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Clean ultrahigh vacuum without traces of atmospheric gasses can be obtained in all metal chambers with soft metal gaskets only. However it is much easier to handle demountable joints with elastomeric gaskets. This is a cost effective solution where frequent opening of a system is needed. Elastomeric gaskets are also used in ultrahigh vacuum valves when high number of closing cycles is required.
Our experience is that there are always remains of atmospheric gasses present in residual atmosphere even if we use only a single elestomeric seal. This can often mislead to a conclusion that there is an air leak present in the system.
Most common gasket material is Viton. Very limited data on permeability of Viton can be found in vacuum literature. For modeling of time dependent phenomena related to outgassing of Viton diffusion constant has to be known also. We were not able to find such data in literature; therefore we performed extensive study of air diffusion in a Viton gasket. Diffusion constant, solubility and permeation rate of three main components of dry air (N2, O2 and Ar) were measured in a temperature range from room temperature up to 150ºC. Details of the experimental setup and measurement method will be presented. Calibration and overall uncertainty of the system will be discussed also. |