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Basic knowledge of temperature sensor

The basic concept of temperature measurement

1. Temperature definition:

Temperature is a physical quantity that characterizes the degree of hot and cold of an object. Temperatures can only be indirectly measured by certain characteristics of the object as a function of temperature, and the scale used to measure the temperature of the object is called temperature. It specifies the starting point (zero point) of the temperature reading and the basic unit of measurement temperature. At present, the international temperature is more Fahrenheit temperature scale, Celsius temperature scale, thermodynamic temperature scale and international practical temperature scale.

 

Celsius standard temperature (℃) provides: Under standard atmospheric pressure, the melting point of ice is 0 degrees, the boiling point of water is 100 degrees, the middle divided into 100 equal parts, each equal to 1 degree Celsius, the symbol is ℃.

 

Fahrenheit temperature scale (° F) states that ice has a melting point of 32 ° C and a boiling point of 212 ° C at standard atmospheric pressure. 180 parts are divided equally between 1 ° F and 1 ° F.

 

Thermodynamic temperature scale (symbol T), also known as Kelvin temperature scale (symbol K), or absolute temperature scale, which provides the molecular motion stops when the temperature is absolute zero.

 

International temperature scale: International practical temperature scale is an internationally agreed temperature scale, it is close to thermodynamic temperature scale, and the replication of high precision, easy to use. At present, the international standard of temperature scale is "International Practical Temperature Scale 1968-1975 Revised Edition" adopted by the 15th International Right Congress in 1975, which is recorded as IPTS-68 (REV-75). However, due to a certain shortage of temperature of IPTS-68, the International Committee of Measurement was authorized by Resolution 7 of 18th International Conference of Metrology to adopt the International ITS-90 and ITS-90 temperature scales in 1990 instead of IPS-68. Since January 1, 1994, our country has fully implemented ITS-90 international temperature scale.

 

1990 International Temperature Scale:

 

a. Temperature units: Thermodynamic temperature is one of the basic physical quantities of an International Unit. Its unit, Kelvin, is defined as 1 / 273.16 of the thermodynamic temperature of the triple point of water, using the difference from 273.15K (freezing point) to indicate the temperature This method is still retained. By definition, the magnitude of Celsius equals Kelvin, and the temperature difference can also be expressed in degrees Celsius or Kelvin. The International Temperature Scale ITS-90 also defines the International Kelvin Temperature (symbol T90) and International Celsius (symbol t90).

b. International temperature scale ITS-90 general rule: ITS-90 from .065K up to Planck's radiation law using monochrome radiation actually measurable maximum temperature. ITS-90 is formulated in such a way that at full range, any best estimate of T at temperature acceptance is much more convenient, more sophisticated and highly repeatable than measuring the thermodynamic temperature directly Sex

c. ITS-90 definition:

The first temperature zone is between 0.65 K and 5.00 K and the T90 is defined by the relationship between the vapor pressure of 3He and 4He and the temperature. The second temperature zone is between 3.0K and Ne three-phase point (24.5661K) T90 is defined as a neon gas thermometer.

The third temperature zone is to balance the neon triple point (13.8033K) to the silver freezing point (961.78 ° C). The T90 is defined by a platinum resistance thermometer and is indexed using a defined set of defined interpolation methods. Phase freezing point (961.78 ℃) above the temperature, T90 is Planck's radiation law to define, reproduce the instrument for the optical pyrometer.

 

Second, the classification of temperature measuring instruments

 

Temperature measuring instruments by temperature measurement can be divided into two types of contact and non-contact. Generally speaking, the contact thermometer is relatively simple and reliable, and the measurement accuracy is high. However, due to the fact that the temperature measuring element and the medium to be measured need sufficient heat transfer, it takes a certain amount of time to reach the thermal balance, so there is a delay phenomenon of temperature measurement , At the same time subject to high temperature materials, can not be applied to high temperature measurement. Non-contact temperature measurement instrument is measured by the principle of thermal radiation temperature, measuring elements do not need to be in contact with the measured medium, a wide temperature range, not subject to the upper limit temperature, it will not undermine the temperature field of the measured object, The reaction speed is generally faster; however, the measurement error is large due to the influence of external factors such as the emissivity of the object, the measurement distance, soot, and moisture.

 

Third, the choice of sensors

National standard GB7665-87 under the sensor is defined as: "can feel the provisions of the measured and in accordance with certain laws converted to the available signal devices or devices, usually composed of sensitive components and conversion components." The sensor is a kind of detection device that can sense the information to be measured and transform the information detected by the detection into electrical signals or other required forms of information output according to certain rules to meet the requirements of information transmission, processing, storage, Display, record and control requirements. It is to achieve automatic detection and automatic control of the most important part.

