The so-called “vacuum” refers to a gaseous state where the pressure in a given space is less than 101325 Pascal (i.e. a standard atmospheric pressure around 101KPa).
The standard or unit of vacuum measurement is called vacuum level. In the international general standards and China’s current standard in force, the most commonly used unit in vacuum measurements is Pascal ( Pa.). 1 Pa is equal to 1N/㎡. The unit used in technical atmosphere is Bar.1Bar=1kgf/cm2. The unit now used less often is torr. 1Torr is defined as 1/760atm.
International general classification of vacuum levels
The international standard usually divides the vacuum into four levels: low vacuum, medium vacuum, high vacuum, and ultra-high vacuum. In absence of strict criteria, the vacuum levels are roughly divided as follows:
Low Vacuum Level （< 105~102Pa） —— The pressure difference obtained by low vacuum is used to clamp, lift and transport materials, as well as to vacuum and filtrate (e.g. vacuum cleaner, vacuum suction cups, etc.)
Medium Vacuum Level （< 102~10-1Pa） —— Medium vacuum is generally used to remove the gas or moisture that is retained and dissolved in the material, as well as for vacuum heat insulation and isolation (e.g. food vacuum freezing, vacuum drying, vacuum packaging, etc.)
High Vacuum Level （< 10-1~10-5Pa） ——vacuum smelting, vacuum coating, and manufacturing of vacuum devices can be done by using the characteristics of low density of residual gas and weak chemical action of any substance in high vacuum. A typical product is vacuum insulated glass. Vacuum insulated glass refers to the glass consisting of two or more pieces of flat glass with micro supporting spacers inside and edges sealed to create vacuum chamber in between. In principle, the vacuum level inside the vacuum insulated glass is lower than 10-1pa, so that the factors such as heat transfer by gas and sound transmission can be ignored.
Ultra-high Vacuum Level （< 10-5Pa） —— In the ultra-high vacuum state, there is almost no atoms or molecules. It can be used for simulation of a space environment, as well as researches on surface physics and surface chemistry, etc.