Explain the working principle of 8 different thermometers

1. Gas thermometers: Hydrogen gas or helium gas is often used as a temperature measurement material. Because the liquefaction temperature of hydrogen gas and helium gas is very low, close to absolute zero, it has a wide range of temperature measurement. This kind of thermometer is very accurate and used for precision measurements.

2. Resistance thermometer: It is divided into metal resistance thermometer and semiconductor resistance thermometer, and it is made according to the characteristics of resistance change with temperature. Metal thermometers are mainly used for platinum, gold, copper, nickel and other pure metals as well as neodymium iron and phosphor bronze alloys; semiconductor thermometers mainly use carbon and germanium. Resistance thermometers are easy to use and have been widely used. Its measurement range is around -260°C to 600°C.

3. Thermocouple thermometer: It is a widely used temperature measuring instrument in industry. Made using thermoelectricity. Two different wires are welded together to form a working end, and the other ends are connected to a measuring instrument to form a circuit. When the working end is placed at the measured temperature and the working end and free end temperature are different, an electromotive force occurs, and thus a current flows through the loop. Through the measurement of the electrical quantity, using the known temperature, the temperature of another place can be measured. It is suitable for the two substances with larger temperature difference, and is mostly used for high temperature and low turbidity measurement. Some thermocouples can measure temperatures as high as 3000°C, and others can measure cryogenic temperatures near absolute zero.

4. High-temperature thermometer: refers to a thermometer that is specifically designed to measure temperatures above 500°C. There are photometric thermometers, colorimetric thermometers, and radiation thermometers. The principle and construction of high-temperature thermometers are more complex and will not be discussed here. Its measurement range is from 500°C to more than 3000°C and is not suitable for measuring low temperatures.

5. Analog thermometer: It is shaped like a dashboard thermometer, also known as the thermometer, used to measure room temperature, is made of the principle of thermal expansion and contraction of the metal. It uses a bimetal as a temperature sensing element to control the pointer. The bimetal is usually riveted together with a copper sheet and iron sheet with the copper sheet on the left and the iron sheet on the right. Because the thermal expansion and contraction of copper is much more than that of iron, when the temperature rises, the copper pulls the iron piece to the right, and the pointer deflects to the right (points to high temperature) under the driving of the bimetal; As the temperature becomes lower, the pointer is deflected to the left (pointing to low temperature) driven by the bimetal.

6. Glass tube thermometer: Glass tube thermometer uses the principle of thermal expansion and contraction to achieve temperature measurement. Due to the difference in the expansion coefficient of the temperature measuring medium, the boiling point and the freezing point, our common glass tube thermometers mainly include kerosene thermometers, mercury thermometers, and red pen water thermometers. His advantages are simple structure, easy to use, relatively high measurement accuracy, and low price. The disadvantage is that the upper and lower limits of the measurement and the accuracy are limited by the quality of the glass and the properties of the temperature measuring medium. And can not be far away, fragile.

7. Pressure type thermometer: The pressure type thermometer uses the liquid in the closed container, and the gas or saturated vapor generates a volume expansion or pressure change as a signal. Its basic structure is composed of three parts: a warm package, a capillary tube, and an indication table. Pressure thermometers have the advantages of simple structure, high mechanical strength, and no fear of vibration. It is inexpensive and does not require external energy. The disadvantages are: the temperature range is limited, generally -80 ~ 400 °C; heat loss, a large response time is slow.

8. Mercury thermometer: Mercury thermometer is a kind of expansion thermometer. The freezing point of mercury is -38.87°C and the boiling point is 356.7°C. It is used to measure the temperature in the range of 0--150°C or 500°C. It can only be used as a local Supervised instrumentation. Using it to measure temperature is not only simple and intuitive, but also can avoid the error of external remote thermometer.

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