**2. Traditional temperature sensors**

## **2.1 Expansion type temperature sensor**

The thermal expansion and contraction of most substances can be harnessed for temperature measurement purposes. Depending on the physical state of the working material, temperature sensors can be categorized into gas, glass tube liquid, and solid thermometers (such as bimetallic thermometers). One such thermometer, the mercury thermometer, is a traditional and widely used glass liquid thermometer as shown in **Figure 1** based on the thermal expansion effect. By measuring the volume change of the thermometer with changes in temperature, the mercury thermometer is able to accurately determine temperature. Due to its simple structure and lack of external energy requirements, the mercury thermometer is often used as a household thermometer. Its accuracy ranges between 0.005 and 4°C, which is primarily determined by the precision of the capillary tube manufacturing process.
