The first band below atmosphere, usually taken as 1000 down to 1 mbar (roughly 750 to 0.75 Torr). Molecules collide with each other far more often than with the walls, so gas moves in viscous flow and pumps behave like ordinary compressors. Rotary vane, scroll, dry screw and Roots pumps work here, and most industrial jobs such as vacuum packaging, degassing of liquids and vacuum lifting never leave this range.
Load lock only needs rough vacuum, the scroll gets it down to a few torr in under a minute.
Also heard as
- low vacuum
- coarse vacuum
Do not mix it up with
- Rough out: Rough vacuum is a pressure range, while roughing out is the act of pumping a chamber down through that range to crossover.
Related terms
Medium vacuum
The band between about 1 and 10⁻³ mbar, where gas flow shifts from fluid-like to molecule-by-molecule.
Viscous flow
Flow regime in which molecules collide mostly with each other, so the gas moves as a continuous fluid.
Roughing pump
Pump that takes a chamber from atmosphere down to the pressure where the high vacuum pump can take over.
Rough out
To pump a vented chamber from atmosphere to crossover with the roughing pump, before the high vacuum valve opens.