In order to ensure that the Mineral insulated thermocouple calibration can meet the relevant requirements of the specification, the main work is to reduce the uncertainty of the thermocouple calibration as much as possible. Precision thermocouple calibration furnace has a wide uniform temperature zone, wide corresponding jack temperature uniform temperature zone and good temperature stability. 1. Issues Issues with the previous calibration furnace is that the furnace body used a horizontal tubular heating furnace structure, while it used a single-section heating wire made of unequal distances for heating. This heating method can effectively ensure that the central area obtains a wide isothermal working area. However, there are serious problems in the actual application process. The worst issue is short distance of the isothermal zone. Therefore, when performing calibration work, not only the heat conduction of the tested thermocouple will affect the actual temperature field, but also the external heat transfer of the test furnace itself, which will lead to certain errors in the results of the thermocouple test. For the above effects, it can be specifically manifested as inconsistent heat exchange conditions between the calibration furnace and each tested thermocouple, and at the same time, there are certain differences in the heat balance points at different times. (1) .Influence of temperature field For the heat exchange status and heat balance point between the furnace and the tested thermocouple, the change of the temperature field in the calibration furnace working area will affect the results. Specifically, the most direct result of the change in the temperature field is the impact on the temperature of the measuring end of the standard thermocouple and the calibrated thermocouple, temperature will also vary with the change of position. The first thing to do when calibrating an assembled thermocouple is to disassemble the protective tube and then place the thermal electrode and insulating tube with the standard thermocouple in the calibration fire for comparison. Through a certain comparison, it can be known that the temperature of the measuring end between the standard thermocouple and the calibrated thermocouple is so small that it is almost negligible, which will not cause very large uncertainty. When the calibration diameter of the calibrated MI thermocouple is very large, the measuring end node of the thermocouple will need a very large distance because it is in the metal protective tube, and then the measuring end of the standard thermocouple will appear, which will cause temperature differences. In the actual calibration process, when MI thermocouple are tied together, the phenomenon of axial heat conduction will occur at this time, and the temperature field at this time will be much worse than the temperature field environment at no-load. At the same time, in the working area of the horizontal furnace itself, there will be radial con...

1. Principle of thermocouple measurement We measure temperature based on thermoelectric effect, which consists of two ends conductors or semiconductors with different compositions forming a loop. When the two ends have temperature gradient, there will be fluent circuit in the loop. At the same time, there will be an electric force between the ends. 2. Reasons of temperature deviation (1) . Unstable thermoelectric characteristics    After using for a long time, affected by the environment, thermocouple will have certain changes, which means there will be a certain deviation between the measured temperature and the real temperature. (2) . Measuring points                     The choice of measuring points is significant, including location and depth. The measuring points should be avoided to be affected by strong sources of electromagnetic field interference and not too close to heating services. The material of sheath also affects the measuring accuracy. Metal sheath has good thermal conductivity, which needs to be inserted deeper to avoid heat loss. (3) . Disturbances during measuring process Thermocouple temperature measurements can be subject to interference from the external environment, such as electromagnetic fields, vibration, or radiation (4) . Deviation due to thermal inertia Due to delay of measuring, the measured temperature fluctuation will be less than real temperature fluctuation. The large measurement hysteresis, the smaller the amplitude of the thermocpuple fluctuation and the greater the difference from the real furnace temperature. When using thermocouple with long thermal response time to measure or control temperature, the temperature of equipment is almost unaffected by the fluctuation, but the real environment temperature will have a serious fluctuation. To get more specific results, we should choose thermocouple with small diameter sheath, thin hot electrode and quick hot response time. If we need fast and accurate measuring results, bare wire thermocouple can be applied. But it can be easily contaminated and destroyed. We need to change it timely. (5) . Contamination & corrosion of hot electrode Thermocouple wire is thin, soft and easily deformed. When producing the wire, temperature or other reasons will result in uneven thickness of the wire or serious coiling. In addition, the repeated processing for the thermocouple will make thermocouple distorted or lose homogeneity. Therefore, wire should be pulled straight before measuring. When it’s contaminated or oxidized, the surface will turn black and dark. The greater the degree of contamination on the surface, the greater change of hot electrode characteristics and the greater the deviation. (6) . Deviation caused by improper compensation Thermocouple temperature measurement principle indicates that when the temperature of cold ...

