Temperature measurement and temperature measurement instrument

First, the basic concept of temperature measurement Temperature is the physical quantity that characterizes the degree of cold and heat of an object. The temperature can only be measured indirectly through certain characteristics of the object with changes in temperature, and the scale used to measure the value of the object temperature is called the temperature scale. It specifies the starting point (zero point) for reading the temperature and the basic unit for measuring the temperature. At present, the internationally used temperature scales include the Fahrenheit temperature scale, the Celsius temperature scale, the thermodynamic temperature scale, and the international practical temperature scale.

The Fahrenheit temperature scale (oF) stipulates: 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 is divided into 1 degree of newspaper and the symbol is oF.

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

The thermodynamic temperature scale, also known as the Kelvin temperature scale, or absolute temperature scale, specifies that the temperature when the molecular motion is stopped is absolutely zero, and the symbol is K.

The international practical temperature scale is an internationally agreed temperature scale. It is close to the thermodynamic temperature scale, and it has high reproduction accuracy and is easy to use. At present, the international common temperature scale is the "International Practical Temperature Scale of 1968 - Revision of 1975" passed by the 15th International Conference on Weights in 1975, and it is recorded as: IPTS-68 (Rev-75). However, because of IPTS-68 temperature indications, there are certain deficiencies. The International Metrology Committee authorized the seventh resolution of the 18th International Metrological Conference to the 1989 Conference to adopt the 1990 international ITS-90 and ITS-90 temperature scales instead of IPTS-68. Since January 1, 1994, China has fully implemented the ITS-90 international temperature standard.

The introduction of the International Temperature Scale (ITS-90) in 1990 is as follows.

1. Temperature unit The thermodynamic temperature (symbol T) is the basic amount of the master's physical quantity. Its unit is Kelvin (symbol is K) and is defined as 1/273.16 of the thermodynamic temperature of the water triple point. Since the previous temperature scale definition uses a difference from 273.15K (freezing point) to represent temperature, this method is still retained.

By definition, the degree 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).

2. The general rule of the international standard ITS-90 ITS-90 Up to Planck radiation law from 0.65K uses the actual measurable maximum temperature of monochromatic radiation. ITS-90 was developed in such a way that the T90 value at any temperature is very close to the best estimate of T at the temperature scale adoption in the full range. Compared with the direct measurement of the thermodynamic temperature, the T90 measurement is much more convenient and more convenient. It is precise and highly reproducible.

3. Definition of ITS-90 The first temperature zone is between 0.65K and 5.00K. T90 is defined by the relationship between the vapor pressure and temperature of 3He and 4He.

The second temperature zone is 3.0K to the triple point (24.5661K). The T90 is defined by a helium gas thermometer.

The second temperature zone is between the equilibrium hydrogen triple point (13.8033K) and the silver solidification point (961.78°C), and T90 is defined by a platinum resistance thermometer. It uses a set of defined fixed points and uses prescribed interpolation to index.

In the temperature zone above the freezing point of silver (961.78°C), T90 is defined according to Planck's radiation law, and the reproducing apparatus is an optical pyrometer.

Second, the classification of temperature measurement instruments Temperature measurement instruments according to the temperature measurement methods can be divided into contact and non-contact two categories. In general, the temperature measuring instrument for contact-type temperature measuring instruments is relatively simple and reliable, and the measuring accuracy is high; however, since the temperature measuring element and the measured medium need to perform sufficient thermal exchange, it takes a certain time to reach the heat balance, so there is measurement. The temperature delay phenomenon, while limited by high temperature materials, cannot be applied to very high temperature measurements. The non-contact meter temperature measurement is based on the principle of thermal radiation. The temperature measurement element does not need to be in contact with the measured medium. It has a wide range of temperature measurement, is not limited by the upper limit of the temperature, and does not destroy the temperature field of the measured object. The reaction speed is generally faster. However, due to external factors such as the emissivity, measurement distance, smoke and dust, and moisture of the object, the measurement error is large.

Third, thermocouple thermocouple is one of the most commonly used temperature detection devices in the industry. Its advantages are:

1 high measurement accuracy. Because the thermocouple is directly in contact with the measured object, it is not affected by the intermediate medium.

2 wide measurement range. Commonly used thermocouples can be measured from -50 to +1600°C. Some special thermocouples can measure at least -269°C (such as gold, iron, nickel-chromium) and up to +2,800°C (such as tungsten-germanium).

3 simple structure, easy to use. Thermocouples are usually composed of two different types of wires, and are not limited by size and the beginning. They have protective sleeves and are very convenient to use.

1. Basic principle of thermocouple temperature measurement Weld two different material conductors or semiconductors A and B to form a closed loop, as shown in Figure 2-1-1. When there is a temperature difference between the two sticking points 1 and 2 of the conductors A and B, an electromotive force is generated between the two sticking points 1 and 2 so that a current of a size is formed in the loop. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.

2. Types and structures of thermocouples (1) Types of thermocouples Thermocouples can be classified into standard thermocouples and non-standard thermocouples. The called standard thermocouple refers to a thermocouple whose national standard stipulates the relationship between thermoelectric potential and temperature, tolerance, and a unified standard indexing table. It has its own display instrument for selection. Non-standardized thermocouples are less than standard thermocouples in terms of range of use or magnitude, and there are generally no uniform indexing tables, which are mainly used for measurement in certain special occasions.

