Thermocouple full introduction from thermocoupl factory

Thermocouple is a commonly used temperature measuring element in the temperature measuring instrument. It directly measures the temperature and converts the temperature signal into thermoelectric ELECTROmotive force signal, which is then converted into the temperature of the measured medium through the electrical instrument (secondary instrument). The appearance of various thermocouples is often different due to the need, but their basic structure is generally the same, usually by the hot electrode, insulation sleeve protection tube and junction box and other main parts, usually with display instruments, recording instruments and electronic regulator matching use.

The basic principle of temperature measurement is that two different material conductors form a closed loop. When there is a temperature gradient at both ends, an electric current will flow through the loop. An electromotive force, the thermoelectric electromotive force, exists between the two ends, which is called the Seebeck effect. Two homogeneous conductors of different composition are thermal electrodes, with the working end at a higher temperature and the free end at a lower temperature, which is usually at a constant temperature. According to the thermoelectric electromotive force and the function relation of temperature, the thermocouple dividing meter is made. The indexing table is obtained under the condition that the free end temperature is 0℃. Different thermocouples have different indexing tables.

When the third metal material is connected in the thermocouple circuit, the thermoelectricity generated by the thermocouple will remain unchanged as long as the temperature of the two contact points of the material is the same, that is, it will not be affected by the third metal access circuit. Therefore, in the thermocouple temperature measurement, access to the measuring instrument, measured thermoelectric emf, you can know the temperature of the measured medium. When measuring the temperature of a thermocouple, it is required that the temperature of its cold end (the measuring end is the hot end, and the end connected with the measuring circuit through the lead wire is called the cold end) should remain unchanged, and the thermoelectric potential is proportional to the measured temperature. If the cold end (environment) temperature changes during measurement, the accuracy of measurement will be seriously affected. Some measures should be taken at the cold end to compensate for the influence caused by the temperature change at the cold end, which is called thermocouple's cold end compensation normal. Special compensation wire for connection with measuring instrument.

Calculation method of thermocouple cold end compensation:

From millivolt to temperature: measure the cold end temperature, convert it to the corresponding millivolt value, and add it to the thermocouple's millivolt value to convert the temperature;

From temperature to millivolt: the actual temperature and the cold end temperature are measured and converted into millivolt values respectively. After subtraction, the millivolt values are obtained and the temperature is obtained

Temperature conditions

Is a temperature sensing element, is a primary instrument, thermocouple direct measurement of temperature. A closed loop consisting of two conductors of different materials, with different electron densities, produces electron diffusion due to different materials, and generates electric potential after stable equilibrium. When gradient temperature exists at both ends, there will be current generation in the loop, generating thermoelectric emf, the greater the temperature difference, the greater the current will be. The temperature will be known when the thermoelectric emf is measured. A thermocouple is actually an energy converter that converts heat energy into electrical energy.

Thermocouple technical advantages: thermocouple temperature measurement range is wide, performance is more stable; High measuring precision, thermocouple and the measured object direct contact, not affected by the medium; The thermal response time is fast and the thermocouple responds flexibly to the temperature change. Large measurement range, thermocouple from -40~+ 1600℃ can be continuous temperature measurement; Thermocouple performance reliable, good mechanical strength. Long service life, convenient installation.

A couple must be composed of two different types of conductor (or semiconductor) data that fit a request. There must be a temperature difference between the measuring end and the reference end of the thermocouple.

A and B, conductors or semiconductors of two different materials, are welded together to form A closed circuit. When there is A temperature difference between two persistent points 1 and 2 of conductors A and B, an electromotive force is generated between the two and A large current is formed in the loop. This phenomenon is called thermoelectric effect. Thermocouples use this effect to work.

The main features

1. Simple assembly

Easy to replace;

2. Pressure spring type temperature sensor, good seismic performance;

3. High measurement accuracy;

4. Large measuring range (-200 ° C ~ 1300 ° C, -270 ° C ~ 2800 ° C in special cases);

5. Fast thermal response time;

6. High mechanical strength and good pressure resistance;

7. High temperature resistance up to 2800 degrees;

8. Long service life

The working principle of

Two conductors of different composition (called thermocouple wires or hot electrodes) are connected to a synthetic circuit,

When two junctions have different temperatures, an electromotive force is produced in the circuit. This phenomenon is called thermoelectric effect, and this electromotive force is called thermoelectric potential. Thermocouples make use of this principle to measure the temperature. One end is called the working end (also known as the measuring end) and the other is called the cold end (also known as the compensation end). The cold end is connected to a display meter or matching meter that indicates the thermoelectric potential generated by the thermocouple.

A thermocouple is actually an energy converter, which converts heat energy into electrical energy and uses the thermoelectric potential generated to measure the temperature. As for the thermoelectric potential of a thermocouple, the following questions should be noted:

1. The thermoelectric potential of a thermocouple is the difference of the temperature function at both ends of the working end of the thermocouple, not the function of the difference of the temperature between the cold end and the working end of the thermocouple;

2, thermocouple generated by the size of the thermoelectric potential, when the thermocouple material is uniform, and the length and diameter of the thermocouple has nothing to do with the thermocouple material composition and temperature difference at both ends;

3, when the thermocouple of the two thermocouple wire material composition is determined, the thermocouple thermoelectric potential size, only with the thermocouple temperature difference; If the temperature at the cold end of the thermocouple remains constant, the thermoelectric potential of the incoming thermocouple is only a single-valued function of the temperature at the working end. A and B, conductors or semiconductors of two different materials, are welded together to form A closed loop, as shown in the figure. When there is A temperature difference between two persistent points 1 and 2 of conductors A and B, an electromotive force is generated between them and A large or small current is formed in the circuit. This phenomenon is called thermoelectric effect. Thermocouples take advantage of this effect.

