Temperature sensor refers to the sensor that can sense temperature and convert it into usable output signal.Temperature sensor YesTemperature measuring instrumentThe core part of the, a wide variety.It can be divided into contact type and non-contact type according to the measurement methodSensor materialandelectronic componentCharacteristics are divided into thermal resistance andThermocoupleTwo types.
The thermometer achieves thermal balance through conduction or convection, so that the indicated value of the thermometer can directly represent the temperature of the measured object.Generally, the measurement accuracy is high.Within a certain temperature measurement range, the thermometer can also measure the temperature distribution inside the object.But for moving objects, small targets or objects with small heat capacity, largemeasurement error, common thermometers areBimetal thermometer、Liquid in glass thermometer、Pressure thermometer、Resistance thermometer, thermistor andThermoelectric coupleEtc.They are widely used in industry, agriculture, commerce and other sectors.In daily life, people often use these thermometers.With the wide application of cryogenic technology in national defense engineering, space technology, metallurgy, electronics, food, medicine, petrochemical and other sectorsSuperconducting technologyTo measure the temperature below 120KLow temperature thermometerDevelopment, such as low temperatureGas thermometer、Steam pressure thermometer、Acoustic thermometer、Paramagnetic salt thermometer、Quantum thermometer、Low temperature thermal resistanceandLow temperature thermocoupleEtc.Low temperature thermometers require temperature sensing elements with small volume, high accuracy, good reproducibility and stability.Utilize porousHigh silicaThe thermal resistance of carburized glass sintered from glass carburizing isLow temperature thermometerIt is a temperature sensing element for measuring the temperature in the range of 1.6~300K.
Contactless
itsSensitive elementNon contact with the measured object, also known as non-contact temperature measurementmeter。This instrument can be used to measure the temperature of moving objects, small targets and objects with small thermal capacity or rapid (transient) temperature changessurface temperature , can also be used to measure the temperature distribution of the temperature field.
Contact temperature sensor
The most commonly used non-contact thermometers are based on the basic law of black body radiation, calledRadiation thermometer。Radiation thermometryIncluding brightness method (seeOptical pyrometer）Radiation method (seeRadiation pyrometer）And colorimetry (seeColorimetric thermometer）。Various radiation temperature measurement methods can only measure the corresponding photometric temperature, radiation temperature or colorimetric temperature.Only the measured temperature of a blackbody (an object that absorbs all radiation but does not reflect light) is the true temperature.If the true temperature of the object is to be measured, the emissivity of the material surface must be corrected.The emissivity of material surface is not only determined by temperature andwavelengthAnd also relates to the surface state, film andMicrostructureSo it is difficult to measure accurately.In automatic production, radiation thermometry is often used to measure or control the surface temperature of some objects, such as steel strip rolling temperature, roll temperature, forging temperature and various molten metals in metallurgySmelting furnaceorcrucibleTemperature in.In these specific cases, it is quite difficult to measure the emissivity of the object surface.For solidssurface temperature For automatic measurement and control, an additional reflector can be used to form a blackbody cavity with the measured surface.The effect of additional radiation can improve the effective radiation and effective emissivity of the measured surface.Pass by using effective emission coefficientmeterThe real temperature of the measured surface can be obtained by correcting the measured temperature accordingly.The most typical additional mirror is a hemispherical mirror.The diffuse radiation of the measured surface near the center of the sphere can be reflected back to the surface by the hemispherical mirror to form additional radiation, thus improving the effective emission coefficientεIs the surface emissivity of the material,ρIs the reflectivity of the reflector.as forGasandliquidFor radiation measurement of real temperature of medium, insertHeat resistant materialTube to a certain depth to form a blackbody cavity.It is calculated thatheat balanceThe effective emission coefficient of the rear cylinder cavity.stayautomatic measurement This value can be used to correct the measured cavity bottom temperature (that is, the medium temperature) in and control to obtain the true temperature of the medium.
