Abstract:
A testing device for measuring leakage from packages, the device having an internal chamber for receiving the package and a cover for sealing the package into the chamber. A gas passage from the chamber to a gas detector carries gas to be measured, and an air passage into the chamber provides a purging passage, and a gas inlet passage though the cover admits gas into the upper part of the chamber.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for measuring the amount of gas that leaks through a package, including the package seals. A thin film membrane typically is adhesively attached over an open top of such packaging, the membrane having a characteristic of being relatively porous to the passage of certain gases but a barrier to the passage of bacteria. The present invention also relates to a method for measuring the amount of gas that leaks through sealed packages of the above-described types. More specifically, the invention relates to gas leakage through packages that have been sealed by a cover of porous material fabricated as a mat of polyethylene fibers. This material acts as a permeable membrane to gases, but an impermeable membrane to bacteria. The membrane comprises a layer having pores which provide a tortuous path to the passage of bacteria; the material is commonly sold under the trademark designation “TYVEC.” The packages which use this material are typically semi-rigid plastic cases which protect medical devices and appliances after manufacture and before actual use. 
     The invention relates to U.S. Pat. No. 5,939,619, issued Aug. 17, 1999, entitled “Method and Apparatus for Detecting Leaks in Packaging,” and U.S. Pat. No. 6,050,133, issued Apr. 18, 2000, entitled “Method and Apparatus for Detecting Leaks in Packaging.” Both of these patents are owned by the assignee of the present invention. The present invention also relates to co-pending application entitled “Method for Measuring Gas Leakage From Sealed Packages,” Ser. No. 09/676,621, filed Oct. 2, 2000, and owned by the assignee of the present invention. The present invention permits a measurement of leakage that is wholly non-destructive to the package. 
     Gas sterilization is widely used for medical devices that must be sterile at the time of use, but cannot be subjected to sterilization by the application of high temperatures. Examples of such medical devices include cardiac pacemakers and catheter-based monitoring devices such as blood pressure probes. Typically, the medical device is sealed within a package that is permeable to gases but impermeable to bacteria. The package is then placed in a gas sterilization chamber, and a sterilizing gas such as ethylene oxide is introduced into the gas-permeable package to achieve sterilization. The sterilizing gas is then removed from the package, leaving the interior of the package sterile and non-toxic. 
     In a typical design, the medical device is placed within a thermoformed rigid plastic tray equipped with a flat sealing flange. A sheet of gas-permeable membrane, such as DuPont TYVEK® 1073-B (medical grade) brand membrane, which is available from E.I. duPont de Nemours &amp; Co., is then sealed to the sealing flange, typically by using an adhesive. The integrity of the seal is critically important to maintaining sterility. Leaks can result from incorrect setting of parameters in the automated sealing process, or from physical defects such as burrs on the face of the sealing equipment or pin holes in the plastic tray. 
     According to the known practice described in the prior art patents listed herein, a temporary barrier is formed over the gas-permeable layer, wherein the temporary barrier has an aperture with the gas-permeable layer to temporarily seal the gas-permeable layer except where the aperture is located. A tracer gas is applied under low pressure through the aperture so that it can enter into the interior chamber of the package. The entire package is placed into a larger sealed second chamber, and the concentration of tracer gas in the second chamber is measured, outside the package, to thereby measure the amount of tracer gas which has leaked through the package, presumably via leaks in the sealing flange, although leakage can also occur through pinhole defects in the plastic tray itself. 
     The methods described in the foregoing patents provide very accurate measurements and evaluations for sealed packages, under controlled conditions. However, in many applications, it is not necessary to achieve a high degree of accuracy in the leakage measurement, but is desirable to provide a quick evaluation of leakage as a production line test, to determine whether packages are leaking excessively. Excessive leakage is defined in terms of leakage beyond a predetermined range of acceptability, as a pass/fail parameter, and the precise degree of leakage does not need to by quantified. Methods for making this type of determination should produce results more quickly and at a lesser cost. 
     The present invention provides a pass/fail test which can be quickly performed at considerably less cost in terms of test equipment and testing time, because it relies on measurement of internal package gas pressure, and specifically pressure drop, caused by leakage of gas from within the package. The method of the present invention can be performed in a short time, perhaps 30-60 seconds, using very much less expensive equipment than prior art methods. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an apparatus for measuring the gas leakage through a package which has been prepared to hold a sterilized object in isolation from ambient air and potentially harmful gases. The invention also comprises a method for making leakage measurements using the aforementioned apparatus, comprising the steps of 
     It is a principal object and advantage of the present invention to provide a method for measuring gas leakage from a package. 
     It is another object and advantage of the present invention to provide a leakage measurement apparatus which does not degrade or otherwise harm the package being measured or its contents. 