Structure requirements

In order to ensure the reliability of the thermocouple,

To work stably, its structure is required as follows:

1. The welding of the two hot electrodes of the thermocouple must be firm;

2. The two hot electrodes should be well insulated from each other to prevent short circuit;

3. The connection between compensation wire and free end of thermocouple should be convenient and reliable;

4. The protective sleeve shall be able to ensure sufficient isolation between the hot electrode and harmful media.

The main classification

1. Classification by fixed device type

Thermocouple as the main means of temperature measurement, the use is very wide, and there are a variety of requirements for the fixed device and technical performance, so the fixed device of thermocouple is divided into six kinds: no fixed device type, screw type, fixed flange type, movable flange Angle square form, cone protection pipe type six kinds.

2. Classification by assembly and structure

According to the performance of thermocouple structure can be divided into: detachable thermocouple, flameproof thermocouple, armored thermocouple and pressure spring fixed thermocouple and other special purpose thermocouple.

Common species

Common thermocouple can be divided into standard thermocouple and non - standard thermocouple two categories. So-called standard thermocouple is to show the thermocouple that national standard stipulated its thermoelectric potential and temperature relation, allow error, and the thermocouple that has unified standard cent meter, it has the display instrument with its form a complete set to be able to be chosen. Non-standardized thermocouple in the scope of use or order of magnitude are less than the standard thermocouple, generally there is no unified grading table, mainly used for some special occasions of measurement. Standardized thermocouple China from January 1, 1988, thermocouple and thermal resistance are all produced in accordance with IEC international standards, and designated S, B, E, K, R, J, T seven standardized thermocouple as the unified design of China thermocouple.

In theory, any two different conductors (or semiconductors) can be configured into a thermocouple, but as a practical temperature measuring element, the requirements are manifold. In order to ensure the reliability of engineering technology and sufficient measurement accuracy, not all materials can be composed of thermocouple. Generally, the basic requirements for the electrode materials of thermocouple are as follows:

1. Within the temperature measuring range, thermoelectric properties are stable and do not change with time. It has sufficient physical and chemical stability and is not easy to be oxidized or corroded;

2. Low resistance temperature coefficient, high conductivity and low specific heat;

3. The thermoelectric potential generated in the temperature measurement should be large, and the thermoelectric potential and temperature show a linear or nearly linear single-value function relationship;

4. Good material replication, high mechanical strength, simple manufacturing process and low price.

Welding method

Before welding the measuring end of a thermocouple [2], the top of the welded hot electrode should be twisted into a twist shape or the two ends should be aligned. (1) Gas welding method: first twist the top of the hot electrode into a twist shape, then slightly heat and dip in flux, then use acetylene flame to make the measuring end melt into a ball, and then quickly put the measuring end away from the flame into hot water. This method is easy to operate, so it is widely used. (2) Arc welding: Arc welding is the use of high temperature arc to melt the measuring end of a thermocouple into a ball. Common arc welding alternating current and direct current arc welding two kinds. Ac arc welding is generally used to weld base metal thermocouples. Before welding should also dip in the top of the hot electrode with flux, after welding to remove the flux on the spot. Dc arc welding is generally used to weld precious metal thermocouples. (3) Butt welding: align the top of the hot electrode, slightly some pressure, and then switch on the power to make the contact surface melt together. This welding method is more convenient (4), DC argon arc welding: DC argon arc welding is developed in recent years is a more ideal method of welding thermocouple. Usually dc argon arc welding thermocouple welding machine is protected by argon gas, arc initiation with high frequency, DC welding, and can carry out butt welding. It is composed of power supply, DC welding power supply 1, welding gun 2, high-frequency oscillator 3, welding power and fixture 4 and other parts, this welding device is characterized by convenient use, fast welding speed, no contamination, no pores, welding end bright and beautiful, and can be a multi-purpose machine. It can weld a variety of metal materials and different specifications of a variety of thermocouple, thermocouple in the choice of brand to choose a thermocouple jack. (5) Salt bath welding method: The welding device consists of a voltage regulator (3-5KW), a graphite crucible and a welding clamp. The crucible acts as a power source - electrode, and the hot electrode acts as another electrode. Before welding, clean the oxide on the surface of the hot electrode to be welded, twist it into a twist shape, place the carbon rod on the barium chloride, turn on the power to melt the barium chloride, then contact the thermocouple hot electrode with the barium chloride molten solution for arc starting and leave quickly, and wash the barium chloride particles on the measuring end and the hot electrode with hot water.

