1、 Basic concepts of temperature measurement
1. Temperature definition:
Temperature is a physical quantity that represents the degree of heat and cold of an object. The temperature can only be measured indirectly by some characteristics of the object changing with the temperature, and the scale used to measure the temperature value of the object is called the temperature scale. It specifies the starting point (zero point) of temperature reading and the basic unit of measuring temperature. At present, the temperature scales widely used internationally include Fahrenheit temperature scale, Celsius temperature scale, thermodynamic temperature scale and international practical temperature scale.
Centigrade temperature scale (℃): under standard atmospheric pressure, the melting point of ice is 0 ° C, the boiling point of water is 100 ° C, and the middle is divided into 100 equal parts, each equal part is 1 ° C, and the symbol is ° C.
The Fahrenheit temperature scale (℉) stipulates that under standard atmospheric pressure, the melting point of ice is 32 degrees, the boiling point of water is 212 degrees, and the middle is divided into 180 equal parts, each equal part is 1 degree Fahrenheit, and the symbol is ℉.
Thermodynamic temperature scale (symbol T) is also called Kelvin temperature scale (symbol K), or absolute temperature scale. It specifies that the temperature when the molecular motion stops is absolute zero.
International temperature scale: the international practical temperature scale is an international agreement temperature scale, which is close to the thermodynamic temperature scale, with high repetition accuracy and easy use. At present, the internationally used temperature scale is the 1968 international practical temperature scale - 1975 revision, which was adopted by the 15th International weight conference in 1975, and is marked as IPTS-68 (rev-75). However, due to certain inaccuracy in the temperature of IPTS-68, the seventh resolution of the 18th International metrology conference of the international metrology Committee authorized the 1989 meeting to adopt the international ITS-90 in 1990, and ITS-90 temperature scale replaced ips-68. China has fully implemented ITS-90 international temperature standard since January 1, 1994.
International temperature scale in 1990:
a. Temperature unit: thermodynamic temperature is a basic physical quantity, and its unit is Kelvin, which is defined as 1 / 273.16 of the thermodynamic temperature of the three-phase point of water. The difference with 273.15k (freezing point) is used to express the temperature, so this method is still retained. According to the definition, the degree of Celsius is equal to Kelvin, and the temperature difference can also be expressed in degrees Celsius or Kelvin. The international temperature scale ITS-90 defines both the international Kelvin temperature (symbol T90) and the international Celsius temperature (symbol T90).
b. General rules of international temperature scale ITS-90: ITS-90 is from 0.65k up to the maximum temperature that can be measured by Planck radiation law using monochromatic radiation. ITS-90 is formulated in such a way that, in the full range, the measurement of T90 is much more convenient, more precise, and has high reproducibility than the direct measurement of thermodynamic temperature when any optimal estimation value of T is adopted.
c. Definition of ITS-90:
The first temperature range is between 0.65k and 5.00k, and T90 is defined by the relationship between the vapor pressure and temperature of 3He and 4He.
The second temperature range is between 3.0k and neon triple point (24.5661k). T90 is defined by helium gas thermometer.
The third temperature range is from the three-phase point of hydrogen (13.8033k) to the freezing point of silver (961.78 ℃). T90 is defined by the platinum resistance thermometer, which uses a set of defined interpolation methods to divide. In the temperature range above the freezing point of silver (961.78 ℃), T90 is defined according to Planck's radiation law, and the reproducing instrument is an optical pyrometer.
2、 Classification of temperature measuring instruments
Temperature measuring instruments can be divided into contact type and non-contact type according to temperature measuring methods. Generally speaking, the contact temperature measuring instrument is simple, reliable and has high measurement accuracy; However, because the temperature measuring element and the measured medium need sufficient thermal bonding, and it takes a certain time to achieve thermal balance, there is a delay phenomenon in temperature measurement, and at the same time, due to the limitation of high-temperature resistant materials, it cannot be applied to very high temperature measurement. The non-contact instrument temperature measurement is based on the principle of thermal radiation. The measuring element does not need to contact with the measured medium, and the temperature measurement range is wide. It is not limited by the upper limit of temperature measurement, nor will it damage the temperature field of the measured object, and the reaction speed is generally relatively fast; However, due to the influence of external factors such as the emissivity of the object, the measurement distance, smoke and moisture, the measurement error is large.
3、 Selection of sensors
The national standard gb7665-87 defines the sensor as "a device or device that can sense the specified measured signal and convert it into a usable signal according to a certain law, usually consisting of a sensitive element and a conversion element". The sensor is a kind of detection device, which can sense the measured information and convert the detected information into electrical signals or other required forms of information output according to a certain law to meet the requirements of information transmission, processing, storage, display, recording and control. It is the primary link to realize automatic detection and automatic control.
（1） Modern sensors differ greatly in principle and structure. How to reasonably select the sensor according to the specific measurement purpose, measurement object and measurement environment is the first problem to be solved when carrying out a certain quantity. After the sensor is determined, the corresponding measuring method and measuring equipment can also be determined. The success or failure of the measurement results depends largely on the rationality of the sensor selection.
