impedance of the protection tube; if the measured medium is a melt, there will be slag deposition in the use process, which not only increases the response time of the thermocouple, but also makes the indicated temperature low.Therefore, in addition to regular verification, in order to reduce the error, it is necessary to carry out spot check frequently.For example, imported copper smelting furnace is not only equipped with continuous temperature measuring thermocouple, but also equipped with consumption type thermocouple temperature measuring device to calibrate the accuracy of continuous temperature measuring thermocouple in time.

III. precautions for thermocouple temperature measurement

1.

Influence of thermocouple wire heterogeneity

(1) Thermocouple material is not homogeneous When the thermocouple is verified in the measuring room, according to the regulations, the depth inserted into the verification furnace is only 300 mm.Therefore, the verification results of each thermocouple can only reflect or mainly reflect the thermoelectric behavior of 300 mm long thermocouple wires from the measurement end. However, when the thermocouple is long, most of the thermocouple wires are in the high temperature zone. If the thermocouple wires are homogeneous, the measurement results are independent of the length according to the uniform circuit rule.However, thermocouple wires are not

homogeneous, especially in the case of poor homogeneity and temperature gradient of low-metal thermocouple wires, then local thermoelectric EMF will be generated, which is called parasitic EMF.The error caused by parasitic potential is called inhomogeneous error.

In the existing verification regulation of precious metal and low-cost metal thermocouples, the heterogeneity of thermocouples has not yet been regulated. Only in the standard of thermocouple wire material, there are certain requirements for the heterogeneity of thermocouple wire.The non-uniform thermoelectric EMF of low-cost metal thermocouples is calculated by the head tail verification method.Normal thermocouple wire production plants, according to the requirements of national standards, produce non-uniform thermoelectric EMF products that meet the requirements.

(2) Heterogeneity of thermocouple wire after use For the new thermocouple, even if the non-uniform thermoelectric potential energy meets the requirements, but repeated processing and bending will cause processing distortion and loss of homogeneity of the thermocouple; and the thermocouple in use will also cause thermoelectric potential change due to the degradation of the thermocouple wire for a long time under high temperature.

For example, the thermocouple inserted into the industrial furnace will degrade along the length direction of the thermocouple wire and increase with the temperatureWhen the degraded part is in the place with temperature gradient, the parasitic electromotive force will be superposed in the total thermoelectric force and the measurement error will appear.

In practice, the author found that some thermocouples (mostly low-cost metal thermocouples) that have passed the verification by the Metrology Department are unqualified when they are used on site, and they are still qualified when they are returned to the Metrology Department for verification.

The main reason is the inhomogeneity of the coupling wire.The technicians who produce thermocouples have realized that the unqualified rate of thermocouples also increases with the increase of their length, which is the influence of the heterogeneity of heating thermocouple wire.In a word, the error caused by inhomogeneity, i.e. parasitic electromotive force, depends on the degree of inhomogeneity of thermocouple wire itself and the magnitude of temperature gradient, so it is very difficult to quantify it.

2.

Shunt error of armored thermocouple

(1) Shunt error The armored thermocouple used in carburizing furnace of Wazhou group is not allowed for only one week.In order to explore the reasons, the author has visited the site and found no abnormality, and took it from the furnace and passed the verification by the measuring room.So what's the problem?

Finally, according to the field installation characteristics of the thermocouple, it is found that the above problems are caused by the shunt error of armored thermocouple.

The so-called shunt error is that when the temperature of the furnace is measured by armored thermocouple, when there is a temperature distribution of more than 800 ℃ in the middle part of the thermocouple, because of the decrease of its insulation resistance, the thermocouple indication appears abnormal phenomenon.According to the rule of homogeneous circuit, the temperature measurement with thermocouple is only related to the temperature at both ends of the measuring end and the reference end, but not to the temperature distribution in the middle.However, because the insulation material of armored thermocouple is powder like MgO, the insulation resistance will drop by an order of magnitude every 100 ℃ when the temperature rises. When the temperature of the middle part is high, there must be leakage current, which will cause shunt error in the output potential of thermocouple.

