DEFINITION OF VACUUM
THE DEFINITION of vacuum is used to describe any pressure below one standard atmosphere, or 14.7 PSIA. The internationally accepted unit of vacuum measurement is the Torr, after Torricelli. One Torr is equivalent to 1 mm of mercury ABSOLUTE; One (1) standard atmosphere will support a column of mercury 760 mm high against absolute (perfect) vacuum.
In the United States, vacuum is expressed in both inches of mercury with 29.92" indicating perfect vacuum (zero pressure) or in terms of absolute vacuum (Torr or mm) with "0" indicating perfect vacuum. This gets confusing especially when converting from the 0-30" scale to the absolute pressure scale using mm or Torr's.
= 14.7 PSIA
= 760 Torr
= 760 mm Hg
= 760,000 microns
Please see the following figure relating to how vacuum is measured