1. The basic concept of temperature measurement
1. Temperature definition:
Temperature is a physical quantity that characterizes the degree of heat or cold of an object. Temperature can only be measured indirectly through certain characteristics of objects that change with temperature, and the scale used to measure the temperature of an object is called a temperature scale. It specifies the starting point (zero point) of temperature readings and the basic unit of temperature measurement. At present, the most widely used temperature scales in the world are Fahrenheit, Celsius, thermodynamics and international practical temperature scales.
The Celsius temperature scale (°C) stipulates that 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 divided into 1°C, the symbol is °C.
The Fahrenheit temperature scale (℉) 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 and each equal part is 1 degree Fahrenheit. The symbol is ℉.
Thermodynamic temperature scale (symbol T) is also called Kelvin temperature scale (symbol K), or absolute temperature scale, which stipulates that the temperature at which the molecular motion stops is absolute zero.
International temperature scale: The international practical temperature scale is an international agreement temperature scale, which is similar to the thermodynamic temperature scale, and has high reproducibility and is easy to use. The current international temperature scale is the “1968 International Practical Temperature Scale-1975 Revised Edition” adopted by the 15th International Conference on Power in 1975, which is recorded as: IPTS-68 (REV-75). However, due to the unpredictable temperature of IPTS-68, the International Committee of Weights and Measures authorized the 1989 meeting to pass the 1990 International ITS-90, ITS-90 temperature scale to replace IPS-68 in the seventh resolution of the 18th International Conference on Metrology. my country has fully implemented the ITS-90 international temperature scale since January 1, 1994.
International temperature scale in 1990:
a. Temperature unit: Thermodynamic temperature is a basic physical quantity. Its unit Kelvin is defined as 1/273.16 of the thermodynamic temperature of the triple point of water. The difference with 273.15K (freezing point) is used to represent the temperature, so it is still retained this way. By 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 standard ITS-90 defines both the international Kelvin temperature (symbol T90) and the international Celsius temperature (symbol t90).
b. The general rules of the international temperature standard ITS-90: ITS-90 increases from 0.65K to the highest temperature actually measurable by the Planck radiation law using monochromatic radiation. ITS-90 is formulated in such a way that in the full range, any best estimate of T when the temperature is adopted, the measurement of T90 is much more convenient and more precise than the direct measurement of thermodynamic temperature, and has a high degree of repeatability. Presentability.
c. Definition of ITS-90:
The first temperature zone 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 zone is between 3.0K and the triple point of neon (24.5661K). T90 is defined by a helium gas thermometer.
The third temperature zone is between the triple point of flat hydrogen (13.8033K) and the freezing point of silver (961.78°C). T90 is defined by a platinum resistance thermometer, which uses a set of defined interpolation methods to index. For the temperature zone above the freezing point of silver (961.78°C), T90 is defined by Planck’s law of radiation, and the reproduction instrument is an optical pyrometer.
2. Classification of temperature measuring instruments
Temperature measuring instruments can be divided into two categories: contact type and non-contact type. Generally speaking, contact temperature measuring instruments are relatively simple, reliable, and have high measurement accuracy; but because the temperature measuring element and the measured medium need to be fully thermally exchanged, it takes a certain time to reach thermal equilibrium, so there is a delay in temperature measurement. At the same time, it is limited by high temperature resistant materials and cannot be used for high temperature measurement. Non-contact instrument temperature measurement is based on the principle of thermal radiation to measure temperature. The measuring element does not need to be in contact with the measured medium. The temperature measurement range is wide. It is not limited by the upper limit of temperature measurement and will not damage the temperature field of the measured object. The reaction speed is generally relatively fast; however, the measurement error is relatively large due to the influence of external factors such as the emissivity of the object, the measurement distance, smoke, and moisture.