The pressure increase ip produced by opening and closing bakeable ultrahigh vacuum valves of different commercial sources has been determined using mass spectrometric and other standard techniques. Typical results show:1) Dependence of Δp on geometry and material of bellow seat and gasket and their previous treatment. 2)Δp (Bellows;gasket closing, opening) Approaches after Initial operation a constant value. 3) Δp (closing)< Δp (opening for the gasket), 4) Δp (closing) same order of magnitude as Δp (opening for the bellows). 5) Pressure bursts correspond in most cases to an equivalent increase of the partial pressure of H2. 6) Δp (bellows) shows weak dependence on background pressure, Δp (gasket) is pressure independent.
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