How Different Thermometers Work

1. Gas thermometer: Hydrogen or helium is mostly used as the temperature measuring substance. Because the liquefaction temperature of hydrogen and helium is very low, close to absolute zero, it has a wide temperature measurement range. This kind of thermometer has high accuracy and is mostly used for precision measurement.

2. Resistance thermometer: It is divided into metal resistance thermometer and semiconductor resistance thermometer, which are made according to the characteristic of resistance value changing with temperature. Metal thermometers are mainly used for platinum, gold, copper, nickel and other pure metals and rhodium iron, phosphor bronze alloys; semiconductor thermometers are mainly used for carbon, germanium and so on. Resistance thermometers are easy to use and reliable, and have been widely used. Its measuring range is about -260 ℃ to 600 ℃.

3. Thermocouple thermometer: It is a widely used temperature measuring instrument in industry. Made with thermoelectric phenomena. Two different metal wires are welded together to form a working end, and the other two ends are connected to a measuring instrument to form a circuit. Put the working end at the measured temperature. When the temperature of the working end and the free end are different, the electromotive force will appear, so there is a current through the loop. Through the measurement of electrical quantities, the temperature at another location can be determined using the temperature at a known location. It is suitable for two substances with large temperature difference, mostly used for high temperature and low turbidity measurement. Some thermocouples can measure high temperatures up to 3000 ° C, and some can measure low temperatures close to absolute zero.

4. High-temperature thermometer: refers to a thermometer specially used to measure the temperature above 500 ℃, there are light measuring thermometer, colorimetric thermometer and radiation thermometer. The principle and structure of the high-temperature thermometer are relatively complicated, and will not be discussed here. Its measuring range is 500 ℃ to 3000 ℃ above, it is not suitable for measuring low temperature.

5. Pointer thermometer: It is a thermometer shaped like an instrument panel, also known as a cold and heat meter. It is used to measure room temperature and is made by the principle of thermal expansion and contraction of metal. It uses a bimetallic strip as a temperature sensing element to control the pointer. The bimetallic sheet is usually riveted together with a copper sheet and an 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 obvious than that of iron, when the temperature rises, the copper sheet pulls the iron sheet to bend to the right, and the pointer deflects to the right (pointing to high temperature) under the driving of the bimetallic sheet; otherwise , The temperature becomes lower, the pointer deflects to the left (pointing to low temperature) under the driving of the bimetallic strip.

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

7. Pressure thermometer: The pressure thermometer uses the liquid in a closed container, and the volume expansion or pressure change of the gas or saturated steam after heating is used as the measurement signal. Its basic structure is composed of three parts: a thermostat, a capillary tube and an indicator. The advantages of the pressure thermometer are: simple structure, high mechanical strength, and no fear of vibration. The price is low and no external energy is required. Disadvantages: the temperature measurement range is limited, generally -80 ~ 400 ℃; large heat loss response time is slow.

8. Mercury thermometer: Mercury thermometer is a kind of expansion thermometer. The freezing point of mercury is: -38.87 ℃, the boiling point is: 356.7 ℃. It is used to measure the temperature within the range of 0--150 ℃ or 500 ℃. It can only be used as Instruments for local supervision. Using it to measure temperature is not only simple and intuitive, but also avoids the error of an external remote thermometer.