Understanding Vacuum Pressure: Uncovering the Units and Measurement Methods

When discussing vacuum pressure, it is important to understand that the unit of vacuum pressure is typically measured in terms of pressure measurements, such as Pascals (Pa), millimeters of mercury (mmHg), Torr, bar, inches of mercury (inHg), and more. These units help us quantify the absence or deficiency of atmospheric pressure within a system.

Common Pressure Units

In vacuum applications, pressure is measured below atmospheric pressure. Commonly used units include:

Pascals (Pa): The International System of Units (SI) unit of pressure. Millimeters of Mercury (mmHg): Widely used in medical, laboratory, and meteorological settings. Torr: Equivalent to mmHg, 1 Torr is defined as 1 mmHg. Bar: Approximately equal to atmospheric pressure (101325 Pa). Inches of Mercury (inHg): Commonly used in meteorology and aviation.

Vacuum Units and Measurement

Vacuum units can be defined in two ways:

In common language, vacuum is taken as negative pressure below standard atmospheric pressure. The units commonly used here are mm of Hg or inHg, with these referring to the column of Hg supported by the gas/air in a Torricelli tube inverted in a free mercury dish by air around it. In vacuum technology, absolute pressure of air or gas is measured, either with or without partial vapor pressure of components. Perfect vacuum means zero absolute pressure (0 Torr).

Units of Measurement in Vacuum Technology

For rough vacuum, the common unit adopted is mm Hg, referred to as Torr. For finer vacuums, millitorr (MilliTorr) or microtorr (MicroTorr) are used, with one being of the order of 10^-3 Torr and the other 10^-6 Torr, respectively. At the extreme, one can measure vacuum pressures of the order of 10^-12 mmHg.

Perfect vacuum, theoretically, means the total absence of air or gas. While this is never observed in practice and is never achieved on Earth, in the universe, space can have a vacuum of 10^-15 to 10^-18 Torr. The closest we can get in laboratories is around 10^-18 Torr.

Abuse of Terminology

There is confusion in using the term vacuum pressure. In vacuum technology, it is recommended not to use this term. Instead, one should either call it vacuum or pressure. Vacuum is the absence of air or gases, achieved by removing air from a system to create a pressure below atmospheric conditions, measured as negative pressure relative to atmospheric pressure.

Most Bourdon vacuum gauges function based on this principle, conventionally carrying a negative sign when measuring downward below atmospheric pressure. An absolute vacuum is theoretically the complete absence of air or gas; in practice, it is measured from a zero point, with 760 mmHg (Torr) corresponding to atmospheric pressure and 0 Torr representing absolute vacuum.

While perfect vacuum (absolute zero) is never achieved in practice, the closest we can get in laboratories is around 10^-6 to 10^-12 Torr. Significant industrial processes, such as semiconductor manufacturing, involve the use of vacuum, emphasizing the importance of accurate pressure and vacuum measurements.