Editing Vacuum (section)

=== Measurements relative to 1&nbsp;atm ===
[[File:McLeod gauge 01.jpg|upright|thumb|A glass McLeod gauge, drained of mercury]]
The [[SI]] unit of pressure is the [[pascal (unit)|pascal]] (symbol Pa), but vacuum is often measured in [[torr]]s, named for an Italian physicist Torricelli (1608–1647). A torr is equal to the displacement of a millimeter of mercury ([[mmHg]]) in a [[manometer]] with 1&nbsp;torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured on the [[barometer|barometric]] scale or as a percentage of [[atmospheric pressure]] in [[bar (unit)|bars]] or [[atmosphere (unit)|atmospheres]]. Low vacuum is often measured in [[millimeters of mercury]] (mmHg) or pascals (Pa) below standard atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure.

In other words, most low vacuum gauges that read, for example 50.79&nbsp;Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of 0&nbsp;Torr but in practice this generally requires a two-stage rotary vane or other medium type of vacuum pump to go much beyond (lower than) 1&nbsp;torr.