Thermal resistance is the most commonly used temperature in the middle and low temperature zonedetector。Thermal resistance temperature measurement is based on metalconductorThe resistance oftemperatureThe increase of this characteristic is used for temperature measurement.Its main characteristics are high measurement accuracy and stable performance.amongPlatinum thermistorMeasurement ofaccuracyIt is the highest. It is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.Thermistors are mostly made of pure metal materials, most of which are platinum and copper. In addition, they have begun to usenickel、manganeseAnd rhodium and other materials.There are many kinds of temperature sensing materials commonly used for metal thermistors, the most commonly used is platinum wire.In addition to platinum wire, there are copper, nickel, iron, iron nickel and other metal thermal resistance materials for industrial measurement.
1. The pressure spring type temperature sensor has good anti vibration performance;
2. High temperature measurement accuracy;
3. High mechanical strength, good resistance to high temperature and pressure;
4. ImportFilm resistanceElement, reliable and stable performance.
working principle
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Thermal resistance (Figure 2)
The temperature measurement principle of thermal resistance is based onconductororsemiconductorThe resistance value of varies with the temperature to measure the temperature and temperature related parameters.Thermistors are mostly made of pure metal materials, most of which are platinum and copper, and they have begun to usenickel、manganeseandrhodiumAnd other materials.Thermal resistance usually needs to transmit resistance signal to computer control device or other secondary instruments through leads.
Main types
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Ordinary thermal resistance
It can be seen from the temperature measurement principle of the thermal resistance that the change of the measured temperature is directly measured by the change of the thermal resistance valuewirewayThe change of resistance will affect the temperature measurement.
Armored thermal resistance
Armored thermal resistanceIt is composed of temperature sensing element (resistance body), lead wireInsulating materialThe outer diameter of the solid body composed of stainless steel casing is generally φ 2 - φ 8mm, and the minimum diameter can reach φ mm.Compared with ordinary thermal resistance, it has the following advantages:
1. Small size, no air gap inside,Thermal inertiaThe measurement hysteresis is small;
The end face thermal resistance temperature sensing element is wound with specially treated resistance wire, which is closely attached to thethermometerEnd face.Compared with the general axial thermal resistance, it can reflect the actual temperature of the measured end face more accurately and quickly, and is suitable for measuring the end face temperature of bearing shells and other parts.
Flameproof thermal resistance
Flameproof thermal resistance passes through theJunction box, and make its shell explosivemixed gas The explosion caused by spark or arc is limited in the junction box, and the production site will not lead to explosion.Flameproof thermal resistance can be used in Bla -- B3c area withExplosion hazardous areaTemperature measurement of.
Principle of temperature measurement
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Thermal resistance (Figure 3)
Temperature measurement principle of thermal resistance andThermocoupleThe difference in temperature measurement principle of is that the thermal resistance is based on the thermal effect of the resistance, that is, the resistance of the resistance changes with the temperature.Therefore, the temperature can be measured as long as the resistance change of the temperature sensing thermistor is measured.Mainly metal thermistors and semiconductorsThermistorTwo types.
The resistance value and temperature of the metal thermistor can generally be expressed by the following approximate relationship, that is
Rt=Rt0[1+α（t-t0）]
Where, Rt is the resistance value at temperature t;Rt0 is the resistance value corresponding to the temperature t0 (usually t0=0 ℃);α is the temperature coefficient.
Where, Rt is the resistance value when the temperature is t;A. B is a constant depending on the structure of the semiconductor material.
In comparison, the temperature coefficient of thermistor is larger,normal atmospheric temperatureThe resistance value under is higher (usually above several thousand ohms), but the interchangeability is poor, the nonlinearity is serious, and the temperature measurement range is only about - 50~300 ℃, which is widely used for temperature detection and control of household appliances and automobiles.Metal thermistors are generally suitable for temperature measurement in the range of - 200~500 ℃. They are characterized by accurate measurement, good stability and reliable performance. They are widely used in process control.
From the perspective of the change of resistance with temperature, most metal conductors have this property, but not all can be used as thermal resistance for temperature measurement. The general requirements for metal materials as thermal resistance are: as large and stable as possible, the temperature coefficient and resistivity should be large (reduce the size of the sensor under the same sensitivity)It has stable chemical and physical properties within the temperature range of use, good replication of materials, and the resistance value should have a constant value as the temperature changesFunctional relation(preferably in a linear relationship).
practical application
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Thermal resistance (Figure 4)
The most widely used thermal resistance materials are platinum and copper:rtd High precision, suitable for neutral and oxidizing media, good stability, certain nonlinearity, the higher the temperature, the smaller the resistance change rate;Copper resistance within the temperature measurement rangeInternal resistanceThe value is linear with temperature, and the number of temperature lines is large. It is suitable for non corrosive media, and it is easy to be oxidized if it exceeds 150.The most commonly used ones in China are R0=10 Ω, R0=100 Ω and R0=1000 ΩGraduation They are Pt10, Pt100 and Pt1000 respectively; copper resistance has R0=50 Ω and R0=100 Ω, and their graduation numbers are Cu50 and Cu100.Pt100 and Cu50 are the most widely used.
