1、 Basic concepts of temperature measurement

1. Temperature definition:

Temperature is a physical quantity that characterizes the degree of coldness or heat of an object. Temperature can only be indirectly measured through certain characteristics of an object that change with temperature, and the scale used to measure the temperature value of an object is called a temperature scale. It specifies the starting point (zero) for temperature readings and the basic unit for measuring temperature. The commonly used temperature scales currently include Fahrenheit, Celsius, thermodynamic, and practical temperature scales.

The Celsius temperature scale (℃) stipulates that at standard atmospheric pressure, the melting point of ice is 0 degrees and the boiling point of water is 100 degrees. It is divided into 100 equal parts, with each part being 1 degree Celsius and the symbol being ℃.

The Fahrenheit temperature scale (℉) stipulates that at standard atmospheric pressure, the melting point of ice is 32 degrees Celsius, the boiling point of water is 212 degrees Celsius, and there are 180 equal parts in between, with each equal part being 1 degree Fahrenheit and the symbol being ℉.

The thermodynamic temperature scale (symbol T), also known as the Kelvin temperature scale (symbol K), or temperature scale, specifies the temperature at which molecular motion stops at zero degrees.

Temperature scale: The practical temperature scale is a protocol temperature scale that is similar to the thermodynamic temperature scale, with high reproducibility and easy use. The current commonly used temperature scale is the "1968 Practical Temperature Scale -1975 Revised Edition" adopted at the 15th Weights and Measures Conference in 1975, denoted as IPTS-68 (REV-75). However, due to certain uncertainties in the temperature of IPTS-68, the Metrology Committee authorized the adoption of the ITS-90 temperature scale in 1990 to replace IPS-68 at the 1989 meeting in Resolution 7 of the 18th Metrology Conference. China has fully implemented the ITS-90 temperature scale since January 1, 1994.

The 1990 temperature scale:

a、 Temperature unit: Thermodynamic temperature is a fundamental physical quantity, with its unit in Kelvin defined as 1/273.16 of the thermodynamic temperature at the triple point of water. The temperature is expressed using the difference from 273.15K (freezing point), so this method is still retained today. According to the definition, the magnitude of Celsius is equal to Kelvin, and temperature difference can also be expressed in Celsius or Kelvin. The ITS-90 temperature scale defines both Kelvin temperature (symbol T90) and Celsius temperature (symbol t90).

b、 The general rule of ITS-90 temperature scale: ITS-90 is the highest temperature that can be measured using monochromatic radiation from 0.65K upwards to Planck's radiation law. ITS-90 is formulated in such a way that, at the full range, any estimated value of T * adopted for temperature measurement is much more convenient, precise, and highly reproducible compared to directly measuring thermodynamic temperature with T90.

c. Definition of ITS-90:

*The 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 defined by the helium gas thermometer T90 between 3.0K and the neon triple point (24.5661K).

The third temperature range is between the triple point of Pingheng hydrogen (13.8033K) and the solidification point of silver (961.78 ℃), and T90 is defined by a platinum resistance thermometer, which uses a set of prescribed interpolation methods for division. The temperature range above the silver solidification point (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 two categories based on temperature measurement methods: contact and non-contact. Generally speaking, contact temperature measuring instruments are relatively simple, reliable, and have high measurement accuracy; However, due to the need for sufficient thermal exchange between the temperature measuring element and the measured medium, it takes some time to reach thermal equilibrium, so there is a delay in temperature measurement. At the same time, due to the limitations of high-temperature resistant materials, it cannot be applied to very high temperature measurements. Non contact instrument temperature measurement is based on the principle of thermal radiation. The measuring element does not need to be in contact with the measured medium, and the temperature measurement range is wide. It is not limited by the upper limit of temperature measurement and does not damage the temperature field of the measured object. The reaction speed is generally fast; However, due to external factors such as the emissivity of the object, measurement distance, smoke and moisture, the measurement error is relatively large.

3、 Selection of sensors

The definition of sensors in the national standard GB7665-87 is: "A device or apparatus that can sense a specified measured object and convert it into a usable signal according to certain rules, usually composed of sensitive elements and conversion elements. A sensor is a detection device that can sense the measured information and convert it into electrical signals or other required forms of information output according to certain rules, in order to meet the requirements of information transmission, processing, storage, display, recording, and control. It is the primary step in achieving automatic detection and control.

（1） Modern sensors vary greatly in principle and structure. How to select sensors reasonably based on specific measurement purposes, measurement objects, and measurement environments is the first problem to be solved when measuring a certain quantity. After the sensor is determined, the corresponding measurement methods and equipment can also be determined. The success or failure of measurement results largely depends on whether the selection of sensors is reasonable.

