Working principle of five common thermometers
1. Liquid expansion thermometer is made according to the nature of liquid's expansion and contraction. The most common is the glass tube liquid thermometer, which uses the principle that the volume of liquid in the glass tube expands with the increase of temperature It consists of liquid memory, capillary, scale and safety bubble. Liquid can be: Mercury, alcohol, toluene, etc.
2. The solid expansion thermometer is made of two materials with different coefficient of linear expansion. Common types are: stem thermometer (generally made of solid materials with large expansion coefficient), bimetal chip thermometer (its temperature sensing element is made of two kinds of metal sheets with different expansion coefficient firmly combined).
3. Pressure type thermometer is a kind of mechanical instrument which can judge the temperature value by measuring the pressure of the working medium in the closed volume. The working medium of pressure thermometer can be gas, liquid or steam。
4. Thermocouple thermometers are widely used in industrial production. Two conductors of different components (called thermocouple wires or thermo-electric poles) are connected to a composite circuit at both ends. When the temperature of the junction is different, electromotive force will be generated in the circuit. This phenomenon is called thermo-electric effect, and this electromotive force is called thermo-electric potential. The thermocouple uses this principle to measure temperature.
5. Resistance thermometer; with the increase of temperature, the resistance of conductor or semiconductor will change. There is a single functional relationship between temperature and resistance. The method of measuring temperature by using this functional relationship is called thermal resistance thermometer. The conductor or semiconductor used for temperature measurement is called thermal resistance.