Abstract:
A method for testing the sealing tightness of a specimen, containing a gas, in a film chamber made from flexible material. The film chamber is evacuated by a vacuum pump and the gas flow flowing out of the film chamber and generated by the vacuum pump is measured during the evacuation and is examined with respect to a possible leak of the specimen.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the United States national phase of International Application No. PCT/EP2014/067591 filed Aug. 18, 2014, and claims priority to German Patent Application No. 10 2013 217 288.5 filed Aug. 29, 2013, the disclosures of which are hereby incorporated in their entirety by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for testing the tightness of a gas-containing specimen in a film chamber of a flexible material. 
     Description of Related Art 
     Film chambers are test chambers testing the tightness of specimens such as food packages or other flexible packages. Here, a film chamber is made at least in part of a soft flexible material which clings to the specimen in the film chamber when the film chamber is evacuated. A vacuum is generated inside the film chamber in the external environment of the specimen. This vacuum expels a filling gas contained in the specimen through possible leaks into the external environment of the specimen inside the film chamber. The atmospheric pressure outside the film chamber prevents the internal pressure of the specimen from causing the specimen to burst in the vacuum. 
     Conventionally, two alternative methods are known for testing the tightness of a specimen in a film chamber. In one method, a test gas is added to the filling gas in the specimen and the test gas partial pressure is measured in the gas flow of the vacuum system of the film chamber. Here, the test gas partial pressure serves as the measure for the leakage rate. 
     The other test method is independent of filling gas. In a first step the film chamber is evacuated and then a valve to the vacuum pump system is closed. Thereafter, in a second step, the pressure increase over time is measured inside the film chamber and outside the specimen. This pressure increase serves as a measure for the leakage rate of the specimen. Due to the two steps “evacuation” and “pressure measurement” that have to be performed one after the other, this method is disadvantageous for industrial tightness tests, since these require short cycle rates in tightness testing. It is another disadvantage that in case of a large leakage of the specimen the filling gas will be pumped out from the specimen already during the evacuation phase. Thus, after evacuation, the specimen is also evacuated and will erroneously be considered tight during the pressure measurement phase. 
     It is an object of the invention to provide an improved method for testing the tightness of a specimen in a film chamber. 
     SUMMARY OF THE INVENTION 
     In the test method of the present invention, the gas flow from the film chamber, generated by the vacuum pump, is measured already during evacuation and is checked for a possible leak in the specimen already during evacuation. Here, it is possible to omit the measurement of the pressure conventionally performed after evacuation during the accumulation of the gas flowing from a possible leak in the specimen. Preferably, the gas flow measured during evacuation is compared with the course of the gas flow in the case of a tight specimen. The gas flow in the case of the tight specimen serves as a reference gas flow. This reference gas flow may be measured and stored prior to the actual pressure measurement. The leakage rate of the specimen may then be determined by subtracting the reference gas flow from the measured gas flow. 
     The measurement may be performed using a flow sensor in the exhaust gas flow of the vacuum pump or, as an alternative, using a differential pressure sensor at a throttle point, which sensor is arranged in the pipe system that connects the film chamber and the vacuum chamber, so as to evacuate the film chamber by means of the vacuum pump. 
     Prior to measurement, a ratio of the gas quantity in the film chamber outside the specimen and the gas quantity inside the specimen should be reduced, for example by adapting the volume of the film chamber to the volume of the specimen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a detailed explanation of embodiments of the invention with reference to the drawings. In the Figures: 
         FIG. 1  shows a first embodiment of the present invention; 
         FIG. 2  shows a second embodiment of the present invention; 
         FIG. 3  shows a third embodiment of the present invention; 
         FIG. 4  shows a fourth embodiment of the present invention; 
         FIG. 5  shows a fifth embodiment of the present invention; and 
         FIG. 6  shows a sixth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In all embodiments, the film chamber  12  is formed by two flexible films  14 ,  16  that enclose the specimen  18  and are provided with an O-ring seal  22  in the region of their edge  20  surrounding the specimen  18 . The O-ring seal  22  is positioned between both films  14 ,  16  in a manner abutting the same and prevents gas from flowing into the film chamber  12  via the edge portion  20  of the two films  14 ,  16 . The lower film  16  has a connector  24  for a gas-carrying pipe line  26  that is connected with a vacuum pump  28  in order to evacuate the film chamber  12 . 
     The specimen  18  typically is a flexible food package illustrated in an oval shape in the Figures for reasons of simplicity. The specimen  18  is filled with a filling gas and the volume of the film chamber  12  is adapted to the volume of the specimen  18 . That means that the volume remaining in the film chamber  12  outside the specimen  18  is small. 
     In  FIG. 1 , a flow sensor  30  is arranged downstream of the vacuum pump  28  in order to measure the gas flow in the exhaust gas flow of the vacuum pump  28 . In  FIG. 2 , the flow sensor  30  in the pipe system  26  connecting the film chamber  12  and the vacuum pump  28  is arranged in parallel with the vacuum pump  28 . In  FIG. 3 , the flow sensor  30  in the pipe system  26  connecting the film chamber  12  and the vacuum pump  28  is arranged in series with the vacuum pump  28 . In this case, the flow sensor  30  is situated exactly between the film chamber  12  and the vacuum pump  28 . In  FIG. 4 , a pressure sensor  32  in the pipe system  26  connecting the film chamber  12  and the vacuum pump  28  is arranged in parallel with the vacuum pump  28 . The embodiment of  FIG. 4  corresponds to the embodiment of  FIG. 2  except for the flow sensor  30 . 
     In the embodiment of  FIG. 5 , the pressure sensor is arranged downstream of the vacuum pump  28 , and in the embodiment of  FIG. 6  it is included in the pipe system that connects the film chamber  12  and the vacuum pump  28 . In the embodiments of  FIGS. 5 and 6 , the pressure sensor  30  is respectively arranged in parallel with a throttle point  34  in order to measure the pressure drop over the throttle point  34 . 
     It is common to all embodiments that the measurement of the gas flow, i.e. either by means of the flow sensor  30  or the pressure sensor  32 , is performed already during the evacuation of the film chamber  12  by means of the vacuum pump  28 . Thus, the difference to the conventional test method according to the pressure increase method is that the measurement is not performed in a separate step after evacuation. The expenditure of time for testing the tightness of the specimen  18  is thereby reduced significantly. In all embodiments, the total gas flow Q total  pumped out of the film chamber  12  during the testing process is composed of the gas quantity Q chamber  from the film chamber  12  outside the specimen  18  and the gas quantity Q leak  from the specimen  18 :
 
