ABSTRACT
Although the devices developed to measure vacuum are frequently referred to as pressure gauges, in many applications of vacuum technology the use of the concept of pressure as a force per unit area is not relevant. In just a few low-vacuum applications, such as vacuum molding, pressure is a quantity of explicit importance because they exploit the mechanical force created by the difference between atmospheric pressure and a pressure of about 100 mbar. And, of course, all vacuum vessels operating at essentially zero internal pressure must be constructed to withstand the large forces placed on them by the external atmosphere. But for most applications of vacuum, it is n, the number density of molecules in the residual gas, that makes a vacuum suitable for its intended purpose by determining, for example, the scattering of accelerated particles out of the beam in a high-energy particle physics experiment, the mean free path λ in a vacuum-coating application, or the residual gas impact rate J on surfaces in a surface science experiment. We know, however, that n and p are related by the simple equation p = nkT, and with the understanding that a fixed temperature of reference is taken, room temperature, 295 K, the familiar and convenient pressure p serves as a measure of vacuum over its whole range.