A comparative study

First, the basic concept of temperature measurement

As one of the seven basic physical quantities in the International system of Units (SI), temperature is a physical quantity that measures the degree of coolness and heat of an object and occupies an extremely important position in production and scientific experiments. From the perspective of energy, temperature is a physical quantity that describes the energy release state between different degrees of freedom of the system. From the point of view of thermal equilibrium, temperature is a physical quantity describing the degree of coolness and heat of the thermal equilibrium system. From the microscopic point of view, temperature indicates the intensity of irregular motion of molecules in the system. Objects with high temperature have high average kinetic energy, while objects with low temperature have low average kinetic energy.

In the early days, people started from the sense of human organs, and they could distinguish the high and low temperature by the degree of cold and hot they felt or touched. The results are often unreliable and inaccurate. The research shows that the properties of almost all substances are related to temperature, such as size, volume, density, hardness, elastic transverse magnitude, destructive strength, electrical conductivity, magnetic conductivity, and optical radiation intensity, etc., and the temperature can be measured by using these properties and their variation with temperature. In other words, temperature can only be measured indirectly by certain properties of an object that vary with temperature, and the ruler used to measure the temperature of an object is called a temperature scale. It specifies the starting point of temperature reading (zero) and the basic unit of temperature measurement. At present, Fahrenheit, Celsius, thermodynamics and practical temperature scales are widely used in the world.

Introduction of main temperature measurement methods

There are two types of temperature measurement: contact type and non-contact type.

1. The method of temperature measurement, in which the sensor is placed in the same thermal balance state as the object and the sensor maintains the same temperature with the object, is called contact temperature measurement method. For example, mercury thermometers, pressure thermometers and bimetal thermometers that use the principle of thermal expansion of the medium. In addition, the characteristics of the electrical parameters of the object with the change of temperature are used to detect the temperature. Examples include thermal resistors, thermistors, electronic temperature sensors, and thermocouples.

Contact thermometers are simple, reliable and have high measuring accuracy. However, due to the sufficient heat exchange between the temperature measuring element and the measured medium, it needs a certain time to reach the heat balance, so there is a delay in temperature measurement, and at the same time, due to the limitation of high temperature resistant materials, it cannot be used in very high temperature measurement.

2. Non-contact instrument temperature measurement is based on the principle of thermal radiation to measure the temperature, the temperature measuring element does not need to contact with the measured medium. This method can be used to measure the temperature by the relationship between the surface thermal radiation intensity and the temperature. There are total radiation method, partial radiation method, brightness method of single wavelength radiation power and colorimetric method to compare the radiation power of two wavelengths. The non-contact instrument has a wide range of temperature measurement, is not limited by the upper limit of temperature measurement, and will not destroy the temperature field of the measured object, and the reaction speed is generally relatively fast; However, due to the influence of external factors such as emissivity of the object, measuring distance, smoke, water and gas, the measurement error is relatively large.

Iii. Standard requirements

Virtually all CSA and UL electrical product standards require temperature rise testing and specify test conditions such as product input power, load requirements, and test environment. Test methods, such as installation location and use of temperature measurement methods; Test time; Judgment criteria, maximum temperature rise and incidental tests. In the test method, the temperature measurement method will be specified, usually the thermocouple requirements are 30AWG(0.51 square centimeter), iron - constantan (grading number J) or copper - constantan (grading number T) and the matching recording instrument.

Loading requirements

Attention should be paid to the thermocouple installation for accurate temperature measurement.

Safe and easy to test and maintain, and does not affect the operation of equipment and production operations. To meet the above requirements, the following points should be paid attention to when choosing the installation position and insertion depth of the thermocouple:

1. In order to make sufficient heat exchange between the measuring end of the thermocouple and the measured medium, the location of the measuring point should be reasonably selected. As far as possible, no thermocouple or thermal resistance should be installed near the dead corner of the valve, elbow, pipe and equipment.

2. A thermocouple with a protective sleeve has heat transfer and heat dissipation losses. In order to reduce measurement errors, the thermocouple should have sufficient insertion depth:

(1) For the thermocouple measuring the fluid temperature in the center of the pipeline,

Generally, the measuring end should be inserted into the center of the pipe (vertical installation or inclined installation). If the pipe diameter of the measured flow body is 200mm, the depth of thermocouple insertion should be 100mm.

(2) for high temperature and high pressure and high velocity of temperature measurement (such as main steam temperature), in order to reduce cases of fluid resistance and prevent cases under the effect of fluid fracturing, the way protecting tube shallow inserted or use hot thermocouple, shallow inserted type thermocouple protection tube, its depth into the steam pipe should be no less than 75 mm; The standard insertion depth of hot-sleeve thermocouple is 100mm;

(3) If the flue gas temperature needs to be measured, although the flue diameter is 4m, the thermocouple insertion depth of 1 m can be;

(4) When measuring elements are inserted at a depth of more than 1m, they should be installed as vertically as possible, or with support frames and protective sleeves.

The neglected problem of folding

I. Impact of response time

The basic principle of the contact method is that the temperature measuring element should reach a thermal balance with the measured object. Therefore, the temperature needs to be kept for a certain time in order to achieve thermal balance between the two. The length of holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the structure and measurement conditions of the sensor, which vary greatly. Therefore, appropriate heating rate and time of heat balance should be selected according to different types of thermocouples in the daily verification process.