Wiring mode
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Thermal resistance (Figure 5)
Thermal resistance is a primary element that converts temperature change into resistance value change. Usually, it needs to transmit resistance signal to computer control device or other primary instruments through leads.Industrial thermal resistanceInstalled on the production site, there is a certain distance from the control room, so the lead pair of thermal resistancemeasurement result Will have a greater impact.[1]
There are three main ways to lead the thermal resistor:
Two-wire systemThe method of connecting a lead wire at both ends of the thermal resistance to lead out the resistance signal is called two-wire system: this lead wire method is very simple, but because there must be lead wire resistance r in the connecting lead wire, the size of r is related to the material and length of the lead wire, so this lead wire method is only suitable for occasions with low measurement accuracy
Three wire system: The method of connecting one lead at one end of the root of the thermal resistance and two leads at the other end is called three wire system. This method is usually used together with the bridge, which can better eliminate the impact of lead resistance, and is the most commonly used method in industrial process control.
Four wire system: The way of connecting two wires at the root and two ends of the thermal resistance is called four wire system, in which two leads provide the thermal resistance with constant current I, convert R into voltage signal U, and then lead U toSecondary instrument。It can be seen that this lead way can completely eliminate the influence of lead resistance, and is mainly used for high-precision temperature detection.
Three wire connection method is adopted for thermal resistance.The three wire system is used to eliminate the resistance of the connecting wiremeasurement error。This is because the circuit for measuring thermal resistance is generally an unbalanced bridge.As a bridge arm resistance of the bridge, its connecting wire (from the thermal resistance to the central control room) also becomes a part of the bridge arm resistance, which is unknown and followsambient temperatureChange, resulting in measurement error.The three wire system is adopted. One conductor is connected to the power supply end of the bridge, and the other two are connected to the bridge arm where the thermal resistance is located and its adjacent bridge arm, so that the measurement error caused by the resistance of the conductor line is eliminated.
Installation method
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Installation requirements
Thermal resistance (Figure 6)
For the installation of thermal resistance, attention shall be paid to the advantages of accurate temperature measurement, safety, reliability and convenient maintenance, and it shall not affect the equipment operation and production operation.To meet the above requirements, pay attention to the following points when selecting the installation position and insertion depth of the thermal resistor:
1. In order to ensure sufficient heat exchange between the measuring end of the thermal resistance and the measured medium, the position of the measuring point should be reasonably selected, and the installation of thermal resistance near the dead corner of valves, elbows, pipes and equipment should be avoided as far as possible.
2. Thermal resistance with protective sleeve has heat transfer and heat dissipation losses. In order to reduce measurement error, thermocouple and thermal resistance should have sufficient insertion depth:
1) For the thermal resistance measuring the fluid temperature in the center of the pipeline, its measuring end should generally be inserted into the center of the pipeline (vertical installation or inclined installation).If the pipe diameter of the fluid to be measured is 200mm, the insertion depth of the thermal resistor should be 100mm;
2) For temperature measurement of high-temperature, high-pressure and high-speed fluids (such asMain steamTemperature), in order to reduce the resistance of the protective sleeve to the fluid and prevent the protective sleeve from breaking under the action of the fluid, the shallow insertion of the protective tube or the use of the hot sleeve thermal resistance can be adopted.The shallow inserted thermal resistance protective sleeve shall be inserted into the main steam pipe at least 75mm deep;The standard insertion depth of hot sleeve thermal resistance is 100mm.
3) If it is necessary to measure the temperature of flue gas in the flue, although the diameter of the flue is 4m, the insertion depth of the thermal resistance is 1m.
4) When the insertion depth of the measuring element exceeds 1m, it shall be installed vertically as far as possible, or the support frame and protective sleeve shall be installed.
Installation notes
Thermal resistance (Figure 7)
1. Thermal resistance shall be vertically installed on horizontal or vertical pipes as far as possible, and protective casing shall be provided during installation to facilitate maintenance and replacement.