Thermocouple verification devices are generally developing in the direction of automation, and the verification efficiency continues to improve. Automatic constant temperature, automatic recording and generation of verification records and verification certificates that meet the requirements of the national metrology verification regulations have become very attractive and important. Combined with the advantages of fast and accurate computer data processing, the application of the intelligent thermocouple calibration device integrated with the microcomputer system greatly reduces the labor intensity of the calibration personnel and improves the reliability of data processing at the same time. 1. Progress in Theory and Technology 1.1. Multi-zone heating and temperature control technology By transforming the temperature control method of the tube resistance furnace, the three-stage temperature control method reduces the measurement uncertainty during calibration. There is a physical partition heating scheme, the heating is divided into three sections and controlled independently, which solves the problem of excessive peak current, obtains a longer central uniform temperature zone, and realizes multiple working modes (long furnace, short furnace and Annealing furnace). There is also a time-sharing patented technology, which divides the heating process into 3 time stages, and solves the problem of excessive peak current by re-series configuration of heating wires in time-sharing heating. 1.2. Short Length Thermocouple The current technical specification for temperature field test of thermocouple verification furnace is JJF1184-2007 "Technical Specification for Temperature Field Test of Thermocouple Calibration Furnace". The test of the temperature field of the horizontal tube type verification furnace provides the basis, and also provides the standard for the test of the vertical tube type verification furnace and annealing furnace. The specification clearly distinguishes the verification furnace and the short furnace. Traditional measurement equipment and measurement methods are often unable to accurately measure the indication deviation of short thermocouple in high temperature areas. 1.3. Control and Automation Using the new thermocouple automatic detection mode combined with computer technology, automatic control technology and digital technology, the overall framework of multi-parameter data acquisition, data correction, data storage and report judgment is given, and the thermocouple automatic verification system is further developed. The system runs stably and is easy to operate, which can meet the testing requirements of thermocouple measuring instruments. 1.4. Furnace Technology A. Enhanced heat radiation technology: In order to reduce energy consumption and reduce atmospheric pollution, the heating furnace has been enhanced with heat radiation technology using black body technology. In this work, the furnace lin...

NiCrSi-NiCrMg thermocouple is a type of nickel-based alloy temperature measurement material, and it is also a major achievement in the research of base metal thermocouple alloy materials in the world. 1. Advantages The positive of N-type thermocouple is a nickel-chromium-silicon alloy containing chromium and silicon, and the negative is a nickel-based alloy containing silicon. The main features are as follows: (1) High temperature oxidation resistance, good long-term stability In view of the problems of uneven alloy composition and thermal potential drift caused by the preferential oxidation of Cr and Si elements in the nickel-chromium of K-type thermocouple, the content of Si and Cr in the positive of N-type thermocouple is increased, so that the oxidation mode of the nickel-chromium alloy is changed fromthe internal oxidation is transformed into external oxidation, so that the oxidation reaction is only carried out on the surface, and the solute elements Mg and Si are added to the negative electrode, so that the passivation film between the metal and the oxide is denser. And because it forms a diffusion barrier with preferential oxidation, it prevents the "edge erosion" phenomenon from diffusing to the inside and inhibits further oxidation from occurring. In addition, Mn is no longer added to the N-type, and its high-temperature stability and service life are significantly improved compared with the K-type. (2) Good thermal cycle stability When K-type thermocouple is used cyclically in the range of 250-550°C, because of the change of its microstructure, it forms a short-range ordered structure, which makes its thermoelectric potential unstable, while N-type thermocouple can eliminate this short-term instability. In the Ni-Cr binary alloy, the Cr content ranges from 5% to 30%, and there is an order-to-disorder transition of the atomic lattice structure. However, within this composition range, there is a very small area, when the Cr content is about 14% to 16%. A nickel-chromium alloy with a Cr content of 14.2% will not be affected by the transition from order-to-disorder in the structure, causing a large change in the thermoelectric potential value. During the heating and cooling process of N-type thermocouple in the range of 250-550°C, the change of the thermoelectric potential does not exceed 10uV. (3) Regular change of thermoelectric potential N-type thermocouple suppresses the magnetic transition in the range of 250-550°C, and there is no obvious irregular change of thermoelectric potential. The negative pole of K-type thermocouple undergoes a magnetic transition at about 170°C. The negative electrode alloy composition of N-type thermocouple basically does not contain Mn, Al, Co and other elements, but the Si content is greatly increased, thereby inhibiting the magnetic transition of the new alloy and reducing its magnetic transition temperature below room temperature. The electric ...