Ii. Influence of insulation resistance

Wear-resisting thermocouple [4] at high temperature, its insulation resistance decreases sharply with the increase of temperature, so leakage current will be generated. This current will flow into the instrument through the insulation with reduced insulation resistance, which will make the instrument unstable or generate measurement error. Therefore, do not ignore the insulation resistance of the thermocouple before installing the furnace, only when it meets the requirements of verification regulations, can the temperature tolerance test.

The length of the thermocouple

The length of the thermocouple is not less than 750mm, which is clearly stipulated in the verification regulation of 'Low metal Thermocouple for Work'. The reason why the length of the thermocouple is stipulated is that the thermocouple should have a wide enough temperature gradient after leaving the temperature measuring area. The thermoelectric EMF of the thermocouple is also generated in this area, and the most basic way to effectively prevent the heat from the thermocouple's hot end (measuring end) to the cold end (wiring end) is to have enough distance from the cold end of the thermocouple. Generally speaking, the error caused by insufficient thermocouple length is negative, and the correction is positive. The shorter the length is, the greater the error will be. Therefore, the length of the thermocouple needs to be determined before the furnace verification.

Four, thermocouple wire bending

Thermocouple wire is thin and soft, easy to deformation, when the couple wire folding, distortion and other plastic deformation of the hot electrode wire stress, it will change the thermoelectric characteristics of the thermocouple, so that the accuracy of deformation thermocouple measurement results affected. For this reason, the thermocouple must be straightened before verification.

Three, thermocouple wire is contaminated

Thermocouple wire is polluted, or even be oxidized, will make the surface of the hot electrode wire is not bright, dark black, then the thermoelectric characteristics of the hot electrode is very unstable, the accuracy of the measurement data is poor, therefore, to clean the contaminated electrode, eliminate the pollution layer.

Matters needing attention

A. The thermocouple [10] shall be installed as close as possible to the temperature control point to be measured. To prevent heat from passing along the thermocouple or from affecting the measured temperature in the protective tube, the thermocouple shall be immersed in the measured flow at a depth of at least 10 times the diameter. When measuring the temperature of the solid, the thermocouple should be in close contact with or against the material. In order to minimize the thermal conductivity error, the temperature gradient near the contact should be reduced.

B. The temperature measurement point should be representative. For example, when measuring the fluid temperature in the pipeline, the measuring end of the thermocouple should be at the maximum flow velocity in the pipeline. In general, the protective sleeve end of the thermocouple should cross the velocity center line.

C. When the temperature of the gas in the pipe is measured by a thermocouple, if the wall temperature of the pipe is obviously higher or lower, the thermocouple will radiate or absorb heat to it, thus significantly changing the measured temperature. At this point, a radiation shield can be used to keep the temperature close to the gas temperature, using the so-called shield thermocouple.

D. Pay special attention to the use of compensation wires in actual use. Usually connected to the compensation wire between the instrument and the junction box, its thermoelectric properties and the use of thermocouple or similar, and the thermocouple connected after will not produce a large heating potential, will not affect the thermoelectric circuit of the total thermoelectric potential. If a compensating wire is replaced by a normal wire, it will not compensate and thus reduce the accuracy of temperature measurement. Therefore, the use of units in the installation of the instrument line should pay attention to: compensation wire and thermocouple connection, polarity do not connect the opposite, otherwise the temperature error will increase.

Q&A

1: The thermoelectric potential of a thermocouple is the difference between the temperature function at both ends of the thermocouple's working end and the cold end, rather than the function of the temperature difference at both ends of the thermocouple's cold end and the working end; [5]

2: thermocouple generated by the size of the thermoelectric potential, when the thermocouple material is uniform, and the length and diameter of the thermocouple is independent.

3: when the thermocouple of the two thermocouple wire material composition is determined, the thermocouple thermoelectric potential size, only with the thermocouple temperature difference; If the temperature at the cold end of the thermocouple remains constant, the thermoelectric potential of the incoming thermocouple is only a single-valued function of the temperature at the working end. A and B, conductors or semiconductors of two different materials, are welded together to form A closed loop, as shown in the figure. When there is A temperature difference between two persistent points 1 and 2 of conductors A and B, an electromotive force is generated between them and A large or small current is formed in the circuit. This phenomenon is called thermoelectric effect. Thermocouples take advantage of this effect.

The correct use of

Correct use of thermocouple can not only accurately get the temperature value, to ensure that the product qualified,

But also can save the thermocouple material consumption, both save money and can ensure the quality of products. Incorrect installation, thermal conductivity and time lag are the main errors in the use of thermocouple.

1. Error caused by improper installation

If the thermocouple installation position and insertion depth cannot reflect the true temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and heating place, inserted depth should be at least 8 ~ 10 times the diameter of the protective tube; The gap between the thermocouple's protective sleeve and the wall is not filled with insulation material, which causes the heat overflow or cold air invasion in the furnace. Therefore, the gap between the thermocouple's protective sleeve and the hole of the wall of the furnace should be blocked by refractory mud or asbestos rope, so as not to affect the accuracy of temperature measurement due to the convection of hot and cold air. The cold end of the thermocouple is too close to the furnace so that the temperature exceeds 100℃; The installation of thermocouple should avoid strong magnetic field and strong electric field as far as possible, so the thermocouple and power cable should not be installed in the same conduit to avoid interference and error; The thermocouple cannot be installed in the area where the measured medium has little flow. When measuring the temperature of the gas in the tube with the thermocouple, the thermocouple must be installed against the direction of flow velocity and in full contact with the gas.

