What is vacuum?

Vacuum is generally defined as emptiness of space, or a space absolutely devoid of matter.  In industrial settings, vacuum is considered a space partially exhausted (to the highest degree possible) by artificial means, such as an air pump.  Another common definition is the degree of rarefaction below atmospheric pressure.  

In 1643, Evangelista Torricelli created the first man-made vacuum in history by filling a four-foot, closed-end glass tube with mercury and turning it upside down in a dish of mercury.  Thirty inches of mercury remained standing in the tube held up by the weight of the surrounding atmospheric pressure.  The area in the glass above the mercury was a vacuum.  This, incidentally, was also the first barometer.

 When using the terms vacuum, negative pressure, suction, and so on, we mean a pressure that is lower than atmospheric pressure, which is the pressure of the weight of the air above us.  A column of air in cross section, measured from sea level to the top of the atmosphere, would exert a pressure of approximately 14.7 lb/sq in. (psi).  This is atmospheric pressure at sea level.

 Atmospheric pressure is the working force in all vacuum applications.  This means that the present barometric pressure and altitude above sea level must be taken into consideration in any vacuum application.  As altitude increases, the atmospheric pressure decreases and so does the work force available for vacuum applications.  At sea level in Boston, for example, the atmospheric pressure available is 14.7 psi.  In mile-high Denver, on the other hand, atmospheric pressure is only 12 psi.  As a practical consideration, a vacuum-lifting system that can hold 50 lb at sea level can only hold 42 lb at 5,280 ft.

 There are various expressions used to describe the pressure below atmospheric pressure (vacuum).  They include under pressure, absolute pressure, percent of vacuum, and negative pressure.

 Vacuum is measured in many different units.  They include inches of mercury (in. Hg), kiloPascals (kPa), inches of water (in. H2O), millimeters of mercury (mm Hg), and Torr.  Most commonly used in the United States is inches of mercury vacuum.  The deepest level of vacuum achievable on Earth is 29.921 in. Hg.  A perfect vacuum is one that is 30 in. Hg which would be a condition of having no molecules and is purely theoretical.  Even in outer space a few molecules can be found.  Vacuum pressure controlled by a vacuum pump can be a powerful force.  In a vacuum system, the differential between atmospheric pressure and vacuum pressure creates the ability to lift, hold, move, and perform many types of work.

 The two types of vacuum applications are sealed systems and open systems.  In a sealed or nonporous system, removing air progressively decreases the air density within the confined space.  This causes the absolute pressure of the remaining gas to drop, creating a vacuum.  To achieve a vacuum in an open or porous system, a vacuum unit must have the capacity to remove more atmosphere or air molecules than are able to leak back into the system.

 Vacuum is typically divided into three areas of application usually dependent on the required level of vacuum.

 Low-level vacuum (0 to 6 in. Hg) applications are typically those requiring high flows of air measured in standard cubic feet/min (scfm) and low force (in. Hg).  These applications include ventilation, cooling, and vacuum cleaning and are primarily serviced by blowers.

 Industrial (6 to 29.5 in. Hg) vacuum has the largest number of applications.  Vacuum applications in this range include holding, lifting, and moving.

 Scientific or process (below 29.5 in. Hg) vacuum is deep vacuum for laboratories, microchip manufacturing, ion implementation, and outer-space simulation.  Airflows are minimal and vacuum levels are usually measured in Torr.

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