 Q   total   =Q   chamber ( t )+ Q   leak ( p,t )  (1)
 
     The course over time of these two partial flows is a function of the pressure inside the film chamber  12 . At the beginning of the pumping by means of the vacuum pump  28 , first only the gas from the film chamber  12  outside the specimen  18  will flow to the vacuum pump  28 :
 
 Q   chamber   =S×Δp   chamber   (2)
         S: suction capacity at the film chamber       

     At this time, i.e. at the beginning of the pumping, no gas flows from the specimen  18  yet, since no sufficient driving force exists in the form of a differential pressure Δp specimen  between the pressure inside the specimen  18  and the pressure in the environment of the specimen  18  inside the film chamber  12 :
 
Δ p   specimen =0,
 
where
 
 Q   leak   =L×Δp   specimen   (3)
         L: conductance of the leak channel at the specimen       

     As the pressure p 1  inside the film chamber  12  outside the specimen  18 , the driving force acting on the leak in the specimen  18  increases so that also the leakage rate increases. With large leaks in the specimen  18  and a small gas quantity inside the specimen  18 , the pressure p 2  decreases inside the specimen as well, since the gas from the specimen is also pumped off by the vacuum pump  28 . The specimen is pumped empty and the leakage gas flow ends. In this state, according to the conventional method for pressure increase measurement, a specimen was erroneously determined as being tight. 
     The following generally applies:
 
Δ p   chamber   =p   1   −p   0 ,
 
Δ p   specimen   =p   2   −p   1 ,
         p 0 =pressure at the chamber connection flange (pump nozzle),   p 1 =pressure in the film chamber  12  outside the specimen  18  and   p 2 =pressure inside the specimen  18 .       

     In order that the flow signal of the leakage rate becomes large compared to the flow from the film chamber  12 , the ratio of the gas quantity in the test chamber  12  outside the specimen  18  and the gas quantity inside the specimen  18  must be as low as possible at the start of the process. This may be achieved by adapting the test volume of the film chamber  12  to the specimen  18 . In this regard, the volume of the film chamber  12  is maintained as small as possible. The size of the specimen  18  defines the necessary diameter of the film chamber. The pressure in the film chamber  12  outside the specimen  18  will then drop rather quickly and the flow from the specimen  18  can be measured early on. 
     Whereas in the leakage measurement according to the pressure increase method, in which the pressure increase is measured in the film chamber  12  outside the specimen  18  (1. phase: evacuate, 2. phase: accumulation and pressure measurement), the leakage measurement according to the method of the invention is performed directly in the first and only phase (evacuation). The time per test cycle is reduced thereby.