     Other and further objects and advantages of the invention will become apparent from the following specification and claims and with reference to the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a symbolic diagram of a system for testing for leakage, including the present invention; 
     FIG. 1B shows the type of package which is preferably tested with the invention, after the package has been prepared for testing; 
     FIG. 2 shows an isometric view of the testing cell of the present invention; 
     FIG. 3 shows an isometric exploded view of the components which form the chamber; 
     FIG. 4 shows a top view of the testing cell, with cover removed; 
     FIG. 5 shows a left side cross section view of the testing cell of FIG. 4; 
     FIG. 6 shows a left side view of the testing cell with passages shown in dotted outline; and 
     FIG. 7 shows an underside isometric view of the cover. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing figures, it should be understood that the test apparatus shown in each figure illustrates components which are not necessarily drawn to scale. In the figures, like reference characters refer to the same or functionally similar parts of the respective devices illustrated in each of the figures. 
     Referring first to FIG. 1B, a typical package of the type intended for testing with the present invention is shown, comprising a sealed package  10 . The sealed package  10  includes a tray  11 , a sealing flange  14  with an adhesive sealant  16  applied thereto and a gas-permeable sheet or membrane  18  affixed to the sealing flange  14  by the sealant  16 , all enclosing an interior chamber  12 . The sealant  16  forms a sealing bead around the perimeter of the flange  14  and the perimeter of the gas-permeable sheet or membrane  18 . The gas-permeable membrane  18  is a porous membrane formed of a thermoplastic or paper that allows passage through the membrane of a gas but not larger particles, such as dust, bacteria, etc. In one embodiment of a sealed package that the present invention is usefully applied to, the gas-permeable membrane  18  is a mat of polyolefin fibers available from duPont under the trademark TYVEK. Typically, the gas-permeable membrane  18  has a thickness in the range of between about 0.127 and 0.254 millimeters. In preparing the package  10  for testing, an adhesive tape  61  is applied about the upper edges of the membrane  18 , for purposes which will be hereinafter described. 
     Referring to FIG. 1A, the leak detection apparatus  20  has a housing  22  which encloses a chamber  24 , of sufficient size for holding the package  10 . The housing  22  is made from a non-gas-permeable material. A cover  26  can be clamped over the housing  22 , to tightly seal the adhesive tape  61  against an upper edge of the housing  22 , using an O-ring  51 , after the package  10  is placed into the chamber  24 . Cover  26  has an inlet gas conduit  27  connected to a gas source  28  via a solenoid valve  39  and a needle valve  37 , as will be more described hereinafter. A pair of outlet gas conduits  25   a ,  25   b , are connected through cover  26 , and are typically discharged to atmosphere. The inner end of gas conduit  27  is connected to a suction cup  60 . Suction cup  60  has a passage therethrough, and has a lower suction surface for adhering against the outside of membrane  18  when the cover  26  is closed and clamped against the housing  22 . 
     The housing  22  also has an outlet  30  which is connected to a suitable detector  34 , via a conduit  32 ; FIG. 1A illustrates the detector as infrared (IR) sensor  34 , which is particularly useful for detecting carbon dioxide gas. IR sensor  34  has an electrical output line  35  connected to an amplifier  36 , and amplifier  36  is connected to a threshold detector  38 . Threshold detector  38  is connected to suitable alarm circuits, as desired. IR sensor  34  also has an outlet gas conduit connected to a gauge  40 , which is connected via a flow control valve  42  to a vacuum pump  44 . Vacuum pump  44  exhausts to ambient air. 
     Housing  22  also has a gas inlet  23  which is connected via internal passages in housing  22  to a plurality of outlets along an upper edge of housing  22 , which will be seen more fully hereinafter. Ambient air is conveyed via a solenoid valve  46  to the gas inlet  23 , as will be explained more fully hereinafter. 
     FIG. 2 shows an isometric view of the testing cell of the present invention mounted to a base plate  21 . A cover  26  is hingedly mounted along an edge  29  to housing  22 . Cover  26  has a handle  31  for raising and lowering the cover. A pair of locking pedestals  56 ,  57 , are affixed to base plate  21  adjacent to the cover position, and the locking pedestals have a ball detent mechanism  58 ,  59  which is spring loaded, and which captures the cover  26  in a closed position and which hold the cover  26  securely against the top of housing  22  during testing. A passage  27   a  passes through cover  26  and the inside suction cup  60 , as described earlier, and is adapted for connection to gas conduit  27 ; a passage  25   a ′ and a passage  25   b ′ each pass through the cover  26 , and are adapted for connection respectively to outlet gas conduits  25   a ,  25   b.    
     FIG. 3 shows an exploded view of housing  22 , showing each of the layers that comprise housing  22 . The layers are formed from stacked sections which are adhesively bonded together to form a sealed internal chamber  24  (see FIG.  1 A). These sections include a lower grooved section  22   a , a bottom passages section  22   b , a spacer section  22   c , an upper grooved section  22   d , and an upper lip section  22   e.    
