Thread Content
“Baidu Baike describes “temperature” as follows: Temperature is a physical quantity that indicates the degree of hotness or coldness of an object; on a microscopic level, it represents the intensity of the thermal motion of the molecules in that object. Temperature can only be measured indirectly through certain properties of an object that change with temperature, and the scale used to determine the numerical value of an object’s temperature is called a temperature scale. Temperature defines the starting point (zero point) for temperature readings and the basic unit for measuring temperature. The international unit is the thermodynamic temperature scale (K). Other temperature scales that are widely used internationally include the Fahrenheit scale (°F), the Celsius scale (°C), and the International Practical Temperature Scale. There are several definitions of temperature, depending on the field in question. For example: ◆Physics: Physical phenomena – a form of kinetic energy that indicates the degree of thermal disturbance of molecules within an object or substance ； Any parameter used to measure this phenomenon. ◆Climate: The state of the air that leads to varying degrees of heating. ◆Physiology: Body temperature in humans or animals. Therefore, temperature is an intensity quantity (a quantity used to describe the state of a system whose value does not depend on the amount of material making up the system), which makes it difficult to measure; this encourages the use of practical scales based on repeatable and easily identifiable physical phenomena in order to monitor temperature. Today, the applicable scale is the International Temperature Scale of 1990 (ITS-90). This is the result of a technical improvement that introduces the first scale for temperature measurement since 1927. This scale is obtained based on fixed temperature points (based on phase transitions of pure substances), instruments (thermometers), and interpolation or extrapolation formulas between these points. As the accuracy of the fixed-point temperature improves, this scale inevitably evolves over time, bringing the scale values closer to the thermodynamic temperature. Absolute units and relative units can define two types of units for temperature measurement: absolute units and relative units. ◆Absolute unit: An absolute unit starts from absolute zero and is theoretically the lowest possible temperature. It corresponds to the lowest possible thermal energy level that molecules and atoms in the system can have. -Kelvin (in the International System of Units) is denoted by the letter K, without any degree symbol. It was created by William Thomson. This unit was included in the International System of Units in 1954. The unit of thermodynamic temperature (kelvin) is defined based on the triple point of water at 273.16 K (or 0.01°C). ◆Relative units: Relative units, as they are compared to physical and chemical processes that always produce the same temperature. -Celsius (the International System of Units), also known as degrees Celsius, is denoted by the symbol ℃. The definition of this unit of measurement is that, when two measurements are taken at one atmosphere of pressure, 0°C is designated as the freezing point of water, and 100°C is designated as the boiling point of water. The scale is then divided into 100 equal parts, with each part corresponding to 1 degree. This scale was proposed in 1742 by the Swedish physicist and astronomer Anders Celsius. -Fahrenheit (International System of Units): This unit of measurement is based on the scale between the freezing point and the boiling point of an ammonium chloride solution. In this way, Daniel Gabriel Walther’s proposal in 1724 determined the freezing point (0°C) and boiling point (100°C) of ammonium chloride in water. He used a portable mercury thermometer to measure a mixture of equal amounts of crushed ice and ammonium chloride. This concentrated brine could achieve the lowest temperature in laboratories at that time. Then, he created a mixture of crushed ice and pure water, determined the temperature point at 30oF; later, he set it at 32oF (the melting point of ice), and then exposed the portable thermometer to boiling water vapor to obtain the temperature point of 212oF (the boiling point of water). The difference between two points is 180°F, which is divided into 180 equal parts, thus giving rise to degrees Fahrenheit. ITS-90 is defined based on Planck’s law of monochromatic radiation, above 0.65 K and up to the highest measurable temperature. The temperature measured using this scale (T90) is the closest to the thermodynamic temperature. This means that this scale is universal. ITS-90 covers multiple temperature ranges. Therefore, for each temperature range, it defines fixed temperature points and specific instruments used for making measurements and interpolations between these fixed points. Fixed temperature points correspond to the phase transitions of pure substances. For example, the freezing points of zinc, tin, or silver, the melting point of gallium, or the triple point of oxygen, mercury, or water. Fixed-point temperature (in K) Substance Type of fixed point 3–5 Helium Saturated vapor pressure 13.8033 Helium Triple point Approximately 17 Saturated vapor pressure of helium (or gas thermometer) Approximately 20.3 Saturated vapor pressure of helium (or gas thermometer) 24.5561 Neon Triple point 54.3584 Oxygen Triple point 83.8058 Neon Triple point 234.3156 Mercury Triple point 273.16 Water Triple point 302.9146 Gallium Melting point 429.7485 Indium Freezing point 505.078 Tin Freezing point 692.677 Zinc Freezing point 933.473 Aluminum Freezing point 1234.93 Silver Freezing point 1337.33 Gold Freezing point 1357.77 Copper Freezing point In particular, for the temperatures most commonly measured, ITS-90 defines: ①14 fixed points between 13.803 K (-259.346 °C) and 1234.93 K (+961.78 °C), with the interpolating instrument being a standard platinum resistance thermometer. ②Three fixed points above 1234.93 K (961.78°C), and the temperature is measured by optical pyrometry (by extrapolation using Planck’s law of radiation for one of these three fixed points). Today, temperature is the most widely measured quantity besides time. In industry, this value is particularly important. In fact, it often affects the quality of finished products. Furthermore, temperature is measured and controlled (using controllers, PLCs, or other devices) to ensure a safe process and to manage energy costs. This means you must use temperature sensors suitable for the process, and make as accurate measurements as possible depending on the operating conditions. Two types of temperature sensors, thermocouples and thermal resistors, are widely used to achieve this function. Source: Digital display instruments http://yunrun.com.cn/tech/