Noble metal thermocouple has the advantages of high temperature measurement accuracy, good stability, wide temperature measurement range, and long life. They are widely used in iron and steel, metallurgy, petrochemical, glass fiber, electronics, aviation and aerospace and other fields. However, the strength of Noble metal thermocouple decreases in high temperature environments, and they are sensitive to environmental pollution. It is difficult to adapt to complex environments where bending and short thermal response time are required in narrow spaces. Noble metal Mineral insulated thermocouple is a new type of temperature measuring material developed on the basis of Noble metal assembly thermocouple. The magnesia or other insulating materials, and the pair wire and insulating material are assembled in a precious metal alloy tube and drawn repeatedly. And annealing treatment, processed into a slender non-detachable solid body, which has the advantages of vibration resistance, high pressure resistance, medium chemical corrosion resistance, bending ability, short response time and durability. 1. Structure &Advantages (1) . Structure Noble metal MI thermocouple is mainly composed of precious metal casing, insulating material, and even wire material. Usually, magnesia or other insulating materials are filled between the noble metal outer casing and the even wire, so that the even wire is airtight while maintaining high temperature insulation. state, thereby preventing the thermocouple wire from corroding and deteriorating due to air or gas at high temperature. The hot end of the noble metal MI thermocouple can be made into grounded type, insulated type, exposed type, and the type with a block chamber for measuring the temperature of high-speed transient airflow, as shown in Figure 1. Figure 1 A. Grounded type; B. Insulated type; C. Exposed type; D. Blocked type (2) .Advantages A. Corrosion resistance, due to the protection of the metal tube, the noble metal MI thermocouple wire is protected by air tightness, and can be used even in oxidizing and corrosive environments; B. Good stability of thermoelectric, and the thermoelectric drift is small for long-term use; C. The operating temperature is high up to 1500C; D. The end of the MI thermocouple is airtightly sealed, and there is no residual gas inside, and the thermocouple wire is not easy to deteriorate due to chemical reactions; E. High insulation resistance, noble metal MI thermocouple can reach more than 50 MΩ at room temperature; F. Specifications and tube materials can be processed according to requirements, suitable for various environments and durable; G. Easy to bend, good flexibility, easy to install and assemble, and the installation conditions are not harsh; H. Pressure and vibration resistance; I. Short thermal response time; J. Long life, because of the protection of the outer metal tube, the service life of the thermocouple is longer. 2. Sheath...

Among the two categories of contact and non-contact products, contact thermocouple with a large quantity and a wide range have developed rapidly in the development of modern technology. Increased service life and greatly reduced costs by using new materials. 1. Current Situation The protection tube of industrial thermocouple has high requirements for prolonging the service life of thermode and improving the accuracy of temperature measurement: air tightness (small porosity), sufficient mechanical strength, stable physical and chemical properties, good thermal conductivity. The material selected as the high-temperature protection tube should have comprehensive properties of high temperature resistance and thermal shock resistance due to harsh working conditions( high thermal conductivity, small thermal expansion, and good high-temperature mechanical strength), so that it can withstand rapid cooling and rapid heating. In addition to the above-mentioned properties, the protection tube material for the temperature measurement of molten steel is also required to be resistant to corrosion and erosion of molten metal and steel slag.The protection tube material used in strong corrosion environments such as acid, alkali and salt bath, in addition to the above-mentioned properties. , must also be resistant to corrosion by specific media. 1.1. Metal Protective Sheath Material (1) Ti alloy: the use temperature is generally in the low temperature range below 250 °C, especially the corrosion resistance of seawater is good; (2) Copper and copper alloy: this type of material is mainly used in an environment below 300-500 °C without corrosive atmosphere. It has good thermal conductivity and air tightness, and good processing performance. Copper is easy to oxidize, so it is often plated with nickel or chromium on the surface; (3) Low-carbon heat-resistant steel: mainly used in oxidizing environments with temperatures around 600°C, with poor oxidation resistance and general corrosion resistance. For corrosion resistance, surface modification must be carried out, such as nickel plating on the surface; (4) Stainless steel: there are many varieties of stainless steel with a wide range of applications. The temperature ranges from 800 to 1000°C. It has good acid and corrosion resistance and good mechanical strength. But due to its poor high temperature heat strength, its high wear resistance is poor. In order to reduce the cost of the protective tube, the section inserted into the measured medium is usually made of stainless steel or heat-resistant stainless steel, and the rest is made of ordinary carbon steel, and the two are welded together; (5) Nickel-based super alloy: There are many varieties, the maximum service temperature is 1250°C, and the general application is wide.Because of its high nickel content, the cost of the material is also high. This type of steel has better high temperature oxida...

