According to different usage purposes, various thermometers have been designed and manufactured. The basis for its design includes: utilizing the phenomenon of thermal expansion and contraction of solids, liquids, and gases under the influence of temperature; Under constant volume conditions, the pressure of gas (or vapor) varies with different temperatures; The role of thermoelectric effect; Resistance changes with temperature; The impact of thermal radiation, etc.

Generally speaking, any physical property of any substance that undergoes a monotonic and significant change with temperature can be used to indicate temperature and make a thermometer. Common temperature sensing liquids filled in glass precision thermometers include mercury, alcohol (often dyed blue), kerosene (often dyed red), pentane (used for low-temperature thermometers), and gallium (used for high-temperature thermometers). The most commonly used ones are mercury and alcohol. Due to the boiling point of mercury when heated above 300 ℃ in vacuum, in order to increase its boiling point and prevent oxidation, protective gases such as nitrogen, argon, helium, etc. should be filled into thermometers used for measuring high temperatures.

During the temperature measurement process of a precision glass thermometer, the change in glass volume caused by the post thermal effect will cause a change in the zero point of the thermometer. When the thermometer is not used for a long time, the glass bulb will shrink, causing the mercury level to exceed zero and leading to a rise in zero. As the storage time increases, the value of zero point rise gradually decreases until it stops. This is called the zero point * rise. The reason is that glass has a relatively relaxed structure during the manufacturing process, which has not reached equilibrium and there is also viscous flow. After a long period of time, the glass structure tends to become tighter, causing the temperature sensing bubble of the thermometer to shrink and the zero point to rise. In order to reduce the post thermal effect of glass, artificial aging (aging) and natural aging (aging) methods can be used. Artificial aging is the process of achieving equilibrium in the glass structure through heat treatment; Natural aging is the process of placing semi-finished products, whether filled or unfilled with mercury, at room temperature for six months or a year, and then calibrating the temperature scale.

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