1. Determine the type of sensor based on the measurement object and measurement environment: To carry out a specific measurement work, the first thing to consider is which principle of sensor to use, which requires analyzing multiple factors before determining. Because even if measuring the same physical quantity, there are multiple principle sensors available for selection. Which principle sensor is more suitable depends on the characteristics of the measured object and the usage conditions of the sensor, taking into account the following specific issues: the size of the measuring range; The volume requirements of the sensor for the measured location; The measurement method is either contact or non-contact; The method of signal extraction, wired or non-contact measurement; Is the source of the sensor imported or domestically produced, is the price acceptable, or is it self-developed.

2. Sensitivity selection: Generally, within the linear range of the sensor, it is desirable to have a higher sensitivity because only when the sensitivity is high, the output signal corresponding to the measured change is relatively large, which is beneficial for signal processing. However, it should be noted that the sensitivity of the sensor is high, and external noise unrelated to the measured object is also easily mixed in, which can be amplified by the amplification system and affect the measurement accuracy. Therefore, the sensor itself is required to have a high signal-to-noise ratio to minimize the introduction of factory noise signals from the outside world. The sensitivity of sensors is directional. When the measured quantity is unidirectional and requires high directionality, sensors with lower sensitivity in other directions should be selected. If the measured vector is multidimensional, the smaller the cross sensitivity of the sensor, the better.

3. Frequency response characteristics: The frequency response characteristics of a sensor determine the frequency range being measured, and it is necessary to maintain measurement conditions without distortion within the allowable frequency range. In fact, the response of a sensor always has a certain delay, and it is hoped that the shorter the delay, the better. The frequency response of sensors is high, and the frequency range of measurable signals is wide. However, due to the influence of structural characteristics, the inertia of mechanical systems is large, and sensors with lower frequencies can measure signals with lower frequencies. In dynamic measurement, the response characteristics should be based on the characteristics of the signal (steady-state, random, etc.) to avoid excessive errors.

4. Linear range: The linear range of a sensor refers to the range where the output is proportional to the input. In theory, within this range, the sensitivity remains constant, and the wider the linear range of the sensor, the larger its range, while ensuring a certain level of measurement accuracy. When choosing a sensor, after determining the type of sensor, the first thing to consider is whether its range meets the requirements. However, in reality, no sensor can guarantee linearity, and its linearity is also relative. When the required measurement accuracy is relatively low, sensors with small nonlinear errors can be approximated as linear within a certain range, which will bring great convenience to the measurement.

5. Stability: The ability of a sensor to maintain its performance unchanged after a period of use is called stability. The factors that affect the long-term stability of sensors are not only the structure of the sensor itself, but also the usage environment of the sensor. Therefore, in order for sensors to have good stability, they must have strong environmental adaptability. Before selecting a sensor, it is necessary to investigate its usage environment and choose the appropriate sensor based on the specific usage environment, or take appropriate measures to reduce environmental impact. In some situations where sensors are required to be able to be used for a long time and easily replaced or calibrated, the stability requirements for the selected sensors are more stringent, and they must be able to withstand long-term tests.

6. Accuracy: Accuracy is an important performance indicator of sensors, and it is an important link related to the measurement accuracy of the entire measurement system. The higher the accuracy of the sensor, the more expensive its price. Therefore, as long as the accuracy of the sensor meets the accuracy requirements of the entire measurement system, it is not necessary to choose too high. This way, cheaper and simpler sensors can be selected among many sensors that meet the same measurement requirements. If the measurement purpose is qualitative analysis, sensors with high repeatability accuracy should be selected, and sensors with high measurement accuracy should not be used; If it is necessary to obtain measurement values for quantitative analysis, sensors with accuracy levels that meet the requirements must be selected. For certain special usage scenarios where suitable sensors cannot be selected, it is necessary to design and manufacture sensors themselves, and the performance of homemade sensors should meet the requirements for use.

（2） Temperature sensor:

1. Thermistor: Thermistor is a commonly used temperature detector in the medium and low temperature range. Its main features are high measurement accuracy and stable performance. The measurement accuracy of platinum resistance thermometer is the highest among them. It is not widely used in industrial temperature measurement and has been made into a standard reference instrument.

① Principle and Materials of Thermistor Temperature Measurement: Thermistor temperature measurement is based on the characteristic that the resistance value of a metal conductor increases with temperature. Thermistors are mostly made of metal materials, with platinum and copper being the most commonly used. In addition, materials such as rhodium, nickel, and manganese have been used to manufacture thermistors.

② The composition of a thermistor temperature measurement system: A thermistor temperature measurement system generally consists of a thermistor, connecting wires, and a digital temperature control display meter. Two points must be noted: "The division marks of the thermistor and digital temperature control display meter must be consistent; in order to eliminate the influence of changes in the resistance of the connecting wires, a three wire connection method must be adopted

2. Thermistor: NTC thermistor, with small size, high test accuracy, fast reaction speed, stability and reliability, anti-aging, interchangeability, good consistency and other characteristics. Widely used in fields such as air conditioning, heating equipment, electronic thermometers, liquid level sensors, automotive electronics, electronic calendars, etc.

