A Beginner’s Guide: How to Evaluate Vacuum Level of a Vacuum Pump

vacuum gauge

This article aims to guide you on comprehending the vacuum level of a vacuum pump effectively.


In theory, a vacuum refers to a state where a container is entirely devoid of any substance (zero pressure). However, such an absolute vacuum doesn’t exist in the real world. In practical terms, a vacuum is commonly defined as a state in which the air pressure inside a container is below the atmospheric pressure (101,325 Pa).


The displacement of a vacuum pump refers to its pumping speed or flow rate, typically measured in units of L/s or m³/h. This crucial parameter takes into account the rate of air leakage. It’s easy to comprehend why a vacuum pump with a large displacement can effortlessly achieve the desired vacuum, while a pump with a smaller pumping speed may be slow or even unable to attain the desired vacuum level.

In reality, it is practically impossible to achieve absolutely zero air leakage in a pipe or container. Hence, a vacuum pump with a substantial displacement compensates for the reduction in vacuum level caused by air leakage. This inherent advantage makes it easier to reach the required vacuum value efficiently.

Vacuum Level

The term “vacuum level,” also referred to as “vacuum grade”, is not a strictly professional expression. However, it can occasionally be encountered on the internet. It denotes the gas density indicated by a pressure value in a vacuum state. Vacuum measurements are generally interpreted in two ways: ultimate pressure, which refers to absolute vacuum, and gauge pressure, representing relative vacuum.

To determine the vacuum level, you subtract the absolute pressure from the atmospheric pressure. For instance, the vacuum level can be calculated as atmospheric pressure – absolute pressure.

The absolute ultimate pressure of different types of vacuum pumps varies. For a piston vacuum pump, it is approximately 8,000 Pa, for a liquid ring pump it’s around 3,300 Pa, and for a rotary vane vacuum pump, it is approximately 10 Pa.

Absolute Vacuum and Relative Vacuum

  • Ultimate pressure (absolute)

The absolute pressure is the pressure measured inside a container, and it is always higher than the “theoretical vacuum” value of 0 Pa. In practice, achieving an absolute vacuum of 0 Pa is impossible with any vacuum pump. Hence, the vacuum attained by the pump is always higher than this theoretical vacuum value. As a result, the value of absolute pressure is always positive.

  • Ultimate pressure (relative)

The relative pressure, also known as gauge pressure, is the pressure measured inside a container and is always lower than the atmospheric pressure. This difference occurs because the vacuum pump is actively removing air from the container, leading to a lower pressure inside. As a consequence, the value of a relative pressure or gauge pressure must always be negative, signifying that the internal pressure of the container is consistently lower than the external pressure.

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