Contact temperature sensor
Advantages of non-contact temperature measurement:The upper limit of measurement is not limited by the temperature resistance of the temperature sensing element, so there is no limit to the maximum measurable temperature in principle.For high temperature above 1800 ℃, non-contact temperature measurement method is mainly used.along withInfrared technologyWith the development of, radiation thermometry has gradually expanded from visible light to infrared light, and has been adopted from below 700 ℃ to normal temperature, andresolving powerVery high.
working principle
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Sensor designed by metal expansion principle
The metal will have a corresponding extension after the ambient temperature changes, so the sensor can convert the signal of this reaction in different ways.
Bimetallic sensor
Temperature sensor
Bimetallic sheetIt is composed of two pieces of metal with different expansion coefficient stuck together. With the change of temperature, material A expands more than another metal, causing the bending of the metal sheet.The curved curvature can be converted into an output signal.
Bimetal rod and metal tube sensor
As the temperature rises, the length of the metal tube (material A) increases, but the length of the non expansion steel rod (metal B) does not increase, so the linear expansion of the metal tube can be transmitted due to the change of position.In turn, this linear expansion can be converted into an output signal.
Sensor designed by deformation curve of liquid and gas
When the temperature changes, the volume of liquid and gas will also change accordingly.
Various types of structures can transform this expansion change into a position change, thus generating position change output (potentiometer, inductive deviation, baffle, etc.).
Resistance sensing
The resistance of metal changes with temperature.
For different metals, the change of resistance value is different every degree of temperature change, and the resistance value can be directly used as the output signal.
There are two types of resistance changes
Positive temperature coefficient
Temperature rise=resistance increase
Temperature decrease=resistance decrease
Negative temperature coefficient
Temperature rise=resistance decrease
Thermal resistance
Temperature decrease=resistance increase
Thermocouple sensing
The thermocouple consists of two metal wires of different materials, which are welded together at the end.The temperature of the heating point can be accurately known by measuring the ambient temperature of the unheated part.Since it must have two conductors of different materials, it is called thermocouple.Thermocouples made of different materials are used in different temperature ranges, and their sensitivity is also different.The sensitivity of thermocouple refers to the change of output potential difference when the temperature of heating spot changes by 1 ℃.For most thermocouples supported by metal materials, this value is about 5~40 microvolts/℃.[1]
Thermocouple
Since the sensitivity of the thermocouple temperature sensor has nothing to do with the thickness of the material, very thin materials can also be used to make a temperature sensor.Due to the good ductility of the metal materials used to make thermocouples, this tiny temperature measuring element has a very high response speed and can measure the process of rapid change.
Selection method
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If reliable temperature measurement is required, it is necessary to select the correctTemperature instrument, that is, the temperature sensor.Thermocouple, thermistor, platinum resistance (RTD) and temperature IC are the most commonly used temperature sensors in the test.
The following is an introduction to the characteristics of thermocouple and thermistor temperature instruments.
1. Thermocouple
Contact temperature sensor
Thermocouple is the most commonly used temperature sensor in temperature measurement.Its main advantages are wide temperature range and adaptability to various atmospheric environments, and it is also the cheapest with low price and no power supply.The thermocouple is composed of two different metal wires (metal A and metal B) connected at one end. When one end of the thermocouple is heated,Thermocouple circuitThere is a potential difference in.The measured potential difference can be used to calculate the temperature.
However, there is a nonlinear relationship between voltage and temperature. Since the relationship between voltage and temperature is nonlinear, it is necessary to make a second measurement for the reference temperature (Tref), and use the test equipment software or hardware to process the voltage temperature transformation inside the instrument to finally obtain the thermocouple temperature (Tx).Agilent 34970A and 34980Adata collectorAll have built-in measurement and calculation capability.
In short, thermocouple is the simplest and most common temperature sensor, but it is not suitable for high-precision measurement and application.
2. Thermistor
Temperature sensor (Figure 6)
Thermistors are made of semiconductor materials, mostlyNegative temperature coefficientThat is, the resistance decreases with the increase of temperature.Temperature change will cause large resistance change, so it is the most sensitive temperature sensor.However, the linearity of thermistor is extremely poor, and it has a great relationship with the production process.The manufacturer cannot provide a standardized thermistor curve.
Thermistors are very small in size and respond quickly to temperature changes.However, thermistors need to use a current source, and their small size also makes them extremely sensitive to self heating errors.