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

① High measurement accuracy. Because the thermocouple is in direct contact with the measured object and is not affected by the intermediate medium.

② Wide measurement range. Commonly used thermocouples can measure continuously from -50~+1600 ℃, while some special thermocouples can measure temperatures as low as -269 ℃ (such as gold iron nickel chromium) and as high as+2800 ℃ (such as tungsten rhenium).

③ Simple construction and easy to use. Thermocouples are usually composed of two different types of metal wires, and are not limited by size or opening. They have protective sleeves on the outside, making them very convenient to use.

(1) Basic principle of thermocouple temperature measurement

Weld two different materials of conductors or semiconductors A and B together to form a closed circuit. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between them, resulting in a current of a certain magnitude in the circuit. This phenomenon is called thermoelectric effect. Thermocouples work by utilizing this effect.

(2) Types of thermocouples

Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples.

A standard thermocouple refers to a thermocouple that has a national standard that specifies the relationship between its thermoelectric potential and temperature, allows for errors, and has a unified standard calibration table. It has a matching display instrument available for selection.

Non standardized thermocouples are not as widely used or of the same order of magnitude as standardized thermocouples, and generally do not have a unified calibration table. They are mainly used for measurements in certain special occasions.

Since January 1, 1988, all thermocouples and thermistors in China have been produced according to IEC standards, and the seven standardized thermocouples S, B, E, K, R, J, and T have been designated as unified design thermocouples in China.

(3) Temperature compensation of thermocouple cold end

Due to the fact that the materials of thermocouples are generally expensive (especially when using precious metals), and the distance between the temperature measuring point and the instrument is far, 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 a temperature stable control room and connect it to the instrument terminals. It must be pointed out that the function of the thermocouple compensation wire is only to extend the thermoelectric electrode and move the cold end of the thermocouple to the instrument terminal in the control room. It cannot eliminate the influence of temperature changes at the cold end on temperature measurement and does not have a compensation effect. Therefore, other correction methods need to be used to compensate for the impact of cold end temperature t0 ≠ 0 ℃ on temperature measurement. When using thermocouple compensation wires, attention must be paid to model matching, polarity cannot be connected incorrectly, and the temperature at the connection end between the compensation wire and the thermocouple cannot exceed 100 ℃.

4、 Eight major developments in the field of temperature control in China

China's instruments and meters have kept pace with the development of miniaturization, digitization, intelligence, integration, and networking, and have increased efforts in the development, research, and industrialization of parts with independent intellectual property rights, achieving significant progress. Among them, the significant technological advances worth mentioning mainly include the following eight aspects:

1. Advanced industrial automation instruments and systems have achieved modular and fully digital integration, meeting industrialization requirements and widely used in fields such as steel, electricity, coal, chemical, oil, transportation, construction, national defense, food, medicine, agriculture, and environmental protection, taking a solid step towards having independent intellectual property rights.

The research and industrialization level of intelligent series testing instruments and automatic testing systems have been greatly improved, and automatic testing systems have been established for various industries such as aerospace testing, electromechanical product testing, household appliance testing, earthquake monitoring, meteorological detection, environmental monitoring, etc. The overall level has reached the product level, while the selling price is significantly lower than that of foreign products.

The successful development and mass production of microwave millimeter wave vector network analyzer marks China becoming the second country after the United States to produce such high-precision and cutting-edge instruments.

4. Research and develop unique nano measurement and micro instruments, with the directional preparation of carbon nanotubes and the detection of their structure and physical properties occupying a global position.

5. Complete the complete electrical quantum standard and the 1.5 × 10-5 level national electrical energy standard device, making China's electrical measurement standards at an advanced level.

6. Conducted research and development on scientific instruments with independent intellectual property rights, improving the overall level of scientific instruments in China.

7. We have established a development mechanism that combines industry, academia, and research, as well as domestic and foreign cooperation, and expanded the application fields of scientific instruments. For example, we have successfully developed a spectral instrument for customs anti-counterfeiting tickets. After promoting it in customs nationwide, we have seized a total of 54 billion yuan worth of counterfeit tickets, which has saved the country from huge economic losses. The proportion of domestically produced scientific instruments has increased from 13% during the Eighth Five Year Plan period to 25% by 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, and ultrasound medical instruments have advantages in non-invasive treatment of tumors.

Related links

Method for naming thermocouple models

How to select thermocouples

Principle and Construction of Thermocouples

Basic knowledge of temperature sensors

Anhui Huarun Instrument Qualification and Honor Certificate

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