What is Vacuum?

Atmospheric pressure, the force per unit area exerted against a surface by the weight of the air molecules above that surface, can be measured and expressed in a number of ways. At sea level the standard pressure is 14.7 psia or 29.92" of mercury (Hg) or 760 mm of mercury (Torr). Because the atmospheric pressure varies with weather and altitude, the sea level pressures are used as a reference point. The term "vacuum" describes pressure that is below atmospheric pressure.

nches of mercury ("Hg) are a common measure of vacuum and are expressed in two different ways. One way is in "Hg gauge ("HgV), where the scale starts at 0"Hg (atmospheric pressure) and goes to 29.92" Hg, or full vacuum. The other way is in "Hg absolute ("HgA), where the scale is reversed such that the gauge reads 29.92" Hg at atmospheric pressure and 0" Hg at full vacuum. To relate the two methods 24" Hg gauge pressure at sea level would be 29.92 - 24 = 5.92"Hg absolute pressure.