Abstract:
A gas-selective permeable membrane ( 1 ) utilisable in a leak detector for a gas, more particularly helium, comprising a sheet-like body ( 11 ) on which at least one reduced thickness area ( 15 ) is defined by removing a material from the sheet-like body. This at least one reduced thickness area ( 15 ) being permeable to at least one gas and formed so as that it is partly surrounded by a thicker and substantially gas-impermeable area ( 16 ) ensuring the structural strength of the membrane.

Description:
This application claims Paris Convention priority of Italian application no. TO2003A000032 filed on Jan. 24, 2003. 
   BACKGROUND FO THE INVENTION 
   The present invention relates to a gas-selective permeable membrane, particularly for leak detectors, and to the method for its manufacturing. 
   In the field of leak detection in ducts, tanks etc., the use of apparatuses known as “leak detectors” is widespread. Such apparatuses generally comprise a vacuum-tight chamber equipped with a selective membrane through which only a predetermined gas can flow into the chamber, when the pressure inside the chamber is made significantly lower than the outside pressure. 
   The membranes of the known leak detectors are generally made of quartz or glass with high silica content. Such membranes are permeable to helium if they are brought to a suitable temperature, typically at least 300° C. Use of such membranes has become particularly popular also because helium is a harmless, inert gas that is present in very small amounts in the atmosphere and hence is suitable for use as a test gas for leak detection. 
   An electrical resistor is generally used to bring the membrane to the temperature at which the membrane material becomes permeable. 
   The operation of the leak detectors is as follows: once a sufficient vacuum has been created in the chamber, the detector can absorb, through the selective membrane, an amount of the test gas. If the test gas is present in the surrounding environment, for instance because of a leak from a volume into which said gas has been previously introduced, the gas penetrates into the detector chamber from which it is pumped to the outside by the vacuum pump. The presence of test gas within the chamber results in an increase of the electric current drawn by the vacuum pump if compared to vacuum conditions. The increase of the electric current is signalled by a detector informing of the presence of the test gas and, consequently, of a probable leak in the volume to be tested. 
   To achieve a good sensitivity, the membrane must be very thin, since gas permeability is inversely proportional to the membrane thickness. Moreover, the membrane must resist to high temperatures, since gas permeability is proportional to the membrane temperature. 
   The membranes presently used generally consist of a capillary tube and the electrical resistor for heating the membrane is helically wound around the capillary tube. A leak detector having a capillary tube membrane is disclosed for instance in patent application No. EP 0352371 “Helium leak detector with silica glass probe”. 
   Capillary tube membranes however are fragile, and securing the capillary tube to the vacuum line is difficult. Moreover, the capillary tube shape is not satisfactory in terms of sensitivity, since it is impossible to heat the capillary tube surface wholly and uniformly to the ideal temperature for a good permeability to the test gas. This is due in part to the limitations in possibility of increasing the resistor&#39;s temperature, and the capillary tube is glued to the vacuum line. 
   Moreover, the capillary tube shape increases the chamber volume and, consequently, both the response inertia of the detector in the presence of the test gas, and the time necessary to have the detector again operating after a leak has been detected. 
   Planar membranes have been developed in the past to obviate these drawbacks. 
   These membranes have a composite structure in which a conventional metallic support layer, providing the structural strength, is associated with a thin layer of a material selectively permeable to the test gas. The support layer, which is of a gas impermeable material, has openings or windows through which the permeable layer is exposed at both faces. An example of such a membrane is disclosed in U.S. Pat. No. 3,505,180, in which a hydrogen-permeable layer of palladium is superimposed to a metal support layer provided with openings. 
   Yet, also that solution is not wholly satisfactory because of the different physical properties of the materials forming the membrane. For instance, the different thermal expansion coefficients may compromise the membrane life. Moreover, separation phenomena of the different layers forming the composite structure may occur. The latter drawback is very penalising in terms of permeability to the test gas, since it limits the temperature to which the membrane can be heated. 
   It is a main object of the present invention to provide a selective membrane for leak detectors, allowing overcoming the above drawbacks, as well as a method for producing such a membrane. 
