Using the phenomenon that solids, liquids and gases expand when heated and contract when cooled under the influence of temperature; Under the condition of constant volume, the pressure of gas (or steam) varies with the temperature; The role of thermoelectric effect; The resistance changes with the temperature; The influence of thermal radiation, etc.
1. Gas thermometer: Hydrogen or helium is commonly used as temperature measuring material, because the liquefaction temperature of hydrogen and helium is very low, close to absolute zero, so its temperature measuring range is very wide. This kind of thermometer has high accuracy and is mostly used for precise measurement.
2. Resistance thermometer: it can be divided into metal resistance thermometer and semiconductor resistance thermometer, both of which are made according to the characteristics of resistance changing with temperature. Metal thermometers are mainly made of pure metals such as platinum, gold, copper and nickel and alloys of rhodium, iron and phosphor bronze. Semiconductor thermometers mainly use carbon and germanium. Resistance thermometer is convenient and reliable to use, and has been widely used. The measuring range is about -260℃ to 600℃.
3. Thermocouple thermometer: It is a temperature measuring instrument widely used in industry. Composed of thermoelectric phenomena. Two different metal wires are welded together to form a working end, and the other two ends are connected with a measuring instrument to form a circuit. When the working end is placed at the temperature to be measured, when the temperatures of the working end and the free end are different, electromotive force will appear, so there is current in the circuit. By measuring the electric quantity, we can use the temperature of a known place to measure the temperature of another place. It is suitable for measuring high temperature and low turbidity between two substances with large temperature difference. Some thermocouples can measure high temperatures as high as 3000℃, while others can measure low temperatures close to absolute zero.
4. pointer thermometer: it is a kind of thermometer shaped like a dashboard, also known as thermometer, which is used to measure room temperature and is made by using the principle that metals expand when heated and contract when cooled. It uses bimetal as a temperature sensing element to control the pointer. Bimetal sheets are usually riveted together with copper sheets and iron sheets, with copper sheets on the left and iron sheets 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) driven by bimetal; On the contrary, when the temperature becomes low, the pointer deflects to the left (pointing to low temperature) driven by bimetal.