Subsection 3.2.5 Vacuum
A pressure below atmospheric pressure is called a vacuum. The farther below the atmospheric pressure, the greater the vacuum. Vacuum can not be made greater and greater without limit, however. At some point, the absolute pressure drops as low as possible and a perfect vacuum is achieved. A perfect vacuum is the lowest possible pressure, or the absence of all pressure. When we speak of a “high” vacuum, we are talking about a low absolute pressure.
When measuring a vacuum, there are three possible methods. When vacuum is measured starting with zero at perfect vacuum and increasing as pressure rises, then the greater the value, the smaller the vacuum. This is known as the absolute pressure scale. We can also measure pressure on a scale starting at zero at atmospheric pressure, and indicating negative values when the pressure is below atmospheric (vacuum) and positive values when the pressure is above atmospheric. This is the behavior of a standard pressure gage. And finally, we can measure vacuum on a scale starting with zero at atmospheric pressure and increasing as the vacuum increases. Most vacuum gages operate this way. On this scale no vacuum would be indicated by 0 psig, and a perfect vacuum would be indicated by approximately 14.7 psi vacuum.
The most common scale used on vacuum gages aboard ship display vacuum in inches of mercury, with the vacuum gage scale marked from 0 to 30 inches of mercury. When this gage reads zero, the pressure in the space is the same as atmospheric pressure or, in other words, there is no vacuum. A vacuum gage reading of 29.92 inches of mercury would indicate a perfect (or nearly perfect) vacuum. In actual practice, it is impossible to obtain a perfect vacuum even under laboratory conditions.