     The lower grooved section  22   a  has a plurality of grooves  48  radiating outwardly from outlet passage  30 , and a through passage  23 . The bottom passages section  22   b  has the through passage  23 , and a plurality of through passages  49 , each passage  49  being aligned proximate an end of one of the grooves  48 . Each passage  49  opens through the surface of passages section  22   b , into the lower region of chamber  24 . A spacer section  22   c  is aligned with the bottom passages section  22   b , and also contains the through passage  23 . The upper grooved section  22   d  has a peripheral groove  53  which runs about the perimeter of upper grooved section  22   d  and is in gas flow communication with a passage  52  located proximate one end of section  22   d . Passage  52  connects to through passage  23 , described earlier. The upper lip section  22   e  has an inwardly directed lip  55 , and lip  55  has a plurality of pass-through passages  54  which are aligned above the peripheral groove  53  of upper grooved section  22   d . Finally, upper lip section  22   e  has a raised flange  50  which is dimensioned slightly larger than the outer dimensions of the flange  14  of package  10  (see FIG.  1 B), so that the package  10  can fit inside the raised flange  50 . 
     FIG. 4 shows a top view of housing  22 , with cover  26  removed, and FIG. 5 shows a cross section taken along the lines  4 — 4  of FIG.  4 . In particular, FIG. 5 shows housing  22  is formed of stacked sections, including lower grooved section  22   a , bottom passages section  22   b , spacer section  22   c , upper grooved section  22   d , and upper lip section  22   e . This construction provides an important feature of the present invention, which is to expose the entire lower volume of chamber  24  to access to outlet  30  approximately simultaneously. FIG. 5 also illustrates the gas passages associated with gas inlet  23 . An internal passage  52  passes through the sections, and opens into the peripheral groove  53 , which extends about the entire upper edge of grooved section  22   d . A plurality of small holes are drilled through the lip  55 , providing passages for gas or ambient air entering inlet  23  to be conveyed about the entire peripheral edge formed by lip  55 , and upwardly into the upper volume of chamber  24 . 
     FIG. 6 shows a side view of housing  22 , with the internal passages shown in dotted outline. FIG. 7 shows an isometric view of the underside of cover  26 ; an internal cavity in the underside of cover  26  is surrounded by an O-ring  51 , with dimensions so as to permit O-ring  51  to contact against the top surface of raised flange  50 , and thereby to provide a seal between the cover  26  and the housing  22 . The gas conduit passage  27   a  opens through a suction cup  60 , and suction cup  60  seals against the top surface of a package  10 , as will be hereinafter described. 
     Prior to inserting a package  10  into chamber  24 , an adhesive strip seal  61  is applied about the edge of the package  10  flange  14 , such that the strip seal  61  extends beyond the outside edge of flange  14 . See FIG. 8 for details of this attachment. The entire center section of membrane  18  is left uncovered. Whereas the flange  14  is sized to fit inside the raised flange  50  of upper lip section  22   e , the strip seal engages against the raised flange  50 . When the cover  26  is closed, the O-ring  51  tightly engages against the strip seal  61  to provide a complete seal of the upper surface of the package  10  about the raised flange  50 . 
     In operation, the package is first prepared as described above, and then it is placed inside the housing, with the cover locked down to seal the package in the chamber. The vacuum pump is operated continuously during the test cycle, and the valve  42  and valve  46  are both opened, permitting a flow of ambient air through the chamber and the various conduits as a purging flow. The gas source is preferably carbon dioxide, and this gas is passed into the package through the passage  27  and suction cup  60 , and through the membrane  18 . Excessive gas is relieved to atmosphere via conduits  25   a  and  25   b . After an equilibrium condition has developed, the valve  46  is closed, thereby stopping the flow of ambient air through the chamber  24 . However, the vacuum pump continues to be operated, and any accumulated gas in the chamber will become pumped out of the chamber and toward the IR sensor. Next, the valve  46  is opened, permitting an in-rush of ambient air into the chamber  24  and flushing any residual gas from the chamber into the IR sensor, where it will be detected as a temporary transient signal. If a sufficient level of gas is detected, the threshold detector will trigger an alarm, notifying the operator that a gas leak exists in the package in excess or the permissible limits. The internal radiating pattern of grooves at the bottom of chamber  24  provide flow paths for gases in all parts of chamber  24  to quickly pass through the outlet  30  to the IR sensor  34 , so that the volume of gas conveyed arrives as a slug of gas to the IR sensor  34 , which then produces an output electrical pulse representative of the quantity of gas detected. 
     The present invention may be embodied in other forms without departing from the spirit or essential attributes thereof; and it is, therefore, desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.