Standardized Thermocouples From January 1, 1988, thermocouples and thermal resistances were all produced according to IEC international standards, and seven standardized thermocouples designated as S, B, E, K, R, J, and T were designed for unified design in China. Thermocouples.

(2) The structure of the thermocouple To ensure reliable and stable operation of the thermocouple, the structural requirements for it are as follows: 1 The welding of the two hot electrodes constituting the thermocouple must be firm; 2 The two thermodes should be in good contact with each other Insulation in order to prevent short circuit; 3 The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable; 4 The protection sleeve should ensure that the hot electrode and the harmful medium are fully isolated.

3, temperature compensation of thermocouple cold junction Because thermocouple materials are generally more expensive (especially when using precious metals), and the distance from the temperature measurement point to the instrument is very far, in order to save the thermocouple material, reduce costs, usually using compensation wire The cold junction (free end) of the thermocouple extends into a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the role of thermocouple compensation wire only extends the thermode, so that the cold junction of the thermocouple moves to the instrument terminal of the control room. It does not eliminate the influence of the temperature change of the cold junction on the temperature measurement, and does not compensate. Therefore, other correction methods need to be used to compensate for the influence of temperature at the cold junction temperature t00°C.

When using a thermocouple to compensate for the wire, it must be noted that the model matches, and the polarity cannot be connected wrongly. The temperature of the connecting wire of the compensation wire and the thermocouple cannot exceed 100°C.

Fourth, thermal resistance thermal resistance is the most commonly used temperature detector in the low-temperature region. Its main features are high measurement accuracy and stable performance. Platinum thermal resistance is the highest measurement accuracy. It is widely used not only in industrial temperature measurement but also as a standard reference instrument.

1. Thermistor temperature measurement principle and material Thermistor temperature measurement is based on the characteristic that the resistance value of the metal conductor increases as the temperature increases.

Most of the thermal resistance is made of pure metal materials. At present, platinum and copper are the most widely used materials. In addition, the use of dian, nickel, manganese, and tantalum has been used to manufacture thermal resistors.

2. Structure of thermal resistance (1) The structure and characteristics of the common thermal resistance temperature sensing element (resistor) used in the proficient thermal resistance industry are shown in Table 2-1-11. From the principle of temperature measurement of the thermal resistance, it can be seen that the change in the measured temperature is directly measured by the change in the resistance of the thermal resistor. Therefore, changes in the resistance of the lead wire, such as the thermal resistor body, will affect the temperature measurement. In order to eliminate the influence of lead resistance, the three-wire or four-wire system is adopted in the same way. For details, see the first section of Chapter III of this paper.

(2) Armored Thermal Resistance An Armored Thermal Resistance is a solid body composed of a temperature sensing element (resistor), lead wire, insulating material, and stainless steel bushing, as shown in Figure 2-1-7. Generally 2 ~ 8mm, the minimum can reach mm.

Compared with common thermal resistance, it has the following advantages: 1 small size, no internal air gap, thermal inertia, small measurement lag; 2 good mechanical properties, vibration resistance, impact resistance; 3 can bend, easy to install 4 long service life .

(3) End face thermal resistance The end face thermal resistance temperature sensing element is wound by a specially treated resistance wire, and it is closely attached to the end face of the thermometer. Its structure is shown in Figure 2-1-8. Compared with general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the tested end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.

(4) Flameproof RTD flameproof RTDs are limited to the junction box by a specially constructed junction box in which the explosion of the explosive mixture of its internal enclosure due to sparks or arcing is not induced in the production site. . Flameproof RTDs can be used for temperature measurement in explosion-hazardous areas in Bla-B3c zones.

3, composition of thermal resistance temperature measurement system Thermal resistance temperature measurement system is generally composed of thermal resistance, connecting wires and display instruments. Must pay attention to the following two points: 1 The index number of the thermal resistance and the display instrument must be the same 2 In order to eliminate the influence of the resistance change of the connecting wire, must adopt the three-wire system connection method. For details, see Chapter 3 of this essay.

(2) Armored Thermal Resistance An Armored Thermal Resistance is a solid body composed of a temperature sensing element (resistor), lead wire, insulating material, and stainless steel bushing, as shown in Figure 2-1-7. Generally 2 ~ 8mm, the minimum can reach mm.

Compared with common thermal resistance, it has the following advantages: 1 small size, no internal air gap, thermal inertia, small measurement lag; 2 good mechanical properties, vibration resistance, impact resistance; 3 can bend, easy to install 4 long service life .

(3) End face thermal resistance The end face thermal resistance temperature sensing element is wound by a specially treated resistance wire, and it is closely attached to the end face of the thermometer. Its structure is shown in Figure 2-1-8. Compared with general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the tested end face, and is suitable for measuring the end face temperature of the bearing pad and other parts.

(4) Flameproof thermal resistance flameproof type thermal resistance through the special structure of the junction box, the explosion of the inner shell of the mixture of gas due to spark or arc and other shadow resistor body repair must inevitably change the length of the resistance wire and affect the resistance value, For this reason, it is better to replace the new resistor body. If soldering is used for repair, it must be verified after passing the soldering.