Evacuation of a chamber in the medium vacuum region

In the rough vacuum region, the volume of the vessel is decisive for the time involved in the pumping process. In the high and ultrahigh vacuum regions, however, the gas evolution from the walls plays a significant role. 

Po is the pressure at the beginning of the pumping process p is the desired pressure 

In contrast to equation 2.33b this equation does not permit a definite solution for S, therefore, the effective pumping speed for a known gas 

In practice, therefore, the following method sufficiently high pumping speed  

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