vacuum The degree of vacuum inside an enclosure is expressed in terms of the pressure exerted by the gas inside the enclosure. When the pressure is ~ 10-5torr, it is called low vacuum. A pressure in the range 10-5 torr down to 10-7 torr is called high vacuum and below 10-9 torr, ultrahigh vacuum.
Low vacuum can be achieved by mechanical rotary pumps. Using diffusion pumps pressure ~ 10-6 torr can be obtained. For still lower pressures sorption pumps are used.