Gas Sources and Attainable Pressure in Vacuum Systems

1 Leaks. No system can be free of leaks but leakage must be reduced to the extent that it no longer adversely affects the performance of the vacuum system. 

2 Outgassing. Any material that has been exposed to gas, particularly atmospheric air, will adsorb or otherwise entrap that gas. Under reduced pressure, not only do the inner walls and associated components contribute to the gas load but also any substrates or material being processed. 

3 Permeation. The metals and alloys normally used in vacuum systems can be regarded as impermeable to air even at high temperatures. In some circumstances, however, the permeation of H2, He, 02, air, etc. through elastomers, glasses and some metals must be considered. 

4 Process gas. In, for example, flow-reactors, sputtering systems and systems with flowing purge gas, gas is deliberately introduced. In such cases, the pumping system must deal with this gas load, in addition to 1-3 above, to maintain the pressure. 

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Gas Sources and Attainable Pressure in Vacuum Systems For 95% of the steady-state signal ... 

In Chapter 4, the gas sources and loads which are present in every vacuum system are considered. Since the gas loads from different sources have to be known or estimated to quantify the size of the pump set, their influence on attainable pressure is considered. 

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