Prevakuum

Prevakuum, or pre-vacuum, is a term used in vacuum technology to describe the initial stage of evacuating a chamber or system. This stage uses roughing or forepumps to reduce air pressure from atmospheric conditions to the rough vacuum range, preparing the system for high-vacuum pumping.

The prevacuum range is typically considered to extend from ambient pressure down to around 1 Torr (approximately 133 Pa) and, in some installations, down to about 10^-3 Torr (0.13 Pa) depending on the equipment and measurement standards. The exact boundary between prevacuum and higher vacuum regions varies by device and application.

Commonly, prevacuum is produced by mechanical pumps such as rotary vane pumps, piston pumps, or diaphragm pumps. These pumps may operate alone for small systems or serve as backing pumps for turbomolecular or diffusion pumps in larger setups. The main purpose of the prevacuum stage is to rapidly remove bulk air and moisture, reduce gas load, and allow subsequent high-vacuum pumps to reach much lower pressures more efficiently.

Monitoring during prevacuum often relies on gauges suitable for rough vacuum ranges, such as Pirani or thermal conductivity gauges, with other sensors used as pressures fall toward the high-vacuum region. Proper seals, leak management, and controlling outgassing (sometimes via bake-out) are important to maintain and improve pump-down performance.

Prevacuum is a foundational concept in applications like electron microscopy, mass spectrometry, surface science, thin-film deposition, and semiconductor processing, where staged evacuation enables higher overall vacuum performance.