2. Error caused by poor insulation

If thermocouple insulation, protection tube and wire plate dirt or salt slag too much insulation between thermocouple electrode and furnace wall bad, in the high temperature is more serious, this will not only cause the loss of thermoelectric potential but also introduce interference, the resulting error sometimes up to a hundred.

3. Error introduced by thermal inertia

Because the thermocouple's thermal inertia makes the indicating value of the instrument lag behind the change of the measured temperature,

This effect is particularly pronounced when rapid measurements are taken. So should use the thermocouple with thinner hot electrode and smaller protective tube diameter as far as possible. The protective tube may even be removed when the temperature measuring environment permits. The amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation because of the measurement lag. The larger the measurement delay is, the smaller the amplitude of thermocouple fluctuation is, and the larger the difference is from the actual furnace temperature. When the thermocouple with a large time constant is used to measure or control the temperature, the temperature displayed by the instrument fluctuates very little, but the actual furnace temperature may fluctuate greatly. For accurate temperature measurement, a thermocouple with a low time constant should be selected. The time constant is inversely proportional to the heat transfer coefficient, which is directly proportional to the diameter, material density and specific heat of the thermal end of the thermocouple. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to reduce the size of the thermal end as far as possible. In use, protective sleeves with good thermal conductivity, thin pipe wall and small inside diameter are usually adopted. In more precise temperature measurement, use bare wire thermocouple without protective sleeve, but the thermocouple is easy to be damaged, it should be corrected and replaced in time.

4. Thermal resistance error

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

Fold edit this section of wire and accessories

Large order of bare or insulated thermocouple wire, the cost is very large. One way to economise (especially in the case of precious metal thermocouples) is to use extension wires. Choose pairs of alloy conductors as these extension conductors, which have a net thermoelectric potential very similar to that of the thermocouple itself over a range of temperatures. Such extension wires shall not be used in the high temperature section of the measuring line. The extended conductor may be connected to the line between the medium temperature region and the reference point. The alloy for the extension wire is the leuco, which is cheaper than the high temperature thermocouple material. This becomes especially important when long wires are needed to lead to the reference point.

Choose an extension wire alloy that matches the thermoelectric properties of the thermoelectric couple in a moderate temperature range of approximately 0℃ to a maximum of a few hundred ℃. The upper limit depends on the reactivity of the extension wire and the ambient atmosphere, the type of insulation selected, and the temperature at which the extension wire characteristics begin to deviate significantly from the thermocouple characteristics. Extension wires are needed for some low metal thermocouples as well as precious metal thermocouples. The extended wire of a low metal thermocouple is usually a section of wire of the same nominal composition as the thermocouple, but this section of wire does not meet the required high temperature output technical conditions.

Joints, terminals and other metal fittings manufactured from common thermocouple materials or alloys of similar properties are useful. The combination of thermocouple wires, extension wires, and corresponding accessories may result in an approximately uniform line from the thermal contact to the reference point. Copper wires are usually used on the measuring side of the reference point, and an alloy solder with a thermocouple potential similar to that of copper is available to minimize locally produced thermoelectric potential. [6]

How to wire this section

If the thermocouple [7] wiring is correct, the display temperature will be correct; if the wiring is not correct, it will have an adverse effect. During the temperature measurement process of the material, the thermocouple should accurately measure the temperature, so as to effectively control the whole process. Especially for special occasions, the accuracy of the measured temperature is very high. Temperature measurement in these fields is usually achieved by thermocouple sensors. Thermocouple itself has the advantages of economy and small measurement error.

Thermocouple wiring has positive and negative points, through: in the wiring board above the calibration: +, -, but also the manufacturer has no positive and negative points, so the need for technical personnel to judge by experience

One, is to use color to distinguish, green for the positive, gray for the negative

Two, use a multimeter to measure

Three, look at the temperature trend after wiring

Four, if the magnet can also be determined by the magnet

Five, compensation wire also needs to be connected correctly, there are also positive and negative points

Six, connect the instrument, also should pay attention to the positive and negative points, terminal above are marked

Universal terminals cannot be used for the connection of thermocouples because the guide bars within the terminals are usually made of a special copper material such as electrolytic copper. Unlike the copper material which makes up the thermocouple, and unlike the general combination terminals which are supplied in sheets, thermocouple terminals are provided in pairs. The guide bars for each pair of thermocouple terminals are made of a different metal material. For different types of thermocouple, there are corresponding different terminal. The conductive materials used in the thermocouple terminals shall be exactly the same as the conductive materials used in the thermocouple to ensure the accuracy of the thermocouple signals during transmission. Industrial fabricated thermocouples are often used as temperature transmitters to measure temperatures in conjunction with display meters, recording meters and electronic regulators. It can directly measure the surface temperature of liquids, vapors and gaseous media and solids in a variety of production processes from 0℃ to 1800℃.