(A). Modern sensors vary widely in principle and structure. How to select sensors reasonably according to the purpose of measurement, measurement object and measurement environment is the first problem to be solved when carrying out a certain amount. When the sensor is determined, with the matching measurement methods and measurement equipment can also be determined. The success or failure of a measurement depends to a large extent on the reasonableness of the sensor selection.

1.according to the measurement object and the measurement environment to determine the type of sensor: To carry out a specific measurement, we must first consider what principle of the sensor, which requires analysis of many factors before the determination. Because, even when measuring the same physical quantity, there are various principles of the sensor are available, the principle of one kind of sensor that is more appropriate, the following specific issues need to be considered according to the conditions and the measured sensor characteristics: size range; measured The size of the sensor on the location requirements; measurement methods for contact or non-contact; signal extraction method, wired or non-contact measurement; sensor source is imported or domestic, the price can accept, or self-developed.

2. The choice of sensitivity: Usually, in the linear range of the sensor, the sensor's sensitivity is as high as possible, because only when the sensitivity is high, the output signal corresponding to the measured change is more conducive to signal processing. However, it should be noted that the sensitivity of the sensor is high, and the external noise that is not related to the measurement is also easy to be mixed in. It is also amplified by the amplification system to affect the measurement accuracy. Therefore, the sensor itself needs to have a high signal-to-noise ratio Plant worry signal. Sensitivity of the sensor is directional. When the measurement is a single vector, but also for its high direction of the direction, you should choose the other direction of the sensor sensitivity, if the measurement is multidimensional vector, the cross-sensitivity of the sensor requires the smaller the better.

 

3.The frequency response characteristic: the frequency response characteristic of the sensor determines the frequency range to be measured, undistorted measurement conditions must be maintained within an allowable frequency range, the overall response of the sensor in fact a certain delay, the desired delay as short as possible. The frequency response of the sensor is high, and the measurable frequency range of the signal is wide. Due to the structural characteristics, the inertia of the mechanical system is large, because the frequency of the signal with the low frequency is lower. In the dynamic measurement, response characteristics should be based on the characteristics of the signal (steady state, random, etc.) to avoid over-fire error.

4. Linear range: The linear range of the sensor is the output and the input is proportional to the range. Theoretically, within this range, the sensitivity to maintain a fixed value, the wider the linear range of the sensor, the greater the range, and can guarantee a certain degree of measurement accuracy. In the choice of sensors, when the type of sensor to determine whether the first of its range to meet the requirements. But in fact, any sensor can not guarantee absolute linearity, and its linearity is relative. When the required measurement accuracy is low, the sensor with less nonlinear error can be approximated as linear within a certain range, which brings great convenience to the measurement.

5, Stability: After the sensor is used for a period of time, the ability of its performance to remain unchanged calls stability. Long-term stability of the sensor factors in addition to the structure of the sensor itself, the main sensor is the use of the environment. Therefore, to make the sensor have good stability, the sensor must have a strong ability to adapt to the environment. Before choosing a sensor, it should investigate its use environment and select the appropriate sensor according to the specific use environment, or take appropriate measures to reduce the environmental impact. In some applications require long-term use of the sensor can be easily replaced or calibrated occasions, the choice of the sensor stability requirements more stringent to be able to stand the test of time.

6, Accuracy: Accuracy is an important sensor performance indicators, it is related to the measurement accuracy of the entire measurement system which is an important part. The higher precision of the sensor, its price is more expensive, and therefore, the accuracy of the sensor as long as the overall accuracy of the measuring system can be, not necessarily selected too high, so that you can choose simpler and cheaper in many sensor measurements to meet in the same sensor. If the measurement object is qualitative analysis, selection of a sensor of high accuracy can be repeated, should not use a high-accuracy absolute magnitude; if for quantitative analysis, must obtain accurate measurements, it is necessary to select the sensor accuracy level can meet the requirements. For some special occasions, can not choose the right sensor, you have to design and manufacture of sensors, home-made sensor performance should meet the requirements.

(B) Temperature detector:

1.Thermal resistance: thermal resistance is the most commonly used in a low temperature zone temperature detector. Its main features are high accuracy, stable performance. Among them platinum thermistor measurement accuracy is the highest, it is not widely used in industrial temperature measurement, but also made a standard benchmark.

 RTD temperature measurement principle and material: RTD temperature measurement is based on the resistance of the metal conductor increases with temperature to increase the characteristics of this temperature measurement. Most of the thermal resistance is made of metal materials, the most widely used is platinum and copper, in addition, has now started using rhodium, nickel, manganese and other materials to create thermal resistance.

 Thermal resistance temperature measurement system consists of: RTD temperature measurement system generally by the thermal resistance, connecting wires and digital temperature control display and other components. Must pay attention to two points: "thermal resistance and digital temperature control display sub-degree must be consistent; in order to eliminate the impact of changes in the resistance of the connecting wire, you must take the three-wire connection."