2. When measuring the temperature in the pipe, the length of the element should be on the center line of the pipe (that is, the insertion depth of the protective pipe should be half of the pipe diameter).
3. When installing the temperature moving coil meter, the opening size should be appropriate and the installation should be beautiful and generous.
4. High temperature resistant cable or high temperature resistant compensation wire shall be used in high temperature area.
5. Different measuring elements shall be selected according to different temperatures.Generally, the thermal resistance is selected when the measurement temperature is less than 400 ℃.
6. The wiring shall be reasonable and beautiful, and the needle indication shall be correct.
Main differences
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Thermal resistance (Figure 8)
ThermocoupleBoth thermal resistance and thermal resistance belong to contact temperature measurement in temperature measurement. Although their functions are the same as measuring the temperature of objects, their principles and characteristics are different.
Thermal resistance (Figure 9)
Thermocouple is the most widely used temperature in temperature measurement. Its main characteristics are wide temperature measurement range, relatively stable performance, simple structure, good dynamic response, and remote transmission of 4-20mA electrical signals for automatic control and centralized control.The temperature measurement principle of thermocouple is based onThermoelectric effect。Connect two different conductors or semiconductors into a closed loop. When the temperature at two contacts is different, heat will be generated in the looppotentialThis phenomenon is called thermoelectric effect, also calledSeebeck effect。The thermoelectric potential generated in the closed circuit is composed of two kinds of potential;Thermoelectric potentialAnd contact potential.Thermoelectric potential refers to the potential generated by different temperatures at both ends of the same conductor. Different conductors have differentelectron densityTherefore, the potential they generate is also different, and the contact potential, as its name implies, means that when two different conductors contact, they produce certain electron diffusion because of their different electron density. When they reach a certain balance, the potential formed by the contact potential depends on the material properties of the two different conductors and the temperature of their contact points.There is a standard specification for thermocouples used internationally. It is internationally stipulated that thermocouples are divided into eight different divisions, namely, B, R, S, K, N, E, J and T. The lowest measurable temperature can be 270 ℃ below zero and the highest can be 1800 ℃. Among them, B, R and S belong to platinum series thermocouples. Because platinum belongs to precious metals, they are also called precious metalsMetal thermocoupleThe rest are called cheap metal thermocouples.There are two types of thermocouple structures, common type andArmorType.Ordinary thermocouple is generally composed of thermal electrode, insulating tube, protective sleeve, junction box and other parts, while armored thermocouple isThermocouple wireIt is a solid combination of insulation material and metal protective sleeve after assembly and stretching.However, the electrical signal of thermocouple needs a special wire to transmit, which we callCompensating conductor。Different thermocouples need different compensation wires, whose main function is to connect with thermocouples and keep the reference end of thermocouples away from the power supply, so as to stabilize the temperature of the reference end.The compensation wire is divided into compensation type and extension type. The chemical composition of the extension wire is the same as that of the compensated thermocouple. However, in practice, the extension wire is not made of the same material as the thermocouple, and is generally replaced by the wire with the same electronic density as the thermocouple.The connection between the compensation lead and the thermocouple is generally clear. The positive pole of the thermocouple is connected to the red line of the compensation lead, whilenegative poleConnect the remaining colors.Most of the materials of general compensation conductors areCopper nickel alloy。
Thermal resistance is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.However, his application is limited due to his temperature measurement range. The temperature measurement principle of thermal resistance is based on the characteristic that the resistance value of conductor or semiconductor changes with temperature.It has many advantages. It can also transmit electrical signals remotely, with high sensitivity, strong stability, good interchangeability and accuracy. However, it requires power supply excitation and cannot measure the temperature change instantaneously.Industrial thermal resistance generally adopts Pt100,Pt10，Cu50，Cu100，Platinum thermistorThe temperature measurement range of is generally - 200-800 ℃, and the copper thermal resistance is - 40-140 ℃.Thermal resistance is the same type as thermocouple, but it does not need compensation wire and is cheaper than thermocouple.
There are many installation forms of platinum thermistor, including fixed thread installation, movable thread installation, and fixedflangeInstallation, movable flange installation, movable pipe joint installation, straight pipe joint installation, etc.
