Understanding Vacuum Measurement Units. February 9, ... Inches of mercury (Hg) on a scale of 0 to 30, and the metric equivalent. 3) Millimeters of Hg on a scale of 0 to 760 (A millimeter = 1/1000 th of a meter) ... The mm of Hg was divided into 1000 smaller parts which were called microns. The word micron means a one millionth part of a meter.