According to national regulations, our company produces platinum-platinum-rhodium 30-platinum-rhodium 6, platinum-10-platinum, nickel-chrome-nickel silicon, nickel-Chrome-constantan and so on prefabricated thermocouples in line with IEC international standards.

The assembly of a thermocouple is usually composed of a temperature sensing element, a mounting fixture and a junction box.

Optional model

B, S, K, E

Fold edit the lifetime of this section

Thermocouple is not an endless use of products, it has a certain life span. The lifetime of a thermocouple is closely related to a number of factors that are not easily quantified. Even so, we should pay attention to the use time consciously or unconsciously, so, how to determine the use time of thermocouple?

Only the stability of thermocouple is required in our standard. That is, under a certain temperature through 200h, the use of thermoelectric EMF before and after the change. However, no provision for service life has been found. The Japanese requirements on the service life of thermocouples are based on the continuous service time of thermocouples specified in the Japanese JIS (C-1602-1995) standard. For type B, R and S thermocouple, it is 2000h, and for type K, E, J and T thermocouple, it is 10000h.

In practice, fabricated thermocouples are usually equipped with protective tubes, and only bare wire is used in special cases. Therefore, in most cases, the lifetime of the protective tube determines the lifetime of the thermocouple. The actual service life of a thermocouple must be judged through long-term collection and accumulation of data in the actual use state, so that a more accurate result can be obtained.

Since the armored thermocouple is protected from the outside environment by the casing, the casing material has a great impact on the life of the armored thermocouple, so the thermocouple wire and metal casing must be selected according to the use. When the material is selected, the service life increases with the increase of armored thermocouple diameter. Compared with assembled thermocouple, armored thermocouple has many advantages, but its service life is often lower than that of assembled thermocouple.

When choosing to buy a thermocouple, in addition to considering the performance and price, its life span is also a factor that has to be considered, only every detail is considerate, in order to choose to meet the standards and cost-effective products. In the use of thermocouples, should also pay attention to the use of time, rather than blindly use but do not care about, in order to avoid causing losses.

Anoxic protection principle

The working principle of the thermocouple [8] hypoxia protection device is: when the gas supply and oxygen supply are certain, the flame combustion temperature is also certain. When the oxygen content in the air is sufficient, the flame temperature can reach about 700 ℃. As the amount of oxygen in the air decreases, so does the flame temperature. According to the American standard, the flame temperature in anoxic state does not exceed 450 ℃, and the thermoelectric potential of Type K thermocouple is 1815mV at the relative temperature of 450 ℃. If the thermocouple measured the temperature of the flame is not enough, it can be determined that the oxygen in the air is not enough, you need to alarm and close the control valve. The hypoxia protection function of this scheme is realized as follows: AT89C2051 is adopted in the wide temperature single-chip microcomputer, and it has an analog comparator inside. P110 and P111 are the input pins of the comparator. The 1815mV potential signal is connected to the P111 pin. The voltage signal at the reference end is required to be stable, so a voltage regulator is connected at the VCC. The flame temperature signal measured by the thermocouple is amplified and connected to the P110 pin, and then the two electrical signals are compared with the internal analog comparator. If P110 pin signal is lower than P111 pin signal, then indoor hypoxia is indicated. At this moment, the control valve is closed and the alarm is displayed.

Thermocouple hypoxia protection device mainly consists of single chip microcomputer, thermocouple, explosion-proof thermocouple, potentiometer. The single chip microcomputer AT89C2051 is an efficient single chip microcomputer introduced by American ATMEL Company. Its internal analog comparator, P110 and P111 are the input pins of the comparator. The hypoxia protection function can be realized by using this analog comparator. The instruction of AT89C2051 is fully compatible with McS-51 series microcontroller of INTEL Company, with 2K byte flash programmable erasable READ only memory. The design of circuit can be greatly simplified by using this chip, and other functions of heater can be realized at the same time by using this chip, which is not repeated here. The thermocouple adopts K-type thermocouple, and the thermopotential signal received by it is amplified and processed by the amplifier and then connected to the single chip microcomputer. The thermocouple also plays the role of collecting the temperature signal of constant open flame in the heater. The potential signal of the potentiometer is 1815mV, and its reference end is connected to a voltage stabilizing source 358B-25.

Related introduction

In the production practice, the working end (measuring end) and the cold end (reference end) of the thermocouple are very close to each other, and the cold end is exposed to the working environment, so it is easy to be affected by the temperature fluctuation of the surrounding working environment. Therefore, the temperature of the cold end is difficult to be kept constant, resulting in inaccurate measurement. The practical application is to extend the cold end of a thermocouple to a place where the temperature is lower and more stable with a special compensation wire.

Thermocouple temperature measurement accuracy and thermocouple insertion depth of the object to be tested with a certain relationship, why do you say so we can according to the concrete temperature measuring process to explain, when the thermocouple temperature measurement is along the length direction of the sensor and generate heat flux, the environment if it is in this link below, the object to be tested at this moment along the length direction sensor on the thermal potential there will be a loss, then can cause the thermocouple and and material object to be tested on the temperature there is a temperature difference, caused by temperature is not allowed.