(2) Conditions for the generation of shunt error The armored thermocouple is horizontally inserted into the furnace. Its specification and experimental conditions are as follows: diameter of Φ 4.8mm, length of 25m, length of heating belt in the middle part of 20m, and temperature of 1000 ℃.In this experiment, the temperature difference between the measuring end and the middle part of the thermocouple is 200 ℃.If the temperature of the measuring end is higher than the middle part, a negative error will be generated; on the contrary, a positive error will be generated.If the temperature difference between the two is 200 ℃, then the shunt error is about 100 ℃.This is absolutely not to be ignored. The generation conditions of shunt error are related to the type and diameter of armored thermocouple, as shown in the table.

3.

Influencing factors and Countermeasures of shunt error

The phenomenon of shunt error of armored thermocouple under high temperature is attracting people's attention. Therefore, it is necessary to understand the influencing factors of shunt error and take appropriate measures to reduce or eliminate the influence of shunt error.

(1) Diameter of armored thermocouple For the K-type armored thermocouple (MgO insulated) with a length of 9m, only the middle part of the thermocouple is heated.The experimental results show that the size of the shunt error is inversely proportional to the square root of its diameter (the diameter is too thin, and this rule is not observed), that is, the finer the diameter is, the larger the shunt error is.

When the temperature of the middle part is higher than 800 ℃, the shunt error will be produced for Φ 3.2mm armored thermocouple.But for Φ 6.4mm and Φ 8mm armored thermocouples, when the temperature of the middle part is 900 ℃, no shunt error is found.For armored thermocouples with diameter of Φ 6.4mm (diameter of thermal wire is Φ 1.4mm) and Φ 8mm (diameter of thermal wire is Φ 2.0mm), when the temperature of the middle part is 1100 ℃, the shunt error of armored thermocouple with diameter of Φ 8mm is only half of that of Φ 6.4mm.This value (50%) is approximate to the square ratio (1.42 / 2.02) of the wire diameter of two armored thermocouples, while the square ratio of the wire diameter is the resistance ratio of the wire.Therefore, in order to reduce the shunt error, we should try to choose the armored thermocouple with the coarse diameter.

(2) Temperature in the middle If the temperature of the middle part exceeds 800 ℃, it may produce shunt error, and its size will increase exponentially with the increase of temperature.Therefore, in addition to the measuring end, other parts should be avoided to exceed 800 ℃.

When the temperature of the heating belt in the middle part is higher than 800 ℃, the longer the length of the heating belt is, the farther it is from the measuring end, and the larger the shunt error is.Therefore, the length of heating belt should be shortened as much as possible, and heating should not be far away from the measuring end to reduce the shunt error.

(3) Resistance of thermocouple wire When the diameter of armored thermocouple is the same, the shunt error will increase with the increase of resistance of thermocouple wire.Therefore, it is better to use thermocouple wire with small resistance.For example, the shunt

error of S-type armored thermocouple with the same diameter is reduced by 40% compared with K-type thermocouple.Therefore, S-type thermocouple can be used to measure the temperature field distribution in the furnace. Although the cost is high, it is more accurate.

(4) Insulation resistance At high temperature, the oxide resistivity will decrease exponentially with the increase of temperature, and the shunting error mainly depends on the insulation performance of the high temperature part. The lower the insulation resistance is, the easier the shunting error will occur.When the insulation resistance is increased 10 times or reduced to 1 / 10, the shunt error is also reduced to 1 / 10 or increased 10 times.In order to reduce shunt error, armored thermocouple with thick diameter should be used as much as possible to increase the thickness of insulation layer.If the above measures are invalid, the assembled thermocouple shall be used.

4.

