2.1 General

This section will discuss simple dimensioning questions:

  • What size pump should I select in order to attain a specific pressure in a vacuum vessel within a given period of time?
  • How large should the backing pump be for a high vacuum pump?
  • What do I need to be aware of when pumping high gas loads?
  • What is the influence of piping on the effective pumping speed of a vacuum pump?

All of these questions can naturally not be discussed exhaustively in this chapter. Simple examples will be used and the anticipated results estimated. The technical data of the pumps and components that are used must be taken into consideration for the concrete application in question. And special literature [15, 16, 17, 18, 19] can also be useful for dimensioning.


Every physical technical parameter consists of a numeric value and a unit. The International System of Units (SI, from the French Système international d'unités) has been adopted worldwide and standards have been defined for the basic values of length (m), mass (kg), time (s), thermodynamic temperature (K), amount of substance (mol), electric current (A) and luminous intensity (cd). International regulations concerning the SI system are made by the Bureau International des Poids et Mesures (abbreviated to BIPM) and these are implemented nationally by the metrology institutions in the individual countries.

All other values are derived from these basic values. With few exceptions, the formulas that are used in this compendium contain only the physical technical values and no conversion factors whatever. This means that after expressing the values in SI units, the results will also be in SI units. Examples of SI units are 1 Pa = 1 N m-2 = 0.01 hPa for pressure und 1 m3 s-1 = 3,600 m3 h-1. Used largely throughout the following sections are popular non-SI units; however SI units will be used wherever it seems appropriate due to the required conversion. Exclusive use of SI units would avoid many errors and much conversion effort. Unfortunately, this advantage is only very slowly gaining acceptance throughout the world.