 

2.Thermistor: NTC thermistor, with small size, high precision, fast response, stable and reliable, anti-aging, interchangeability, consistency and so on. Widely used in air conditioners, heating equipment, electronic thermometers, level sensors, automotive electronics, electronic desk calendar and other fields.

Thermocouple: Thermocouple is one of the most commonly used temperature sensing components in the industry. Its advantages are:

 High accuracy. Because the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.

 Wide measuring range. Commonly used thermocouples from -50 ~ +1600 ℃ can be measured continuously, the lowest of some special thermocouple -269 ℃ (such as gold, iron, nickel-chromium), up to +2800 ℃ (such as tungsten - rhenium).

 Simple structure, easy to use. Thermocouples are usually composed of two different wires, and are not limited by the size and the beginning, with a protective sleeve outside, which is very convenient to use.

(1). Thermocouple temperature measurement basic principle

Welding two conductors of different materials or semiconductors A and B to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, thus forming a current of a size in the circuit, which is called the thermoelectric effect. Thermocouple is to use this effect to work.

(2). Thermocouple types

Common thermocouple can be divided into standard thermocouple and non-standard thermocouple two categories.

Standard thermocouple refers to the national standard provides the relationship between thermoelectric power and temperature, allowing error, and a unified standard thermocouple table, it has its supporting display instrument for selection.

Non-standard thermocouple in the use of the range or order of magnitude less than the standard thermocouple, there is no common indexing table, mainly for some special occasions.

China from January 1, 1988 onwards, all thermocouple and RTD according to IEC international standards, and specify the S, B, E, K, R, J, T seven standard thermocouple for the design of a unified thermocouple.

(3). Thermocouple cold junction temperature compensation

Because thermocouple materials are generally more expensive (especially when using precious metals), and the temperature measurement point to the instrument are very far away, in order to save the thermocouple material, reduce costs, usually the compensation wire to the cold end of the thermocouple (free End) extends to a more stable temperature control room, connected to the instrument terminal. It must be pointed out that the role of the thermocouple compensation wire extends only from the thermode so that the cold end of the thermocouple moves to the instrument terminal of the control room and can not itself eliminate the influence of the temperature change of the cold end on the temperature measurement and can not compensate. Therefore, other correction methods need to be used to compensate for the temperature influence when the cold junction temperature t0 ≠ 0 ℃. In the use of thermocouple compensation wire must pay attention to the model match, the polarity can not take the wrong, compensation wire and thermocouple connection end temperature can not exceed 100 ℃.

 

Fourth, China's progress in the field of temperature control eight

China's instrumentation has made remarkable progress in keeping pace with international developments in miniaturization, digitization, intelligence, integration and networking, and has made greater efforts in the development and industrialization of its own intellectual property rights. Among them, the major scientific and technological progress that deserves to be put forward mainly includes the following eight aspects:

1.Advanced industrial automation instrumentation and systems to achieve a modular and fully digital integration, to meet industrial requirements, widely used in steel, electricity, coal, chemical, oil, transportation, construction, defense, food, medicine, agriculture, environmental protection, etc. Areas, to independent intellectual property rights has taken a solid step in the direction.

2. Intelligent Series test equipment and research and industrialization significantly higher levels of automatic test system, the formation of aerospace testing, testing mechanical and electrical products, household appliances test, earthquake monitoring, meteorological exploration, environmental monitoring and other industries such as automatic test system . The overall level of advanced foreign products to reach the level, while the price was significantly lower than the foreign products.

3. Microwave millimeter wave vector network analyzer developed successfully and mass production, marking China as the United States after the world's second to produce such sophisticated instruments of the country.

4. Research and development have their own characteristics of nano-measurement and control of micro-devices, carbon nanotubes directional preparation and the detection of structural and physical properties of the world's leading position.

5. Complete the complete electrical quantum standard and 1.5 × 10-5 national standard electrical equipment so that China's electrical measurement standards in the international advanced level.

6. Undertake research in scientific instruments with independent intellectual property rights and upgrade the overall standard of scientific instruments in our country.

7. The establishment of a combination of production, education and research, the development of mechanisms at home and abroad to broaden the application of scientific instruments, such as the development of successful customs anti-counterfeiting tickets spectrum instruments, the national customs promotion, the cumulative seized counterfeit value of 54 billion yuan worth of The state has saved huge economic losses. The market share of domestic scientific instruments has risen from 13% during the "Eighth Five-Year Plan" period to 25% by the end of the Ninth Five-Year Plan period.

8.High-intensity focused ultrasound tumor treatment system developed and mass production, ultrasonic medical instruments in the non-invasive treatment of tumors with the international leader.