Back pressure
can be expressed in psia or inches of mercury. The most common way of expressing back pressure is inches of mercury ("Hg). As absolute pressure (psia) decreases, the vacuum becomes larger.
As shown here, the absolute pressure scale starts at 0 psia (0 bar). Atmospheric pressure is 14.7 psia (1.01 bar) and a perfect vacuum is 0 psia (0 bar).
However, since vacuum is usually expressed in inches of mercury ("Hg), the scale starts with atmospheric pressure being 0 inches of mercury (0 mm of mercury).