The thermistor measures the absolute temperature on the two lines with good accuracy, but it is more expensive than the thermocouple, and the measurable temperature range is also smaller than the thermocouple.A common thermistor has a resistance value of 5k Ω at 25 ℃, and the resistance changes by 200 Ω every time the temperature changes at 1 ℃.Note that the lead resistance of 10 Ω only causes negligible error of 0.05 ℃.It is very suitable for current control applications requiring fast and sensitive temperature measurements.Small size is beneficial for applications with space requirements, but attention must be paid to prevent self heating errors.
Thermistors also have their own measuring skills.The small size of thermistor is an advantage. It can be stabilized quickly without causing thermal load.However, it is also very weak, and high current will cause self heating.As thermistor is a resistive device, any current source will generate heat due to power on it.Power is equal to the product of the square of the current and the resistance.Therefore, use a small current source.If the thermistor is exposed to high heat, it will cause permanent damage.
Through the introduction of the two temperature instruments, I hope it will be helpful to everyone's work and study.
Selection attention
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Temperature sensor (Figure 7)
1. Whether the temperature of the measured object needs to be recorded, alarmed and automatically controlled, and whether it needs to be measured and transmitted remotely;
2. Size and accuracy requirements of temperature measurement range;
3. Whether the size of temperature measuring element is appropriate;
4. When the temperature of the measured object changes with time, whether the hysteresis of the temperature measuring element can meet the temperature measurement requirements;
5. Whether the environmental conditions of the tested object damage the temperature measuring elements;
6. If the price is guaranteed, is it convenient to use.
Calibration device
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Verification regulation of temperature sensor:
Temperature sensor (Figure 8)
1. JJG229-2010 Verification Regulation of Industrial Platinum and Copper Thermal Resistance
2. JJG833-2007 Verification Regulation of Standard Group Platinum Rhodium 10 Platinum Thermocouple
3. JJG141-2000 Verification Regulation of Working Precious Metal Thermocouples
4. JJG351-1996 Verification Regulation of Working Low Metal Thermocouples
5. JJG368-2000 Verification Regulation of Working Copper Copper Nickel Thermocouple
Calibration standard technology and index of temperature sensor:
2. Parasitic potential of scanning switch: ≤ 0.4 μ V;
Temperature sensor (Figure 9)
3. Temperature range: water tank: (room temperature+5~95) ℃ Oil tank: (95~300) ℃ Low temperature thermostatic tank: (- 80~100) ℃ High temperature furnace: (300~1200) ℃;
4. Temperature control stability: better than 0.01 ℃/10min (oil bath, water bath, low-temperature thermostatic bath);0.2 ℃/min (tubular calibration furnace);
5. Total uncertainty: thermocouple verification, measurement uncertainty better than 0.7 ℃, repeatability error<0.25 ℃;The measurement uncertainty of thermal resistance verification is better than 50mk, and the repeatability error is less than 10mk;
6. Verification quantity: simultaneously check (1-8) thermocouples and (1-7) thermal resistors of the same line at one time;
7. Working power supply: AC220V ± 10%, 50Hz, with good protective grounding;
8. High temperature furnace power: about 2KW;
9. Incubation bath power: about 2KW;
10. Microcomputer measurement and control system power:<500.
Functions and features of temperature sensor calibration device:
1. Calibrate K, E, J, N, B, S, R, T and other types of working thermocouples;
Temperature sensor (Figure 10)
2. Calibrate Pt100, Pt10, Cu50, Cu100 and other working thermal resistances, liquid glass thermometers, pressure thermometers, bimetallic thermometers;
3. Multi way low potential automatic transfer switch, parasitic potential ≤ 0.4 μ V;
4. Control 1-4 high-temperature furnaces;
5. Temperature field test: temperature field test of calibration furnace, oil tank, water tank and low-temperature constant temperature bath can be carried out;
6. Wire system conversion: resistance verification of two-wire system, three wire system and four wire system can be carried out;
7. The software has the functions of comparison experiment, repeatability experiment, temperature field experiment and other related experiments;
8. On Windows 2000/XP platform or above, it has a full Chinese interface and a standard Windows operating system, which is convenient and fast.It can realize:
1) Equipment self inspection and line inspection;
2) The screen displays and saves the temperature control curve ≤ 0.4 μ V;
5) The verification results are saved automatically and cannot be changed manually;
6) Query various thermocouples, thermal resistance scale and other help;
7) All historical verification data, temperature control curve query and statistics of thermocouples and thermal resistors, and intelligent measurement management function.