1. Determine the type of sensor according to the measurement object and the measurement environment: to carry out a specific measurement, it is first necessary to consider the principle of the sensor, which needs to be determined after analyzing various factors. Because, even if the same physical quantity is measured, there are many kinds of sensors with different principles to choose from. Which one is more suitable, the following specific problems need to be considered according to the characteristics of the measured and the use conditions of the sensor: the size of the measuring range; Volume requirements of the measured position on the sensor; The measurement method is contact type or non-contact type; Signal extraction method, wired or non-contact measurement; Whether the source of the sensor is imported or domestic, whether the price is acceptable or self-developed.
2. Sensitivity selection: generally, within the linear range of the sensor, the higher the sensitivity of the sensor, the better, because only when the sensitivity is high, the output signal corresponding to the measured change is relatively large, which is conducive to signal processing. However, it should be noted that the sensitivity of the sensor is high, and external noise unrelated to the measurement is easy to be mixed in, and will also be amplified by the amplification system, affecting the measurement accuracy. Therefore, the sensor itself is required to have a high signal-to-noise ratio, so as to minimize the plant worry signal introduced from the outside. The sensitivity of the sensor is directional. When the measured quantity is unidirectional and the directionality is required to be high, the sensor with low sensitivity in other directions shall be selected. If the measured quantity is multi-dimensional vector, the smaller the cross sensitivity of the sensor, the better.
3. Frequency response characteristics: the frequency response characteristics of the sensor determine the frequency range to be measured, and the measurement conditions without distortion must be maintained within the allowable frequency range. In fact, the response of the sensor always has a certain delay, and the shorter the delay, the better. The frequency response of the sensor is high, and the frequency range of the measurable signal is wide. Due to the influence of the structural characteristics, the inertia of the mechanical system is large, and the frequency of the measurable signal is low due to the sensor with low frequency. In the dynamic measurement, the response characteristics shall be based on the characteristics of the signal (steady state, random, etc.) to avoid excessive error.
4. Linear range: the linear range of the sensor refers to the range in which the output is proportional to the input. Theoretically, within this range, the sensitivity is kept at a constant value, and the wider the linear range of the sensor, the larger the range, and a certain measurement accuracy can be guaranteed. When selecting a sensor, after the type of sensor is determined, it is first necessary to see whether its range meets the requirements. But in fact, no sensor can guarantee absolute linearity, and its linearity is also relative. When the required measurement accuracy is relatively low, the sensor with small nonlinear error can be regarded as linear within a certain range, which will bring great convenience to the measurement.
5. Stability: the ability of the sensor to keep its performance unchanged after a period of use is called stability. In addition to the structure of the sensor itself, the main factor affecting the long-term stability of the sensor is the use environment of the sensor. Therefore, in order to make the sensor have good stability, the sensor must have strong environmental adaptability. Before selecting the sensor, the use environment shall be investigated, and the appropriate sensor shall be selected according to the specific use environment, or appropriate measures shall be taken to reduce the environmental impact. In some cases where the sensor is required to be used for a long time and can be easily replaced or calibrated, the stability of the selected sensor is more strict and it must be able to withstand the test for a long time.
6. Accuracy: accuracy is an important performance index of the sensor, which is an important link related to the measurement accuracy of the whole measurement system. The higher the accuracy of the sensor, the more expensive it is. Therefore, the accuracy of the sensor can only meet the accuracy requirements of the entire measurement system, and it is not necessary to select too high. In this way, the cheaper and simpler sensor can be selected among many sensors that meet the same measurement. If the measurement purpose is qualitative analysis, the sensor with high repetition accuracy can be selected instead of the sensor with high absolute value accuracy; If it is necessary to obtain accurate measurement values for quantitative analysis, it is necessary to select a sensor whose accuracy level can meet the requirements. For some special applications, if a suitable sensor cannot be selected, it is necessary to design and manufacture the sensor by itself. The performance of the self-made sensor shall meet the use requirements.
1. Thermal resistance: thermal resistance is the most commonly used temperature detector in medium and low temperature areas. Its main features are high measurement accuracy and stable performance. Among them, the measurement accuracy of platinum thermistor is the highest. It is not widely used in industrial temperature measurement, and it is made into a standard reference instrument.
① Principle and material of thermal resistance temperature measurement: thermal resistance temperature measurement is based on the characteristic that the resistance value of metal conductor increases with the increase of temperature. Most of the thermal resistors are made of metal materials. Platinum and copper are the most widely used at present. In addition, rhodium, nickel, manganese and other materials have been used to make thermal resistors.
② Composition of thermal resistance temperature measuring system: the thermal resistance temperature measuring system is generally composed of thermal resistance, connecting wire and digital temperature control display meter. Two points must be noted: "the graduation numbers of the thermal resistance and the digital temperature control display meter must be consistent; in order to eliminate the influence of the resistance change of the connecting wire, the three wire connection method must be adopted."
