OVERVIEW

In Chapter 1, the assumption that gases and gas mixtures behave ideally at low pressures (1 bar and below) was stated. (Deviation from this with large amounts of readily condensable vapours under compression near atmospheric pressure was dealt with in Chapter 3.) The ideal gas equation, expressing the relationship between the variables pressure, volume, temperature and amount (number of moles) of gas, together with the expression of pressure in terms of particle number density ( n) and Dalton's law of partial pressures, allow many calculations useful to vacuum technology to be carried out (Examples 1.1 1.5).

The kinetic theory of gases was briefly discussed. It enables the mean or thermal velocity ( c) of gas molecules at a given temperature to be obtained and gas flux to be calculated. From the latter, effusion rates, area-related condensation rates and conductances under molecular flow can be determined (see Examples 1.5 and 1.7 1.10). Calculation of collision frequency (obtained from c, n and the collision cross-section of molecules), enables the mean free path ( l) of particles to be determined. The easily obtained expression for lp is a convenient way of stating the variation of l with p (Examples 1.11 1.15).

From values of l and c, the diffusion coefficient ( D) and dynamic viscosity ( ?) can be calculated (Examples 1.16 1.19). Estimation of the Knudsen ( l/ d) and Reynold's numbers defines the nature of gas flow (viscous, molecular,...