The influence of short range ordered structure change (k state)

When K-type thermocouples are used in the temperature range of 250-600 ℃, due to the change of their microstructure and the formation of short-range ordered structure, they will affect the thermoelectric potential and produce errors, which is called K-state.It is a unique lattice change of Ni Cr alloy. When the content of Cr is in the range of 5-30%, there is an error caused by the order disorder transition of atomic lattice, which varies with the content of Cr and the temperature.Heat the K-type thermocouple from 300 ℃ to 800 ℃, take a point every 50 ℃, and measure the potential at this point.When the temperature is 450 ℃, the maximum deviation is 4 ℃. In the range of 350-600 ℃, all of them are positive deviation.Due to the existence of K-state, the verification results of Ktype thermocouples in temperature rise or temperature fall are inconsistent, so the verification sequence is clearly specified in the verification regulation of low-temperature to high-temperature point by point temperature rise verification; moreover, at the verification point of 400 ℃, not only the heat transfer effect is poor, it is difficult to achieve heat balance, but also just in the maximum range of K-state error.Therefore, it should be very careful to judge whether the point is qualified or not.

The change of short-range ordered structure of Ni Cr alloy exists not only in the Ktype, but also in the E-type thermocouple positive pole, but as a change, the E-type thermocouple is only 2 / 3 of the K-type.In a word, K state is related to temperature and time. When the temperature distribution or thermocouple position changes, its deviation will also change greatly, so it is difficult to make an accurate evaluation of the deviation.

5.

Influence of use atmosphere

(1) Selective oxidation For the Ni Cr alloy containing Fe, if the oxygen partial pressure is lower than a certain value, the CR with high affinity to O2 will undergo selective oxidation, which is the unique grain boundary oxidation of Ni Cr alloy.If the oxide layer on the outer surface is observed with a microscope, green precipitates can be seen, which is usually called "green corrosion".Especially when the temperature is in the range of 800-1050 ℃ and there are reductive gases such as CO and H2 in the system, the positive electrode of K-type thermocouple is more likely to undergo selective oxidation.This kind of low thermal potential caused by the decrease of Cr content has become the limiting factor for the long-term use of K-type thermocouple in the heat treatment industry.

If the gas used is very pure and the system does not contain oxygen, the service life of the thermocouple can be prolonged; if there is an oxide layer on the surface of the thermocouple wire, enough oxygen can still be provided for the selective oxidation of CR.Therefore, clean and polished even wire should be used in non oxidizing atmosphere.At the same time, try to avoid using in inert gas with trace oxygen or air with low oxygen partial pressure.When the length and diameter of the protective tube are large (i.e. the protective tube is very thin), due to the poor air circulation, the residual small amount of oxygen can still provide conditions for the selective oxidation of CR. (2) Countermeasures of selective oxidation In order to prevent or slow down the deterioration of K-type thermocouples caused by selective oxidation, in addition to improving the materials, corresponding countermeasures should be taken in the structure of thermocouples: (a) select the metal with stronger affinity for oxygen than Cr as the getter, seal it in the protective tube to prevent selective oxidation of Cr, or increase the diameter of the protective tube or air blowing method to increase the oxygen content.(b) Assembly thermocouple materialization.The patent product developed by the author - thermocouple for solid carburizing furnace, i.e. the assembled thermocouple with sealed structure, can prevent selective oxidation of Cr. the scheme has been proved effective by more than ten enterprises, such as wachaft group, FAW, Erqi, Epson industrial furnace,

Shenzhong, Shenchi, Qianjiang Motorcycle, etc.The service life is more than 12 months, the user is very satisfied.

(3) Influence of use atmosphere The stability of thermocouples, due to the use of different temperatures and atmospheres, for the same sensor, such as K-type thermocouples, the maximum use temperature also varies with the diameter, and the stability of K-type thermocouples with the same diameter also varies with the structure.When selecting thermocouples, it is necessary to consider the service conditions: common temperature and maximum service temperature; use atmosphere such as oxidation-reduction; anti vibration performance.