Thermal resistance (Figure 10)
The biggest difference between the selection of thermal resistance and thermocouple is the selection of temperature range. Thermal resistance measures low temperatureTemperature sensorGenerally, the measuring temperature is - 200~800 ℃, and the thermocouple is a temperature sensor for measuring medium and high temperature. Generally, the measuring temperature is 400~1800 ℃. If the measuring temperature is about 200 ℃, the thermal resistance measurement should be selected. If the measuring temperature is 600 ℃, the thermal resistance measurement should be selectedK-type thermocoupleIf the measurement temperature is between 1200 ℃ and 1600 ℃, type S orType B thermocouple。
Compared with thermocouple, thermal resistance has the following characteristics:
1. At the same temperature, the output signal is large and easy to measure.
2. The external power supply must be used for resistance measurement.
3. The size of the temperature sensing part of the thermal resistance is large, and the working end of the thermocouple is a small solder joint, so the reaction speed of the temperature measurement of the thermal resistance is slower than that of the thermal couple;
4. The thermal resistance made of similar materials is not as high as the upper limit of temperature measurement of thermocouple.
Difference between thermocouple and thermal resistance:
Thermal resistance (Figure 11)
First, the nature of the signal, the thermal resistance itself is a resistance, and the change of temperature makes the resistance produce positive or negative resistance value changes;The thermocouple generates the change of induced voltage, which changes with the change of temperatureAlthough they are all contact thermometers, their temperature measurement ranges are different. Thermocouples are used in high temperature environments. For example, the measurement range of Pt Rh 30 -- Pt Rh 6 (Type B) is 300~~1600 degrees, and 1800 degrees can be measured in the short term.Type S test - 20~~1300 (short-term 1600), type K test - 50~~1000, short-term 1200)XK type - 50~600 (800), E type - 40~800 (900), J type, T type, etc.This kind of instrument is generally used for high temperature above 500 degrees, and the output thermoelectric potential is very high in low temperature area. When the electric potential is small, the anti-interference measures and secondary meters are required to be very high, otherwise the measurement is not accurate. In addition, in low temperature area, the relative error caused by the change of cold end temperature and environmental temperature is very prominent, and it is not easy to get full compensation.At this time, the temperature range of thermal resistance is generally 200~500, and even lower temperature can be measured (for example, the low temperature of about 1K can be measured with carbon resistance). The platinum thermal resistance Pt100 is normally used (Pt50, 100 and 50 also represent the resistance value of thermal resistance at 0 ℃. BA1 and BA2 are used in the old graduation number,The resistance value of BA1 is 46 ohms at 0 ℃. Copper resistance is also used in industry. The graduation number is CU50 and CU100, but the temperature measurement range is small, ranging from 50 to 150. In some special occasions, there are indium resistance, manganese resistance, etc.).
Second, on-site judgment during work
There are positive and negative thermocouples and compensation wires. First, ensure that the connection and configuration are correct and in operation.The common ones are short circuit, open circuit, poor contact (which can be judged by a multimeter) and deterioration (which can be identified according to the surface color).During inspection, the thermocouple shall be separated from the secondary meter. Use a tool to short circuit the compensation line on the secondary meter. The meter indicates the room temperature and then short circuit the thermocouple terminal. The meter indicates the ambient temperature where the thermocouple is located (no, the compensation line is faulty). Then use the mv gear of the multimeter to roughly measure the thermoelectric potential of the thermocouple (if it is normal, please check the process).
The short circuit and open circuit of the thermal resistance can be judged with a multimeter. During operation, if the short circuit is suspected, just remove a wire end from the resistance end to see the display instrument. If the maximum value is reached, the thermal resistance will be short circuited back to zero, and the wire will be short circuited. When ensuring normal connection and configuration, the meter value will display low or unstable, the possibility of water ingress into the protective tube will display maximum, and the thermal blocking circuit will display minimum short circuit.
Third, from the material point of view, thermal resistance is a kind of metal material with temperature sensitive changes. Thermocouples are bimetallic materials, which are two different metals. Due to temperature changes, potential differences are generated at both ends of two different metal wires.
Fourth, the temperature range detected by the two sensors is different. The thermal resistance generally detects a temperature range of 0-150 degrees (of course, negative temperature can be detected), and the thermocouple can detect a temperature range of 0-1000 degrees (even higher). Therefore, the former is a low temperature detection, and the latter is a high temperature detection.[1]
measuring method
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Thermal resistance (Figure 12)
The principle of thermal resistance thermometer is to use the characteristic that the resistance of conductor or semiconductor changes with temperature.The main advantages of thermal resistance thermometer are: high measurement accuracy and good reproducibility;Large measuring range, especially in low temperature;It is easy to use in automatic measurement and remote measurement.Similarly, thermal resistance also has defects, and its accuracy is not good in high temperature (more than 850 ℃) measurement;Easy to oxidize and not resistant to corrosion.
