Glossary
Vacuum levels
Vacuum levels are broadly defined as
- Rough vacuum: 760 Torr to 1 Torr
- Medium vacuum: 1 Torr to 10-3 Torr
- High vacuum: 10-3 Torr down to 10-7 Torr
- Ultra high vacuum: 10-7 Torr and below
Rough to medium vacuum
Positive displacement pumps are the most effective solution for low vacuums. The effective range of rubber-sealed and plastic-sealed piston pumps is around 500mbar down to 10mbar, whilst a scroll pump might be expected to reach in the order of 0.1mbar. A good rotary vane pump, under ideal conditions, might reach down as far as 0.001mbar. Multi-stage vacuum pumps can increase the speed of evacuation, but the base pressure is still ultimately limited by backstream flows through the mechanical seals, making them unsuitable for high vacuum applications.
High vacuum
Whilst positive displacement pumps will be more than adequate for achieving a rough to medium vacuum (with the base pressure dependent upon the technology chosen), the mechanical limitations inevitably mean than only this partial vacuum can ever be achieved. Using these pumps as backing pumps for molecular pumps, however, allows vacuums to around 0.00001mbar to be achieved, the crossover region between high vacuum and ultra high vacuum. The numbers of pumps, and the combinations of technology will depend upon the evacuation rate and base pressure that need to be achieved, as well as on the gas being removed. However, even vacuum to 0.00001mbar can be achieved with off-the-shelf packages, given attention to the problems of outgassing, etc.
Ultra high vacuum
Ultra high vacuum is the realm of nuclear research and particle acceleration, and can generally only be achieved with custom-built equipment. A typical system might employ three or more different pump technologies to achieve the required vacuum.