This error is generated in the process of heat conduction in the direction of the sensor. The direct reason for this error is that the depth of the thermocouple inserted into the measured medium is directly related to the material of the protection tube. Next, let's look at the cause of thermocouple temperature error caused by the material of protective casing. The reason is whether the thermal conductivity of the protective casing material of the thermocouple is good or not. Metal protection tube because of its thermal conductivity is good, its insertion depth should be a few deep (about 15-20 times of the diameter), ceramic material insulation performance is good, can be inserted shallow (about 10-15 times of the diameter).

When selecting a thermocouple, it is necessary to pay attention to the use environment, measuring range, measuring accuracy, sensitivity and response speed of the thermocouple, as well as the structure, material, compressive strength and insertion depth of the thermocouple protection casing. For the measuring points with higher precision, response speed and sensitivity, the valuable S-type thermocouple must be selected. For the high temperature, response speed and sensitivity of the process measurement point is not very high requirements, choose B type thermocouple: general process parameters measurement, we choose economical and practical Type K or E thermocouple.

Temperature measurement principle

Thermocouple [9] is a kind of temperature sensing element, which can convert temperature signal into thermoelectric potential signal, and through the cooperation of electrical measuring instrument, the measured temperature can be measured.

The basic principle of thermocouple temperature measurement is thermoelectric effect. In A closed loop consisting of conductors A and B of two different materials, A thermoelectric potential is generated in the loop when two contacts of A and B are at different temperatures T and To. This is known as the Seebeck effect.

Conductors A and B are called hot electrodes. The hotter end (T > is called the working end (usually welded together); The lower end (To > is called the free end) is usually at some constant temperature, >.

According to the thermoelectric potential and the temperature function. Thermocouple indexing meter can be made. The indexing table is obtained under the condition of free end temperature To=00C. Different thermocouples have different indexing tables.

When the third metal material is connected in the thermocouple circuit, the thermoelectricity generated by the thermocouple will remain unchanged as long as the temperature of the two contact points of the material is the same, that is, it will not be affected by the third metal access circuit. Therefore, in the thermocouple temperature measurement, access to the measuring instrument, measured thermoelectric potential, you can know the temperature of the measured medium.

In theory, any two conductors can be made into thermocouples, but in practice, not all materials can be made into thermocouples, so the material for the hot electrode must meet the following requirements:

1. Can measure high temperature and apply in a wide range of temperature. After long-term use, the physical, chemical and thermoelectric properties remain stable;

2. Thermocouple materials can generate higher thermoelectric potential under the action of temperature, and the relationship between thermoelectric potential and temperature is better to be linear or approximately linear single-value function;

3, good reproducibility, easy to mass production and exchange, easy to develop a unified grading table;

4. It is required that the resistance temperature coefficient of the material should be small, with high resistivity, good conductivity and small heat capacity;

5. Abundant resources and cheap prices.

6. Good mechanical performance, uniform material;

Fault handling in this section

Fault discrimination method for thermocouple input generation:

After the correct wiring and power on according to the instrument wiring diagram, the instrument first displays the thermocouple dividing number of the instrument.

Then display the measuring range of the instrument, and then the digital tube below the measuring instrument displays the set temperature, and the digital tube above the measuring instrument displays the measured temperature. If the digital tube on the instrument shows the temperature which is not the heating element, but 'OVER', '0000' or '000', indicating that there is a fault at the input part of the instrument, the following tests should be performed:

1) Remove the thermocouple from the input end of the instrument thermocouple, and then short-circuit the input end of the instrument thermocouple with any wire. When the power is switched on, the display value of the digital tube on the instrument is about room temperature, indicating that the internal connection of the thermocouple is open, and the thermocouple of the same type should be replaced. If the above conditions, the instrument in the process of transportation, the input end of the instrument is damaged, to replace the instrument.

2) Remove the thermocouple of the above fault instrument and replace it with the thermocouple connected to the same indexing instrument with normal operation next to it. After power on, when the temperature of the heating element is displayed in the digital tube on the original fault instrument, it indicates that the internal connection of the thermocouple is open and the thermocouple of the same type is replaced. If the condition mentioned above is still the same, it means that the input end of the instrument is damaged in the process of transportation, the instrument should be replaced.

3) Remove the faulty thermocouple from the meter and put it in the measuring ohm (R) *1 gear with a multimeter.

Measure both ends of the thermocouple with two meter sticks of the multimeter. If the resistance value displayed on the multimeter is very large, it means that the internal connection of the thermocouple is open. Replace the thermocouple of the same type. Otherwise, there is a certain resistance value, indicating that there is a problem at the input end of the instrument, the instrument should be replaced.

4) Connect correctly according to the instrument wiring diagram. If the instrument is energized, the digital tube on the instrument will show negative value and other phenomena, indicating that the thermocouple '+' and '-' connected to the instrument are wrong. Just make a fresh switch.

When the instrument is in operation, the temperature displayed in the digital tube of the instrument differs from the actual measured temperature by 40 ° c ~70 ° C. The difference is even greater, indicating that the meter and the thermocouple of the dividing number wrong. According to the corresponding relationship between the thermocouple's temperature, such as degrees B, S, K and E, and the millivolt (MV) value, under the same temperature, the millivolt (MV) value produced is the smallest, S is the smallest, K is the largest, and E is the largest. It can be judged according to this principle.

