Test gas leak detection using a composite membrane

Abstract

A leak detector includes a leak detector inlet to receive a test gas; a vacuum pump coupled to the leak detector inlet; a test gas sensing unit connected through a passage to the leak detector inlet; and a membrane that is permeable to the test gas disposed in the passage between the leak detector inlet and the test gas sensing unit, the membrane having a permeance to the test gas that varies by less than five percent over a temperature range of T 0 −20 K to T 0 +20 K, where T 0 is a design temperature. In addition, methods are provided for making a composite membrane that includes a semi-permeable membrane and a porous membrane. Furthermore, a reference leak, which includes a composite membrane is provided.

1 . A leak detector comprising:
 a leak detector inlet to receive a test gas;   a vacuum pump coupled to the leak detector inlet;   a test gas sensing unit connected through a passage to the leak detector inlet; and   a membrane that is permeable to the test gas disposed in the passage between the leak detector inlet and the test gas sensing unit, the membrane having a permeance to the test gas that varies by less than five percent over a temperature range of T 0 −20 K to T 0 +20 K, where T 0  is a design temperature.   
   
   
       2 . The detector as defined in  claim 1 , wherein the permeance of the membrane has zero slope as a function of temperature at temperature T 0 . 
   
   
       3 . The leak detector as defined in  claim 1 , wherein the membrane comprises a silica layer on a porous glass substrate. 
   
   
       4 . The leak detector as defined in  claim 1 , wherein the membrane comprises a silica layer on a porous alumina substrate. 
   
   
       5 . The leak detector as defined in  claim 1 , wherein the membrane comprises a composite membrane including a semi-permeable membrane in series with a porous membrane. 
   
   
       6 . The leak detector as defined in  claim 1 , wherein the membrane has a permeance to the test gas that varies by less than one percent over the temperature range of T 0 −20 K to T 0 +20 K. 
   
   
       7 . A method for making a composite membrane, comprising:
 selecting a semi-permeable membrane having a permeance S to a test gas and a porous membrane having a permeance P to the test gas to satisfy an equation   
     
       
         
           
             
               
                 
                   
                     
                       P 
                       2 
                     
                      
                     
                       
                          
                         S 
                       
                       
                          
                         T 
                       
                     
                   
                   + 
                   
                     
                       S 
                       2 
                     
                      
                     
                       
                          
                         P 
                       
                       
                          
                         T 
                       
                     
                   
                 
                 = 
                 0 
               
               , 
             
              
             
                 
             
           
         
       
       where T is temperature and wherein the equation is evaluated at a temperature T 0 ; and 
       forming a composite membrane including the selected semi-permeable membrane and the selected porous membrane. 
     
   
   
       8 . The method as defined in  claim 7 , wherein selecting comprises selecting materials and thicknesses of the semi-permeable membrane and the porous membrane to satisfy the equation. 
   
   
       9 . The method as defined in  claim 7 , wherein forming a composite membrane includes forming a silica layer on front and back sides of the porous membrane. 
   
   
       10 . The method as defined in  claim 7 , further comprising incorporating the composite membrane into a passage of a leak detector between an inlet and a test gas sensing unit. 
   
   
       11 . The method as defined in  claim 7 , wherein selecting comprises selecting pore sizes of the porous membrane to satisfy the equation. 
   
   
       12 . The method as defined in  claim 7 , wherein selecting comprises selecting the semi-permeable membrane and the porous membrane such that the net permeance N of the semi-permeable membrane and the porous membrane has zero slope as a function of temperature at the temperature T 0 . 
   
   
       13 . The method as defined in  claim 7 , wherein the semi-permeable membrane comprises a silica layer and wherein the porous membrane comprises a porous glass substrate. 
   
   
       14 . The method as defined in  claim 7 , wherein the semi-permeable membrane comprises a silica layer and wherein the porous membrane comprises a porous alumina substrate. 
   
   
       15 . The method as defined in  claim 7 , wherein selecting comprises selecting a material and a thickness of the semi-permeable membrane and selecting a material, a thickness and a pore size of the porous membrane. 
   
   
       16 . The method as defined in  claim 7 , wherein selecting comprises adjusting parameters of the semi-permeable membrane and the porous membrane so that the composite membrane has a permeance to the test gas that varies by less than five percent over a temperature range of T 0 −20 K to T 0 +20 K. 
   
   
       17 . The method as defined in  claim 7 , wherein selecting comprises adjusting parameters of the semi-permeable membrane and the porous membrane so that the composite membrane has a permeance to the test gas that varies by less than one percent over a temperature range of T 0 −20 K to T 0 +20 K. 
   
   
       18 . A reference leak comprising:
 an enclosure having an opening;   a composite membrane sealing the opening, the composite membrane including a semi-permeable membrane and a porous membrane; and   a test gas sealed within the enclosure.   
   
   
       19 . The reference leak as defined in  claim 18 , wherein the composite membrane has a net permeance with zero slope as a function of temperature at a temperature T 0 , where T 0  is a design temperature. 
   
   
       20 . The reference leak as defined in  claim 18 , wherein the composite membrane has a permeance to the test gas that varies by less than five percent over a temperature range of T 0 −20 K to T 0 +20 K, where T 0  is a design temperature.