2. Thermistor: NTC thermistor has the characteristics of small volume, high test accuracy, fast reaction speed, stability and reliability, anti-aging, good interchangeability and consistency. It is widely used in air conditioning, heating equipment, electronic thermometer, liquid level sensor, automotive electronics, electronic calendar and other fields.
3. Thermocouple: thermocouple is one of the most commonly used temperature detection elements in industry. Its advantages are:
① High measurement accuracy. Since the thermocouple is in direct contact with the measured object, it is not affected by the intermediate medium.
② Wide measuring range. The commonly used thermocouples can be continuously measured from - 50 to 1600 ℃. Some special thermocouples have a minimum of - 269 ℃ (such as gold, iron, nickel and chromium) and a maximum of 2800 ℃ (such as tungsten rhenium).
③ Simple structure and convenient use. Thermocouples are usually composed of two different types of metal wires, which are not limited by size and beginning. They are equipped with protective sleeves, which are very convenient to use.
(1) Basic principle of thermocouple temperature measurement
Two conductors or semiconductors A and B of different materials are welded together 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, electromotive force will be generated between them, and a large current will be formed in the circuit. This phenomenon is called thermoelectric effect. Thermocouples use this effect to work.
(2) . type of thermocouple
Commonly used thermocouples can be divided into standard thermocouples and non-standard thermocouples.
The standard thermocouple refers to the thermocouple whose national standard specifies the relationship between thermal potential and temperature, allowable error, and has a unified standard graduation table. It has its matching display instrument for selection.
Non standardized thermocouples are inferior to standardized thermocouples in the range of use or the order of magnitude. Generally, there is no unified graduation table, which is mainly used for measurement in some special occasions.
Since January 1, 1988, all thermocouples and thermal resistors in China have been produced according to IEC international standards, and seven standardized thermocouples of S, B, e, K, R, J and t have been designated as unified design thermocouples in China.
(3) Temperature compensation of thermocouple cold end
Since the materials of thermocouples are generally expensive (especially when precious metals are used), and the distance between the temperature measuring point and the instrument is very long, in order to save thermocouple materials and reduce costs, compensation wires are usually used to extend the cold end (free end) of the thermocouple to the control room with relatively stable temperature and connect it to the instrument terminal. It must be pointed out that the thermocouple compensation wire only serves to extend the Thermoelectrode and move the cold end of the thermocouple to the instrument terminal in the control room. It can not eliminate the influence of the temperature change of the cold end on the temperature measurement and does not play a compensation role. Therefore, other correction methods should be adopted to compensate the influence of cold end temperature t0 ≠ 0 ℃ on temperature measurement. When using the thermocouple compensation wire, attention must be paid to the model matching, the polarity cannot be wrong, and the temperature at the connection end of the compensation wire and the thermocouple cannot exceed 100 ℃.
4、 Eight advances in temperature control in China
China's instruments and meters keep up with the pace of international development in realizing miniaturization, digitization, intellectualization, integration and networking, and increase the development and industrialization of parts with independent intellectual property rights, and have made remarkable progress. Among them, the major scientific and technological progress worth mentioning mainly includes the following eight aspects:
1. The advanced industrial automation instruments and meters and systems have realized modularization and full digital integration, meeting the industrialization requirements, and are widely used in steel, electricity, coal, chemical, oil, transportation, construction, national defense, food, medicine, agriculture, environmental protection and other fields, making a solid step towards having independent intellectual property rights.
2. The research and industrialization level of intelligent series test instruments and automatic test systems have been greatly improved, and automatic test systems for aerospace testing, electromechanical product testing, household appliance testing, earthquake monitoring, meteorological detection, environmental monitoring and other industries have been established. The overall level reached the level of foreign advanced products, and the price was significantly lower than that of foreign products.
3. The successful development and mass production of microwave and millimeter wave vector network analyzer marks that China has become the second country in the world after the United States that can produce such high-end instruments.
4. Research and develop nano measurement and control and micro instruments with its own characteristics. The directional preparation of carbon nanotubes and the detection of structure and physical properties occupy a leading position in the world.
5. Complete the complete electrical quantum standard and 1.5 × Level 10-5 national electric energy standard device makes China's electric energy measurement standard at the international advanced level.
6. We carried out research on scientific instruments with independent intellectual property rights, and improved the overall level of scientific instruments in China.
7. The development mechanism of combining industry, University and research and combining domestic and foreign countries has been established, and the application field of scientific instruments has been broadened. For example, the spectrum instrument for anti-counterfeiting tickets of the Customs has been successfully developed. After being popularized by the customs throughout the country, a total of 54 billion yuan worth of counterfeit tickets have been seized, which has saved the country huge economic losses. The market share of domestic scientific instruments has increased from 13% during the Eighth Five Year Plan period to 25% at the end of the Ninth Five Year Plan period.
8. The high-intensity focused ultrasound tumor treatment system has been successfully developed and mass produced. The ultrasound medical instruments have international leading advantages in non-invasive treatment of tumors.