Thermocouple compensating wires refer to a pair of wires with insulated layers that have the same nominal value as the thermoelectromotive force of the matched thermocouple within a certain temperature range (including normal temperature), and use them to connect the thermocouple and the measuring device. To compensate for the error caused by the temperature change at the junction between them and the thermocouple. Compensation wires are divided into extension compensating wire and compensation type compensating wire. There are many types of compensation wires for thermocouple. Under different operating temperatures and some special environmental conditions, it is particularly important to choose compensation wires that match the thermocouple. Understanding the related concepts of compensating wires for thermocouple: main model, structure, technical requirements, selection, use, and dealing with some influencing factors in use, which play a very important role in the accurate measurement of temperature by thermocouple. 1. Concept (1) Extension compensating wire. It refers to the nominal chemical composition of the alloy and the nominal value of the thermal electromotive force are the same as the matching thermocouple wire, which is represented by the letter "X" appended to the thermocouple graduation number, such as "KX"; (2) Compensation type compensating wire. It means that the nominal chemical composition of the alloy is different from that of the matching thermocouple wire, but its thermoelectromotive force value is the same as that of the matching thermocouple at 0-100°C or 0-200°C. The value is the same, it is represented by the letter "C" appended to the thermocouple graduation number, such as "KC". Different alloy wires can be applied to thermocouple with the same graduation number, and are distinguished by additional letters, such as "KCA" and "KCB"; (3) Tolerance (or permissible error). The tolerance for compensating wires for thermocouple is the maximum deviation due to the use of compensating wires in the measurement system. The value is expressed in microvolts (μV), and its tolerance is divided into precision grade and common grade. The tolerances of compensation wires for various thermocouple should meet the requirements in Table 1; (4) Reciprocating resistance. It refers to measuring the resistance value of the positive pole and the negative pole of the 1m-long compensation wire at 20°C, and the sum of the resistance values of the positive and negative poles is called the reciprocating resistance of the compensation wire. The reciprocating resistance of the compensation wires for various thermocouple at 20°C should meet the requirements in Table 2. Graduation number Type Thermoelectric potential & tolerance value Measuring end temperature/℃ 100℃ 200℃ thermoelectric potential Tolerance thermoelectric potential Tolerance Ordinary Precision Ordinary Pr...

1. The basic principle of thermocouple temperature measurement Figure 1 : Seeback Effect Figure 1 2. Classification& Feature Criteria for thermocouple selection include cost, maximum and minimum operating temperatures, chemical stability, material compatibility, atmospheric protection, mechanical limitations, exposure time limits, sensor lifetime, sensitivity, and outputEMF. 2.1. Cu-CuNi (Constantan) Alloy Thermocouple (Type T) T-type thermocouple is used at low temperatures. The temperature measurement range is -250~ 350°C. Copper will oxidize rapidly when it is higher than 400 °C. Therefore,we must avoid problems caused by copper electrodes with high thermal conductivity. Since one lead of this thermocouple is copper, no special compensating cable is required. T-type thermocouple has good sensitivity, high stability, fast response speed, and low price. The pair wire has good uniformity and can be made very slender which be bent arbitrarily according to the actual needs of measurement under complex conditions. It has high mechanical strength, good pressure resistance and good applicability in industrial measurement. 2.2. Fe-CuNi Alloy Thermocouple (Type J) J-type thermocouple is popular forit’s high Seebeck coefficient and low price. The maximum temperature of continuous operation in oxidizing gas can reach 800 °C, and it can be well applied in reducing gas at 0-550°C, but it will degrade rapidly whenthe temperature is higher than 550°C. It is mostly used in oil refining and chemical industries that are resistant to hydrogen and carbon monoxide gas corrosion. 2.3. NiCr - CuNi (Constantan) Alloy Thermocouple (Type E) E-type thermocouple provides high output at -250 ~ 900 °C. It combines the advantages of K-type and T-type, and has the advantages of large Seebeck coefficient, good sensitivity, low thermal conductivity, and strong corrosion resistance. 2.4. NiCr-NiSi alloy thermocouple (Type K) K-type thermocouple is the most commonly used thermocouple and is designed for use in oxidizing environments. The maximum continuous use temperature is 1100 °C, although there is drift and loss of accuracy due to oxidation above 800 °C. K-type thermocouple has hysteresis instability in the range of 300 to 600 °C, which may lead to varying degrees of error. 2.5. NiCrSi-NiCrMg alloy (TypeN) N-type thermocouple has strong oxidation resistance at high temperatures up to 1300 °C, good long-term stability of thermoelectromotive force and good reproducibility of short-term thermal cycles. It has good resistance to nuclear radiation and low temperature. Due to the improved linear response stability and the conversion algorithm between thermal electromotive force and temperature, the K-type instability is effectively solved.N-type has a slightly lower voltage-temperature curve than K-type. 2.6. Platinum Rhodium 30 - Plat...