High-Vacuum Seal

Abstract

The invention relates to an improved O-ring seal for use as a high vacuum seal. A limitation of standard O-rig seals is the permeation of water through the O-ring. Especially for instruments in which parts are kept at a cryogenic temperature, such as a cryogenic electron microscope, the presence of water in the vacuum is a problem, as this results in ice growth on the cryogenic parts. As a solution often a double O-ring seal is used, or a metal seal. Both of these solutions have severe draw-backs. The invention proposes to place the O-ring in a channel in which dry gas, such as dry nitrogen, is blown. In this way no water can permeate through the O-ring, resulting in a reduced base pressure and highly reduced ice growth.

We claim as follows: 
     
         1 . A method of providing a seal to maintain a vacuum between a first part and a second part, comprising:
 positioning an O-ring between the first part and the second part, the O-ring having a vacuum facing side and a non-vacuum facing side; and   exposing the non-vacuum facing side of the O-ring to dry gas, the dry gas reducing the amount of moisture permeating through the O-ring from the non-vacuum-facing side to the vacuum facing side.   
     
     
         2 . The method of  claim 1  in which positioning an O-ring between the first part and the second part comprises positioning an O-ring comprising a polymer. 
     
     
         3 . The method of  claim 1  in which exposing the non-vacuum facing side of the O-ring to dry gas includes exposing the non-vacuum facing side of the O-ring to gas having a water content of less than 0.1 g/m3. 
     
     
         4 . The method of  claim 1  in which exposing the non-vacuum facing side of the O-ring to dry gas includes exposing the non-vacuum facing side of the O-ring to nitrogen. 
     
     
         5 . The method of  claim 1  in which the O-ring is positioned exposing the non-vacuum facing side of the O-ring to dry gas includes providing the dry gas via a channel in which the O-ring is positioned, the channel formed in at least one of the first and second parts. 
     
     
         6 . The method of  claim 1  in which exposing the non-vacuum facing side of the O-ring to dry gas includes providing the dry gas via a channel in which the O-ring is positioned, the channel having an inlet and an outlet. 
     
     
         7 . The method of  claim 1  in which exposing the non-vacuum facing side of the O-ring to dry gas includes providing a flow of dry gas in a channel in which the O-ring is positioned. 
     
     
         8 . A vacuum apparatus, comprising:
 a first part;   a second part;   a channel for supporting an O-ring between the first part and the second part, the channel having an inlet for connecting a source of a dry gas; and   an O-ring positioned in the channel and positioned between the first part and the second part to be sealingly connected to each other, the O-ring having a vacuum-facing side and a non-vacuum-facing side;   wherein the channel provides for exposing the surface of the non-vacuum-facing side of the O-ring a dry gas to the radially outer surface of the O-ring, and an inlet to supply the dry gas through the outer channel and out to the atmosphere.   
     
     
         9 . The vacuum apparatus of  claim 8  in which the channel includes an outlet to provide a flow of dry gas from the inlet through the channel to the outlet. 
     
     
         10 . The vacuum apparatus of  claim 8  further comprising a pressure regulator in series with the channel to regulate the flow of dry gas through the outer channels. 
     
     
         11 . The vacuum apparatus of  claim 8  in which the vacuum apparatus is a particle-optical apparatus. 
     
     
         12 . The vacuum apparatus of  claim 11  in which the particle-optical apparatus comprises an electron microscope. 
     
     
         13 . The vacuum apparatus of  claim 8  in which the vacuum apparatus is a cryogenic apparatus. 
     
     
         14 . The vacuum apparatus of  claim 8  in which the apparatus is enclosed in an enclosure, and at least one outlet in communication with the outer channels is fed to the outside of the enclosure so as to avoid suffocation risks.