   It is another object of the present invention to provide a selective membrane for gas detection, having high sensitivity and reliability. 
   The above and other objects are achieved by a membrane for gas detection according to the invention, as claimed in the appended claims. 
   SUMMARY OF THE INVENTION 
   The membrane of the present invention can be kept at high temperature, without risks of loss of integrity, and hence it provides an extremely sensitive and reliable means for leak detection. 
   A gas-selective membrane made of a body comprising a material that is permeable to at least one selected test gas and substantially impermeable to at least another gas. At least one reduced thickness area that is highly permeable to the selected test gas is formed on the body by removing the material from the body according to the detailed description of the method of manufacturing of the membrane. This reduced thickness area is surrounded by thicker area at least partly for structural strength of the membrane. The at least one reduced thickness area is heated by electrical resistor that partly covering the reduced thickness area. 
   An apparatus for gas leak detection having a vacuum-tight chamber with a vacuum pump connected thereto incorporates this gas-selective permeable membrane separating at least a portion of the vacuum chamber from the outside environment. 
   A non-limiting exemplary embodiment of the membrane according to the invention and of the method of manufacturing thereof is disclosed in the detailed description of the invention with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of the membrane for gas detection according to the invention; 
       FIG. 2  is a schematic cross sectional view of the membrane according to the invention; 
       FIG. 3  is a perspective view of a detail of the membrane; 
       FIG. 4  is a schematic view of a leak detector equipped with a membrane according to the invention; and 
       FIGS. 5   a ,  5   b ,  5   c  to  5   d  show the main steps of the method according to the invention for producing the membrane. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 and 2 , there is shown a membrane  1  according to the invention, comprising a body  11  in which dead cavities  13  are formed, which define an equal number of reduced thickness areas  15  on membrane  1 . 
   Body  11  of membrane  1  preferably consists of a sheet-like disc and it is made of a material selectively permeable to gases. 
   For example, quartz, glass with high silica content and palladium are materials selectively permeable to gases. 
   If membrane  1  is used for detecting helium, the material used for producing the membrane will preferably be quartz or glass with high silica content. In such case, the thickness of membrane  1  will preferably be in the range 800 to 900 μm, and reduced thickness areas  15  will be about 10 μm thick. 
   Cavities  13  are preferably circular and have in axial direction an outward-flaring conical cross section. Moreover, said cavities  13  will preferably be formed on a same face  11   a  of membrane  1 . 
   Heating means  17  are provided on the opposite face  11   b  of membrane  1 . An electric resistor adhering to the face  11   b  of membrane  1  and extending through all reduced thickness areas  15  forms heating means  17 . 
   Advantageously, in order to uniformly heat reduced thickness areas  15 , resistor  17  extends along at least a portion of the perimeter of areas  15 , preferably according to a circular path that is located substantially at an intermediate distance between the centre of each area  15  and the outer edge thereof. 
   Thus, areas  15  can be uniformly heated and the temperature required to make the material gas permeable is uniformly obtained over the whole corresponding area  15 . Moreover, resistor  17  is equipped with a pair of terminals  19  for connecting resistor  17  to an electric current source (not shown). 
   Advantageously, according to the invention, both areas  15  and resistor  17  heating them is located within a perimeter defined by an annulus  11   c  having sufficient width to ensure the effective bonding of membrane  1 , for instance by gluing, to the walls of the vacuum-tight chamber of the leak detector. Advantageously, said annulus  11   c  will be substantially “cold” with respect to areas  15 , since it is not run through by resistor  17 . Thus, the adhesion of membrane  1  to the chamber walls will not be harmed. 
   As better shown in  FIG. 3 , resistor  17  comprises a film  17   a  of a conductive material, preferably chromium or in the alternative copper or aluminium, and is bonded to membrane  1  through a layer of adhesive material  17   b , for example of titanium. Conductive layer  17   a  is moreover coated with a protecting layer  17   c , for instance of gold. 
