VACUUM TECHNOLOGY

all processes and physical measurements carried out under conditions of below-normal atmospheric pressure. A process or physical measurement is generally performed in a vacuum for one of the following reasons: (1) to remove the constituents of the atmosphere that could cause a physical or chemical reaction during the process (e.g., vacuum melting of reactive metals such as titanium); (2) to disturb an equilibrium condition that exists at normal room conditions, such as the removal of occluded or dissolved gas or volatile liquid from the bulk of material (e.g., degassing of oils, freeze-drying) or desorption of gas from surfaces (e.g., the cleanup of microwave tubes and linear accelerators during manufacture); (3) to extend the distance that a particle must travel before it collides with another, thereby helping the particles in a process to move without collision between source and target (examples of uses are in vacuum coating, particle accelerators, television picture tubes); (4) to reduce the number of molecular impacts per second, thus reducing chances of contamination of surfaces prepared in vacuum (useful in clean-surface studies). For any vacuum process a limiting parameter for the maximum permissible pressure can be defined. It can be the number of molecules per unit volume (reasons 1 and 2), the mean free path (reason 3), or the time required to form a monolayer (reason 4). At room temperature and normal atmospheric pressure, 1 cubic foot (0.03 cubic m) of air contains approximately 7 1023 molecules moving in random directions and at speeds of around 1,000 miles per hour (1,600 kilometres per hour). The momentum exchange imparted to the walls is equal to a force of 14.7 pounds for every square inch of wall area. This atmospheric pressure can be expressed in a number of units, but until relatively recently it was commonly expressed in terms of the weight of a column of mercury of unit cross section and 760 mm high. Thus, one standard atmosphere equals 760 mm Hg, but to avoid the anomaly of equating apparently different units, a term, torr, has been postulated; one standard atmosphere = 760 torr (1 torr = 1 mm Hg). This term was replaced in 1971 by an SI unit defined as the newton per square metre (N/m2) and called the pascal (one pascal = 7.5 10-3 torr). The first major use of vacuum technology in industry occurred about 1900 in the manufacture of electric light bulbs. Other devices requiring a vacuum for their operation followed, such as the various types of electron tube. Furthermore, it was discovered that certain processes carried out in a vacuum achieved either superior results or ends actually unattainable under normal atmospheric conditions. Such developments included the blooming of lens surfaces to increase the light transmission, the preparation of blood plasma for blood banks, and the production of reactive metals such as titanium. The advent of nuclear energy in the 1950s provided impetus for the development of vacuum equipment on a large scale. Increasing applications for vacuum processes were steadily discovered, as in space simulation and microelectronics. Various kinds of devices have been developed for producing, maintaining, and measuring a vacuum. Several of the more significant types are described below.