Vacuum describes a pressure below the surrounding atmosphere. Rough, medium, high and ultra-high vacuum divide that broad range into smaller regions. As pressure falls, the behaviour of the remaining gas changes, along with the requirements for pumps, gauges, seals and chamber construction.
What do the ranges mean?
The table follows the broad divisions used in Leybold’s references, with extreme-high vacuum shown separately as in its 2024 e-book.
| Vacuum range | Pressure |
|---|---|
| Rough | Atmospheric pressure down to 1 mbar |
| Medium | 1 to 10⁻³ mbar |
| High | 10⁻³ to 10⁻⁷ mbar |
| Ultra-high | 10⁻⁷ to approximately 10⁻¹² mbar |
| Extreme-high | Approximately 10⁻¹² mbar and below |
These boundaries are conventions. References and industries may use different divisions, particularly at the lowest pressures. Specify the number and units when discussing equipment; a range name leaves room for misunderstanding.
A pressure of 10⁻³ mbar is 0.001 mbar. Each further reduction by a power of ten means one-tenth of the previous pressure. “Higher vacuum” means lower absolute pressure. Our pressure-units guide explains the common units.
Rough and medium vacuum
At relatively high pressures, molecules collide frequently with one another. Gas movement through typical pipework behaves broadly as a continuous flow.
As pressure falls, the average distance travelled between molecular collisions increases. This is the mean free path. Medium vacuum often involves a transition towards behaviour dominated by collisions with the surrounding walls. The transition depends on passage dimensions as well as pressure.
For selection, establish how much gas needs removing, at what pressure and how quickly. A pump that reaches the target pressure may still lack the capacity to maintain it while the process releases vapour or introduces gas.
High vacuum
In typical high-vacuum equipment, molecules collide with walls much more often than with one another. This is molecular flow. A long, narrow connection can limit the speed available at the chamber even when the pump has a much higher rating.
Gas released from surfaces also becomes significant. Removing the original air is only part of evacuation.
Turbomolecular and diffusion pumps need suitable backing arrangements. Other high-vacuum technologies capture gas rather than discharge it continuously; their initial evacuation and operating requirements differ. Choose the arrangement for the gas load and process.
Ultra-high vacuum
At ultra-high vacuum, small gas loads become significant. Materials, surface preparation, joints and seals need close attention. Water and other substances retained on surfaces can prevent the intended pressure being reached.
Suitable systems may be baked to help remove gas, using a procedure compatible with every affected component. Specifying a pump with a low ultimate pressure does not establish the performance of the complete installation.
The purpose is often to reduce interference from residual gas. In surface analysis, reducing the arrival rate of molecules helps preserve the surface being measured for longer.
Choose the pressure the process needs
Lower pressure can require longer evacuation and more demanding equipment. Begin with the required operating pressure, acceptable gas composition and evacuation-time limit. If a specification says only “high vacuum”, establish the numerical requirement.
Include those details in your Girovac enquiry, together with chamber volume and a description of the process.
Technical reference: Leybold, Fundamentals of Vacuum Technology, 2016 edition, sections 1.4–1.5 and 2.2.6. Leybold’s online vacuum fundamentals provides further reading.
Additional reference: High, Ultra-high & Extreme High Vacuum: The fundamentals, June 2024, pages 3–8 and 15.
Updated September 2026. Model-specific settings and limits must be checked against the equipment’s operating instructions.