Installation and use
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When installing and using the temperature sensor, the following items should be paid attention to to ensure the best measurement effect:
1. Error caused by improper installation
Temperature sensor (Figure 11)
For example, the installation position and insertion depth of the thermocouple cannot reflect the true temperature of the furnace, in other words, the thermocouple should not be installed too close to the door and heating, and the insertion depth should be at least 8-10 times the diameter of the protective tube;The space between the protective sleeve of the thermocouple and the wall is not filled with thermal insulation material, which causes heat overflow or cold air invasion in the furnace. Therefore, the gap between the protective sleeve of the thermocouple and the hole of the furnace wall should be blocked with thermal insulation materials such as refractory mud or asbestos rope to prevent convection of cold and hot air from affecting the accuracy of temperature measurement;The cold end of the thermocouple is too close to the furnace body to make the temperature exceed 100 ℃;The installation of thermocouples should avoid strong magnetic and electric fields as much as possible, so the thermocouples and power cables should not be installed in the same conduit to avoid introducing interference and causing errors;The thermocouple cannot be installed in the area where the measured medium rarely flows. When the thermocouple is used to measure the gas temperature in the pipe, the thermocouple must be installed against the flow direction and fully contacted with the gas.
2. Error caused by insulation variation
If the thermocouple is insulated, too much dirt or salt slag on the protective tube and pull plate will lead to poor insulation between the thermoelectric dipole and the furnace wall, which is even more serious at high temperatures. This will not only cause the loss of thermal potential but also introduce interference, and the resulting error can sometimes reach up to 100%.
3. Error introduced by thermal inertia
Temperature sensor (Figure 12)
Due to the thermal inertia of the thermocouple, the indicated value of the instrument lags behind the change of the measured temperature, and this effect is particularly prominent in the rapid measurement.Therefore, thermocouples with thinner thermoelectric electrodes and smaller protective tube diameters should be used as far as possible.When the temperature measurement environment permits, the protective tube can even be removed.Due to the measurement lag, the amplitude of temperature fluctuation detected by thermocouple is smaller than that of furnace temperature fluctuation.The greater the measurement lag, the smaller the amplitude of thermocouple fluctuation and the greater the difference from the actual furnace temperature.When a thermocouple with a large time constant is used for temperature measurement or control, the temperature displayed by the instrument may fluctuate slightly, but the actual furnace temperature may fluctuate greatly.In order to measure temperature accurately, thermocouple with small time constant should be selected.The time constant is inversely proportional to the heat transfer coefficient, and is directly proportional to the diameter of the hot end of the thermocouple, the density of the material, and the specific heat. If you want to reduce the time constant, in addition to increasing the heat transfer coefficient, the most effective way is to minimize the size of the hot end.In use, protective sleeves with thin wall and small inner diameter are usually made of materials with good thermal conductivity.In the more precise temperature measurement, the bare wire thermocouple without protective sleeve is used, but the thermocouple is easy to be damaged, so it should be corrected and replaced in time.
4. Thermal resistance error
At high temperature, if there is a layer of coal ash on the protective tube and dust is attached to it, the thermal resistance will increase, which will hinder the heat conduction. At this time, the temperature indication is lower than the true value of the measured temperature.Therefore, keep the exterior of thermocouple protection tube clean to reduce error.