   Referring to  FIG. 4 , there is schematically shown a leak detector, generally denoted  31 . Detector  31  comprises a vacuum-tight chamber  33  obtained by means of a hollow cylindrical body  39 , one end of which is connected to the suction port of a vacuum pump  37 , for example, an ionic pump. The other end of chamber  33  is separated from the outside environment by a gas-selective permeable membrane  1 , of the kind described with reference to the previous Figures. 
   Advantageously, said membrane  1  is bonded to cylindrical body  39  defining chamber  33  along circular rim  41  of said cylindrical body  39 . Membrane  1  is preferably bonded to said rim  41  by gluing peripheral annulus  11   c  of membrane  1 . 
   In the alternative, membrane  1  may be glued to a metal ring, subsequently brazed to rim  41  of chamber  33 . 
   Membrane  1  is preferably mounted so that electric resistor  17  faces the outside of chamber  33 . 
   Moreover, reduced thickness areas  15  are so distributed that annulus  11   c  of the membrane, attached by the gluing to rim  41 , is kept at a sufficiently low temperature in order not to harm the holding of the gluing. 
   The apparatus thus obtained is placed in the environment to be tested, into which a certain amount of test gas might have been previously introduced. An electronic supply unit  19  connected to pump  17  is arranged to detect the presence of test gas, if any, inside chamber  33  thanks to the variation in the current drawn by the pump. 
   Referring to  FIGS. 5   a  to  5   d , the major steps of the method of manufacturing a gas-selective permeable membrane are shown. 
   First, as shown in  FIG. 5   a , a sheet  51  of a material selectively permeable to the test gas, for instance amorphous quartz, is coated with a uniform layer of amorphous silicon  53 . A thin uniform layer  55  of a photosensitive material, (for instance the commercially available material Photoresist HPR504 ARCH Positive) is applied onto layer  53 . Subsequently layer  55  is covered with a lithographic mask  57  having openings  59  in correspondence with the areas of sheet  51  where a reduced thickness is to be obtained. Said mask  57  may be formed by instance by using chromium deposited on optical quartz, or a polyester film commercially available under the name “Mylar®”. 
   The above assembly is exposed to ultra-violet radiation UV perpendicular to sheet  51 , on the side where lithographic mask  57  is provided. 
   The effect of radiation is to remove material from photosensitive layer  55  in the exposed areas, i.e. in the areas corresponding to openings  59  in mask  57 . Thus the pattern of openings  59  in mask  57  is reproduced on photosensitive layer  55 . 
   At the end of the irradiation step, lithographic mask  57  is removed and sheet  51  is submitted to dry etching by means of a plasma, preferably of CF 4 , as shown in  FIG. 5   b . Plasma etching only affects amorphous silicon layer  53  in the exposed areas corresponding to openings  61  in photosensitive layer  55 , so that the pattern of the openings in photosensitive layer  55  is reproduced on amorphous silicon layer  53 . 
   Photosensitive layer  55  is then removed and sheet  51  is submitted to drilling by a ultrasonic drill  63 , as shown in  FIG. 5   c . Ultrasonic drilling only provided within the areas in sheet  51  that are left uncovered by amorphous silicon layer  53 , in correspondence with openings  65 , and creates a plurality of cavities  13  in sheet  51 : thus, an equal number of reduced thickness areas  15 , highly permeable to the test gas, will be defined. 
   A further step of the method according to the invention, shown in  FIG. 5   d , is a wet etching treatment. Sheet  51 , still partly coated with amorphous silicon layer  53 , is placed into a suitable cell  71 , suspended by means of a frame  73  on which sheet  51  is placed while being supported by ring seals  37 . Sheet  51  is immersed into a bath  75  of HF and water, by the action of which cavities  13  are finished by wet etching. 
   Once the processing of the membrane is complete, amorphous silicon layer  53  is removed and, if necessary, the heating resistor is applied. 
   According to another embodiment, the method of manufacturing the membrane is achieved by directly treating a sheet of a material selectively permeable to the test gas, for instance amorphous quartz, by ultrasounds in order to obtain a plurality of reduced thickness areas. According to this method, ultrasonic drills of extremely high precision should be utilised.