1.1.1. Introduction

1.1.2. Expressions for Pressure

1.1.3. Pressure in Coherent Systems of Units

1.1.4. Pressure Units Not Belonging to Coherent Systems

1.2.1. Pressure Scales

1.2.2. Pressure Scale Analogous to the Bel Concept

1.2.3. Pressure Scale Analogous to the Decibel Concept

1.2.4. Pressure Scale Analogous to the pH Concept

1.4.1. Terms Related to Specific Vacuum Applications and Graphic Symbols

1.4.2. Standardization of Interchangeable Parts, Measuring Methods, and Equipment Performance

References

1.1.2 Expressions for Pressure

The pressure units presently in use in vacuum science and technology fall into two categories. In the first, pressure units are grouped within coherent unit systems; the second contains units that do not belong to such systems.

Pressure P is defined as the force F which a gas or vapor exerts perpendicularly on an area A and is expressed by:

=F/A (1.1)

Pressure can also be written

=hρg (1.2)

This expression is obtained by applying the general theorem of the states of perfect liquids to the equilibrium of a liquid column under the conditions of Torricelli’s experiment. Here h is the height of the liquid column, ρ the density of the liquid in the column, and g the acceleration due to gravity at the location of the measurement.

The derived unit of pressure, expressed by Eq. (1.1), is in either of the forms

=[m][l]−1[x]−2 (1.3)

=[F][l]−2 (1.4)

depending on which of the base units length l, mass m, time l, or length l, force F of a coherent system of units have been used to express P.

The derived unit of pressure in Eq. (1.2) expressed in the same units as Eq. (1.3) does not belong to a coherent system.

Figure 1.1 illustrates the derived pressure units in coherent as well as other systems. Some of the pressure units shown, such as pascal, Torr, millimeters of mercury, and bar, are frequently used, others rarely, and some (vac, gaede) never. The vac was proposed to supersede the millimeter of mercury, which does not belong to a coherent system of units (Florescu, 1960, 1961). The unit, having a value 1 vac = 103 dyn/cm2, was not accepted, since it represents another name for the millibar which was in use at the time (Volet, 1960; Bigg, 1960, 1961). The gaede was intended to provide a smaller unit than the picotorr, supposed not to suffice for the measurement of pressures less than 10−12 Torr (Thomas et al., 1959).

The use of units other than pressure in order to characterize the degree of gas rarefaction in the ranges of high and ultrahigh vacuum has been proposed. Worell (1963), Chutbert (1964), and Chuan (1965) proposed the measurement of the molecular density of a gas, and Bobenrieth (1959) suggested the measurement of the “main” spacing between gas molecules. This spacing varies as the third root of the inverse of the number density of gas molecules.

1.1.3 Pressure in Coherent Systems of Units

A coherent system of units is one in which any quantities other than base units are derived by simple multiplication or division, without the use of a numerical factor. The base units of such a system, presumably suited to practical requirements, have to be adaptable to all scientific disciplines. Thus, for instance, in the CGS system arbitrary constants of proportionality between derived and base units have been eliminated, except for electromagnetic and electrostatic quantities. The magnitudes of the units are not practical for all cases. This drawback of the CGS system is eliminated in the Systeme Internationale (SI) units adopted in 1954 by the Conference Generale des Poids et Mesures (CGPM). The SI system, based on the MKSA system, is obviously coherent, but the pressure unit pascal is not matched to practical requirements. Therefore the use of a unit outside the SI, the bar, with submultiples millibar and microbar, is permitted. The bar is recognized by the Comite International des Poids et Mesures (CIPM) as having to be retained either because of its importance in metrology or because of its use in other specialized fields (BS 5555, 1976). Different expressions for pressure in different coherent systems of units are given in Table 1.1. The graph in Fig. 1.2 permits the conversion of torr and millibar to pascals in the range 1.013 × 105 Pa (1 atm) to 10−16 Pa.