Development status
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Temperature sensor (Figure 13)
In recent years, the process of China's industrial modernization andElectronic information industryContinuous high-speed growth has drivenSensingThe rapid rise of the market for electronic appliances.temperatureSensingAsSensingIt is an important class in the device, accounting for the wholeSensingMore than 40% of the total demand.temperatureSensingThe device is a semiconductor device that converts non electrical physical quantities into electrical quantities by using the characteristic that the resistance of NTC changes with temperature, so as to carry out accurate temperature measurement and automatic control.temperatureSensingIt is widely used for temperature measurement and control, temperature compensation, flow rate, flow rate and wind speed measurement, liquid level indication, temperature measurement, ultraviolet and infrared light measurement, microwave power measurement, etc. It is widely used in color TV, computer color display, switching power source, water heater, refrigerator, kitchen equipment, air conditioner, automobile and other fields.In recent years, the rapid growth of automotive electronics and consumer electronics industries has driven China's temperatureSensingThe rapid growth of demand for devices.
In 2020, the human body temperature measurement accuracy of China's domestic infrared temperature sensor will reach 0.1 ℃. High performance ear temperature gun, forehead temperature gun and other non-contact temperature measurement tools are currently one of the main tools for heat screening of large-scale personnel. Its core component, MEMS (Micro Electro Mechanical System) infrared temperature sensor, has been mainly imported for a long time.Recently, the East China Institute of Optoelectronic Integrated Devices of the Electronic Institute of China North Industries Group Corporation has passed the technical breakthrough and has completed various single tests of sensor chips, with the performance index accuracy reaching 0.1 ℃, and the anti-interference ability superior to the current mainstream products in the market (the performance index accuracy is 0.3 ℃).[3]
Main purpose
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Temperature is a physical quantity that represents the degree of cold and hot of objects, and it is a very important and common measurement parameter in the process of industrial and agricultural production.Temperature measurement and control play a very important role in ensuring product quality, improving production efficiency, saving energy, production safety, and promoting the development of the national economy.Due to the universality of temperature measurement, the number of temperature sensors ranks first among various sensors, accounting for about 50%.
The temperature sensor measures indirectly by changing some characteristics of the object with temperature change.The characteristics of many materials and components change with the change of temperature, so there are quite a few materials that can be used as temperature sensors.The physical parameters of the temperature sensor vary with temperature, such as expansion, resistance, capacitance, and electromotive force, magnetic properties, frequency, optical characteristics, and thermal noise.With the development of production, new temperature sensors will continue to emerge.
Due to the wide range of temperature measurement in industrial and agricultural production, from hundreds of degrees below zero to thousands of degrees above zero, temperature sensors made of various materials can only be used within a certain temperature range.
The contact mode between the temperature sensor and the measured medium can be divided into two categories: contact type and non-contact type.The contact temperature sensor needs to maintain thermal contact with the measured medium, so that both can conduct sufficient heat exchange to reach the same temperature.This kind of sensor mainly includes resistance type, thermocouple, PN junction temperature sensor, etc.Non contact temperature sensorIt does not need to contact with the measured medium, but is transmitted to the temperature sensor through the thermal radiation or convection of the measured medium to achieve the purpose of temperature measurement.This type of sensor mainly includesInfrared temperature sensor。The main feature of this temperature measurement method is that it can measure the temperature of materials in motion (such as the bearing temperature of a slow moving train, the temperature of a rotating cement kiln) and objects with small heat capacity (such as the temperature distribution in an integrated circuit).
application area
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Temperature sensor[2]It is the earliest developed and most widely used sensor.The market share of temperature sensors has greatly exceeded that of other sensors.From the beginning of the 17th century, people began to use temperature for measurement.With the support of semiconductor technology, semiconductor thermocouple sensors, PN junction temperature sensors and integrated temperature sensors have been developed in this century.
If two conductors of different materials are connected to each other at a certain point, if the connection point is heated, there will be a potential difference in their unheated parts.The value of this potential difference is related to the temperature of the measuring point at the unheated part and the material of these two conductors.This phenomenon can occur in a wide temperature range. If the potential difference is accurately measured and the ambient temperature of the unheated part is measured, the temperature of the heating point can be accurately known.Since it must have two conductors of different materials, it is called "thermocouple".Thermocouples made of different materials are used in different temperature ranges, and their sensitivity is also different.
Thermocouple sensor has its own advantages and disadvantages. Its sensitivity is relatively low, and it is vulnerable to environmental interference signals andPreamplifierBecause of the influence of temperature drift, it is not suitable for measuring small temperature changes.The sensitivity of the thermocouple temperature sensor is independent of the thickness of the material
