Vacuum degree refers to the thinness of gas in vacuum state, usually expressed by pressure value. The value of vacuum degree indicates that the actual value of the system pressure is lower than the atmospheric pressure. The value expressed from the table is also called the table pressure, which is also called the limit relative pressure in the industry. In practical application, the vacuum degree usually has two expressions: absolute vacuum and relative vacuum. The value read from the vacuum gauge is called vacuum degree. Vacuum degree = atmospheric pressure - absolute pressure (atmospheric pressure is generally 101325pa, and the ultimate absolute pressure of water ring vacuum pump is 3300pa; the ultimate absolute pressure of rotary vane water ring vacuum pump is about 10Pa)
The relative pressure is how much lower the measured internal pressure is than the "atmospheric pressure". It shows that the actual value of system pressure is lower than that of atmospheric pressure. Because the air inside the container is pumped, the internal pressure is always lower than the external pressure of the container. When expressed in terms of relative pressure or gauge pressure, the value must be preceded by a minus sign, indicating that the internal pressure of the container is lower than the external pressure.
The absolute pressure is how much higher the measured internal pressure is than the theoretical vacuum (the theoretical vacuum pressure value is 0Pa). The object of comparison is the absolute vacuum pressure value of theoretical state. Due to the limitation of technology, we can not pump the internal pressure to the absolute vacuum value of 0Pa in any case. The vacuum value of water ring vacuum pump is higher than the theoretical vacuum value. So when we use absolute vacuum, there is no minus sign in front of the value.
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