For the assembled thermocouple, the influence of atmosphere depends on the material of protection tube and thermocouple structure. Therefore, it is necessary to be familiar with and master the physical and chemical properties of various protection tube materials.For example, in the powder metallurgy industry, molybdenum tube is commonly used as thermocouple protection tube, and the application effect is better under the H2 atmosphere of 1600 ℃.However, molybdenum tube is damaged by oxidation in a very short time in an oxidizing atmosphere.Secondly, the appropriate thermocouple should be selected according to the use atmosphere. In the oxidizing atmosphere above 1300 ℃, the platinum rhodium thermocouple should be selected, and the tungsten rhenium thermocouple is better under the reducing and vacuum conditions.

For K-type thermocouple, it is suitable to work in air, O2 and other atmospheres, but when it is used in H2 atmosphere, its surface is reduced by H2, and there is no effect in a short time. For example, when it is exposed in H2 for a long time, at the same time of accelerated reduction, the even wire will grow up and break; in reducing atmospheres such as CO or gas, its degradation will be significantly accelerated and out of tolerance.

For Sheathed Thermocouples, the radius of hydrogen atom is very small, and it is easy to enter the inner part of the thermocouple through the outer shell, which will also accelerate the degradation, resulting in a significant reduction of the thermoelectric potential.

(4) Influence of insulation resistance Under high temperature, the insulation resistance of the insulation material used for thermocouple decreases rapidly with the increase of temperature. Therefore, leakage current will be generated. The current flows into the instrument through the insulation whose insulation resistance has

decreased, which makes the indication of the instrument unstable or produces measurement error, and the recorder may also be disordered.

Deterioration and service life of thermocouples

1.

Degradation of thermocouples

The service life of thermocouples is related to their deterioration. The so-called deterioration of thermocouples is the phenomenon of aging and deterioration of thermocouples after use.The internal grains of thermocouple made of metal or alloy grow up gradually at high temperature.At the same time, there are a small amount of impurities in the alloy, its position or shape will also change, and the reduction or oxidation gas in the surrounding environment will also react.With these changes, the thermoelectric EMF of the thermocouple will also change very sensitively.So the degradation of thermocouple is inevitable.

2.

Thermocouple service life

The degradation of thermocouple is a quantitative process, which is difficult to quantify. It will change with the type, diameter, temperature, atmosphere and time of thermocouple.The service life of thermocouples refers to the time when the deterioration of thermocouples exceeds the allowable error, or even the broken wires cannot be used.

(1) Life of assembled thermocouple Only the stability of thermocouples is required in Chinese standards.That is to say, the change of thermoelectric EMF before and after use is regulated after 200H at a certain temperature.However, no provision for service life has been found.The requirements for thermocouple service life in Japan are based on the continuous use time of thermocouple specified in JIS (c-1602-1995) standard.2000h for B, R, s type thermocouples, 10000h for K, e, J, T type thermocouples.

In practical use, the assembled thermocouple usually has a protective tube, which can only be used as bare wire under special circumstances.Therefore, in most cases, the life of the protection tube determines the life of the thermocouple.To judge the actual service life of thermocouple, it is necessary to collect and accumulate the data in actual use for a long time, so as to give a more accurate result.

(2) Life of armored thermocouple Since the armored thermocouple is protected by a sleeve and isolated from the external environment, the material of the sleeve has a great influence on the service life of the armored thermocouple. The thermocouple wire and metal sleeve must be selected according to the purpose.When the material is selected, its life increases with the increase of the diameter of armored thermocouple.Compared with assembled thermocouple, armored thermocouple has many advantages, but its service life is often lower than assembled thermocouple.

Five conclusion

Thermocouple is the most commonly used temperature sensor in scientific research and production. Although its structure is simple, if it is not noticed in use, it will still produce large measurement error.In view of the problems that are easy to occur in use, the author discusses in detail the main causes of errors in the selection of temperature measuring points, the insertion depth, response time, thermal radiation and thermal impedance of thermocouples, and points out the precautions in the use of thermocouples, such as the heterogeneity of thermocouples, the use atmosphere, the insulation resistance, the selective oxidation of K-type thermocouples, the K-state and the shunt error of armored thermocouples.It is helpful to improve the accuracy of measurement and prolong the service life of thermocouple.