What is vacuum?
Vacuum — A state of a specified space where the gaseous pressure is lower than the atmospheric pressure. In other words, the density of gaseous molecules in this space is lower than that of the atmospheric pressure in the region. In a standard state (0℃, 101325Pa), the gas molecules density is 2.6870*10E25/mE3.
A space totally devoid of gas is called Absolute Vacuum. Absolute Vacuum only exists theoretically.
Vacuum degree—Used to measure how sparse gas is in a vacuum state according to the vacuum technology.
> Vacuum degree is typically denoted by gaseous pressure value.
> Absolute pressure (absolute vacuum degree) values are always positive, because "absolute vacuum" is decoded as 0. The lower the absolute pressure value is the closer to absolute vacuum, and the higher the vacuum degree.
> Absolute pressure plus Vacuum degree equals Ambient atmospheric pressure.
> Relative pressure (surface pressure or relative vacuum degree): Because the ambient atmospheric pressure is decoded as 0, the pressure less than ambient atmospheric pressure is called negative pressure. The bigger the negative value is, the higher the vacuum degree.
> Relative pressure plus Vacuum degree equals 0