The materials used for thermal resistance mainly include platinum, copper, nickel, etc. The main reason for using these materials is that the ratio of their temperature and resistance in the common temperature range is linear. Here we mainly introducePlatinum resistance thermometer。
Platinum is a precious metal with very stable physical and chemical properties, especially strong oxidation resistance. It is easy to purify, has good processability, and can be made into very thin platinum wires. Compared with copper, nickel and other metals, platinum has high resistivity, high reproducibility, and is an ideal thermal resistance material. The disadvantage is that the resistance temperature coefficient is small, and it is fragile when working in reducing media,Prices are also expensive.The purity of platinum is usually expressed by resistance ratio: W (100)=R100/R0
R100 represents the resistance value at 100 ℃;R0 refers to the resistance value at 0 ℃
according toIEC StandardsThe platinum resistor with W (100)=1.3850 and initial resistance value of R0=100 Ω (R0=10 Ω) is used for industryStandard platinum resistanceThe resistance wire of the platinum resistance thermometer with R0=10 Ω is thick, and it is mainly used to measure the temperature above 600 ℃.The resistance and temperature equation of platinum resistance is a piecewise equation:
Rt=R0 [1+At+Bt2+C (t-100 ℃) t3] t indicates that the temperature is - 200 ～ 0 ℃
Rt=R0 (1+At+Bt2) t means 0 ～ 850 ℃
If this equation is solved, the temperature value can be known according to the resistance value, but in actual work, the temperature value can be determined according to the resistance value by referring to the thermal resistance scale.
According to the standard, platinum thermistors are divided into Class A and Class B. The allowable error of Class A temperature measurement is ± (0.15 ℃+0.002 | t |), and the allowable error of Class B temperature measurement is ± (0.3 ℃+0.005 | t |).
The thermal resistance used on site is generally armored thermal resistance, which is composed of thermal resistance body, insulating material and protective tube. The thermal resistance body and protective tube are welded together, and the middle is filled with insulating material, so that the thermal resistance body can be well protected, impact resistant, shock resistant and corrosion resistant.
Three wire platinum thermistor measurement method:
Platinum thermistors have two wire system, three wire system and four wire system. The two wire system has a large error in measurement and is no longer used. Three wire system is generally used in industry and four wire system is generally used in laboratory.This article mainly introduces the wiring of the lower three wire platinum thermistor.Three wire platinum thermistors are connected in parallel with a c terminal at the a terminal of the resistance, so that the resistance leads out to three terminals a, b, and c. In this way, the resistance of the measuring wire itself led in by the b wire can be compensated by the c wire, so that the lead resistance does not change with the temperature and the influence of the lead resistance error introduced will be much reduced.The three wire platinum thermistor has a bridge with variable resistance value in the secondary instrument. According to the different range of the matched platinum thermistor, the platinum thermistor in the bridge of the secondary instrument can be fine tuned for more accurate measurement.
New indexing method of thermal resistance thermometer:
Industrial platinum resistance thermometer is a widely used temperature measuring instrument.For a long time, the calculation method of CVD equation has been widely used in relevant standards or technical specifications at home and abroad to verify and scale.However, the industrial platinum resistance thermometer calibrated by CVD equation has low accuracy, low stability and large uncertainty, so it cannot be used as a transfer standard.
For this reason, most industrial temperature measurement fields or laboratories with low requirements can only use the standard platinum resistance thermometer with high accuracy as the traceability transfer standard. However, due to various conditions in the actual industrial temperature measurement field, the standard platinum resistance thermometer cannot be used, which makes the temperature value transfer and traceability impossible in these places, and the actual measurement and calibration work cannot be carried out.
The feasibility of calibrating and dividing industrial platinum resistance thermometers is discussed, and compared with the calculation results of the temperature resistance relationship given by the commonly used CVD equation.Through the research and analysis of several industrial platinum thermistors of different models and manufacturers in different temperature zones, the experimental results, data curves and measurement errors caused by two different methods of indexing of each thermometer are given.
The experiment proves that the interpolation method of ITS-1990 international temperature scale is feasible for industrial platinum resistance thermometer, and has better accuracy and consistency compared with the calculation method of CVD equation used for industrial platinum resistance calibration graduation.Previously, the national metrology and technology institutions of Italy and Canada conducted the research on the indexing method of industrial platinum resistance by using the interpolation formula of the international temperature scale.
The traditional means to improve the accuracy and stability of industrial resistance temperature measurement focus on component purity, packaging technology and production process;A new idea is proposed from the calculation method, which lays a foundation for the improvement of precision platinum resistance and industrial platinum resistance in the temperature value transmission and traceability system, and can be widely used in the temperature measurement field of industrial platinum resistance.[2]
