Pressure, Vacuum, and Force

Units of pressure and vacuum should be identified as force per unit of area, and pressure units typically are mbar, psi, and kg/cm2. Pressures below 1 torr (= 1 O 3 atm) for many years were described in relationship to standard atmosphere in various ways such as  

Whether one uses millimeters, torr, microns, or millitorr, a unit of force is being determined by a unit of length. To maintain the relationship of force per unit of area used in pressure and vacuum, the SI decided on the term Pascal (one Newton per square meter) as a unit of vacuum. Regrettably, the acceptance of the Pascal has been as successful as metrics in the United States. The numeric relationship between all of these designation can be seen in Table 7.2. 

Vacuum and pressure measurements were all originally made compared to atmospheric pressure, or gauge pressure. The term psig (pounds per square inch-gauge) refers to this comparison. Absolute pressure includes atmospheric pressure (14.7 psi) and is called psia (pounds per square inch-absolute). For example, your tire pressure is 35 psig or 49.7 psia. Generally, unless otherwise identified, the lone identification psi refers to gauge pressure. 

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