Temperature compensation

Since thermocouple materials are generally expensive (especially when using precious metals),

In order to save the thermocouple material and reduce the cost, compensation wire is usually used to extend the cold end (free end) of the thermocouple to the temperature stable control room and connect it to the meter terminal. It must be pointed out that the function of thermocouple compensation wire only extends the hot electrode to make the cold end of the thermocouple move to the instrument terminal in the control room, but it cannot eliminate the influence of temperature change of the cold end on the temperature measurement, so it has no compensation effect. Therefore, other correction methods should be adopted to compensate the influence of cold end temperature t0≠0℃ on temperature measurement. When using the thermocouple compensation wire, it is necessary to pay attention to the matching model, the polarity should not be wrong, and the temperature difference between the compensation wire and the thermocouple connection end should not exceed 100℃.

The main advantages

1. High measurement accuracy. Because of direct contact with the measured object, not affected by the medium.

2. Wide measuring range. Thermocouples in common use can be continuously measured from minus 50 degrees Celsius to 1600 degrees Celsius. Some special thermocouples can measure as low as -269 degrees Celsius (e.g., fe-Ni-Cr) and as high as 2800 degrees Celsius (e.g., tungsten and rhenium).

3. Simple structure and easy to use. A thermocouple is usually made of two different kinds of wire, and is not limited by size or beginning. It has a protective sleeve, which makes it very convenient to use.

Selection method

A thermocouple is two different conductors connected together,

The so-called thermoelectric magnetic force (EMF) is generated when the measurement and reference points are at different temperatures. Connection point Purpose Measurement Connection point is the part of the thermocouple connection point at the temperature to be measured. The reference connection point is the part of the thermocouple connection point that remains above a known temperature or that automatically compensates for temperature changes.

In conventional industrial applications, thermocouple components are generally terminated at the joint. However, the reference points are seldom located on the joints, and the appropriate thermocouple extension line is used to connect to the controlled environment with relatively stable temperature. The connection point type the shell type thermocouple connection point is physically connected (welded) to the probe wall, which provides good heat transfer -- that is, heat is transferred externally to the thermocouple connection point through the probe wall. A case thermocouple is recommended for measuring the temperature of static or flowing corrosive gases and liquids, as well as for some high pressure applications. In an insulating thermocouple, the connection point of the thermocouple is separated from the probe wall and surrounded by a soft powder. Insulating thermocouples provide electrical insulation, although they respond more slowly than shell thermocouples. It is recommended to use an insulating thermocouple to measure the corrosive environment. It is ideal to completely electrically insulate the thermocouple from the surrounding environment through a jacket shield. Exposed end thermocouples allow the top of the connection point to penetrate into the surrounding environment. This type provides optimal response time, but is limited to non-corrosive, non-hazardous, and non-pressurized applications. The response time is expressed in terms of the time constant, which is defined as the time required for the sensor to change 63.2% between the initial value and the final value in a controlled environment. The open-end thermocouple has the fastest response speed, and the smaller the probe jacket diameter is, the faster the response speed will be, but the lower the maximum allowable measurement temperature will be. Extension line A thermocouple extension line is a pair of lines having the same temperature and electromagnetic frequency characteristics as the thermocouple to which it is connected. When the connection is appropriate, the extension line transfers the reference connection point from the thermocouple to the other end of the line, which is usually located in the controlled environment.

When selecting a thermocouple, the following factors should be considered:

1. Temperature range to be measured;

2. Required response time;

3. Type of connection point;

4. Resistance to chemical corrosion of thermocouple or sheath materials;

5. Abrasion resistance or vibration resistance;

6. Installation and restriction requirements, etc.

Applications in instruments

Without the thermocouple, the coal laboratory instrument cannot measure and control the temperature. Thermocouple sometimes also appear contact fracture, so the last spare a thermocouple with coal testing equipment matching, in order to prevent thermocouple problems affecting our testing work. Common thermocouple can be divided into standard thermocouple and non - standard thermocouple two categories. The call standard thermocouple refers to the thermocouple that the national standard stipulates its thermoelectric potential and temperature relationship, permissible error, and has a unified standard dividing meter, it has its supporting display instrument available for selection. Non-standardized thermocouple in the scope of use or order of magnitude are less than the standard thermocouple, generally there is no unified grading table, mainly used for some special occasions of measurement. Standardized thermocouple China from January 1, 1988, thermocouple and thermal resistance are all produced in accordance with IEC international standards, and designated S, B, E, K, R, J, T seven standardized thermocouple for the unified design of thermocouple in China. S, R, B belong to noble metal thermocouple, N, K, E, J, T belong to low metal thermocouple. After understanding the thermocouple classification knowledge, we classify all the thermocouples in the coal laboratory instrument. The service temperature of the sulfur meter is 1150 degrees, and the s-type thermocouple is generally used. The operating temperature of the calorimeter is generally 5~40 degrees. It is generally used for PT100 type thermal resistance. Muffle furnace and drying oven use temperature in general 30~1000 degrees, generally with K - type thermocouple. Type S for muffle furnace heated by silicon carbide bar, type E thermocouple or Type PT100 for individual drying oven. Most of the coal testing instruments can not do without heating, so thermocouple is an indispensable part of the coal testing instruments.