Vacuum is a term used to describe an area, zone, or compartment containing less pressure than atmospheric pressure.
Vacuum is measured in inches of mercury (Hg) in the United States. There are two different scales to measure vacuum.
One scale is referred to as "inches of mercury gauge vacuum" ("HgV), where the measuring range starts at 0 inches of mercury (atmospheric pressure) and goes up to 29.92 inches of mercury, known as perfect vacuum (not achievable on Earth).
The alternative scale is "inches of mercury absolute vacuum" ("HgA), which reverses the "HgV scale, having it instead read 29.92 inches of mercury at atmospheric pressure and 0 inches of mercury at perfect vacuum.
Here's an example of the relationship between inches of mercury gauge and inches of mercury absolute:
24 inches of Hg gauge = 5.92 inches of mercury absolute.
Calculation: 29.92 - 24 = 5.92
It is very important to determine what scale you are using, meaning gauge or absolute. A misunderstanding can result in large errors.
The unit "Torr" is used when working in higher vacuum ranges (low absolute pressure). 1 Torr equals 1 millimeter (mm) of mercury in absolute pressure.
Considering one linear inch equals 25.4 mm, and atmospheric pressure (at sea level) is 29.92 inches of mercury, then using the equation 29.92 inches x 25.4 mm = 760 Torr. 760 Torr is 0 vacuum, while 0 Torr is perfect vacuum.
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