Theoretically, it means that a container does not contain any substance (absence of all pressure, zero pressure). There is no such vacuum in the real world. A vacuum is generally defined as a state in which the air pressure inside a container is below the atmospheric pressure (101 325 Pa).
The displacement is the pumping speed, or flow rate of a vacuum pump. Generally, we measure it with unit of L/s or m³/h. It is the parameter that compensates for the rate of air leakage. It is easy to understand why a vacuum pump with a large displacement can easily achieve the desired vacuum, while a vacuum pump with a small pumping speed is slow or even unable to achieve the vacuum we want? It is impossible to have absolutely no air leakage in a pipe or a container. A large displacement compensates for the decrease of vacuum level caused by air leakage, therefore it’s more easier to get the required vacuum value.
Vacuum level, or vacuum grade, or vacuum degree, is not a professional expression at all. However, it can be found sometimes on internet. It shows the gas density with a pressure value in vacuum state. There are usually two ways to read the vacuum: one is by ultimate pressure (absolute vacuum) and the other is by gauge pressure (relative vacuum). The value read from the vacuum gauge is called the vacuum level. The vacuum level = atmospheric pressure – absolute pressure. The absolute ultimate pressure of a piston vacuum pump is about 8 000 Pa, that of a liquid ring pump is ca. 3 300 Pa, and that of a rotary vane vacuum pump is approximately 10 Pa.
Absolute Vacuum and Relative Vacuum
Ultimate pressure (absolute)
The absolute pressure is the pressure measured inside the container that is higher than the “theoretical vacuum” (0 Pa). Since we are not able to pump the internal pressure to the absolute vacuum of 0 Pa in any case, the vacuum achieved by the vacuum pump is always higher than the theoretical vacuum value. Therefore, the value of an absolute pressure is positive.
Ultimate pressure (relative)
The relative pressure is the pressure at which the measured internal pressure is lower than the atmospheric pressure. The pressure inside the container is always lower than the pressure outside because the air inside is being pumped. The value of a relative pressure or gauge pressure must be negative because the internal pressure of the container is always lower than the external one.
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