Abstract:
Method and apparatus for leak testing a closed container with at least one flexible wall area, whether or not the container undergoing leakage testing is is filled with a product or not. A biasing member is moved towards and onto the flexible wall area in order to apply a predetermined force and the accompanying reaction forces are monitored over time. The instant invention of this leakage or integrity testing system constitutes an improvement over conventional prior art pressure monitoring sytems by instead monitoring a biasing force applied by such biasing member to such flexible wall area at different times over a testing cycle, whereby storage of two biasing force values at different times allows the later processing of a difference signal that is generated by using two measured force signals over time to thus result in both a leak indicative signal and an updated compensating zero offset signal. Furthermore, the moving of the biasing member may be done at a constant rate or predetermined rate, or the moving accomplished by establishing a pressure difference between the inside of the container and its ambient surroundings, such as may be done by evacuating such surroundings by application of vacuum.

Description:
TECHNICAL FIELD 
     The present invention is directed to a method for leak testing closed containers with at least one flexible wall area and to a leak testing apparatus for leak testing a closed container with such flexible wall area, irrespective whether such container is filled with a product or not. 
     BACKGROUND 
     When testing closed containers one known technique is to arrange a container to be tested into a test cavity which is then sealingly closed, then to evacuate the interior space of the test cavity around the container to be tested and to evaluate the time behaviour of the pressure in the surrounding of the container after evacuation has been stopped at a predetermined level. Although this technique is of very high accuracy it necessitates utmost care for reaching such high accuracy. The volume of the test cavity and its shape must snugly fit the outside shape of the container to be tested. On one hand minimising this volume leads to respectively short evacuation time, on the other hand the degree of this minimising largely governs the detection accuracy reached. As a change in pressure in the surrounding of the container is detected as leak indication entity, the smaller than the volume is in which, through a leak, pressure is affected, the higher will be the detection accuracy. 
     Further, accuracy is largely influenced by the degree of vacuum which is established in the surrounding of the container, which makes it necessary, for high accuracy, to provide relatively expensive vacuum pumps, possibly even multiple stage vacuum pumps, if vacuum is to be established down to the level as only reached with turbo vacuum pumps. 
     SUMMARY 
     It is an object of the present invention to provide for a method and apparatus as mentioned above, which remedies for the drawbacks of state of the art leak testing technique using pressure monitoring. This object is resolved by the method of leak testing as mentioned above, comprising the steps of relatively moving a biasing member towards and onto the flexible wall area of the container, stopping such moving and monitoring a biasing force on said container. The biasing force monitored is sampled at a first point in time resulting in a first force measuring signal and is sampled at at least one second subsequent point in time, resulting in a second force measuring signal. There is further generated a difference signal in dependency of said two measuring signals as a leak indicative signal. 
     Thereby, the present invention departs from the recognition that if a container to be tested is biased, leading to either compression or expansion of such container, biasing forces will apply to surfaces applied externally to the wall of the container as reaction forces of the expanded or the compressed container. Such reaction forces may easily be monitored. If such biasing is installed to a predetermined level and then stopped, a tight container will lead to monitoring a constant reaction force according to the biasing level reached. If the container is leaky, there will occur an exchange of medium between the surrounding of the container and its inside, leading to a decrease of the reaction force monitored over time. 
     Thereby, the accuracy of such a technique is largely independent from the volume surrounding the container under test, and is further primarily given by the degree of biasing and the force detecting surface towards which the biased container reacts. 
     In a preferred embodiment of the inventive method, biasing is installed up to a predetermined biasing force. 
     Having reached such predetermined biasing force, it is further proposed to wait for a time span before by sampling the respective first and second force measuring signals are generated, in dependency of which the difference signal is generated. Thereby, in this time span the biased container can stabilise its shape. In one operating embodiment biasing of the container under test is controlled as a function of the difference signal generated, so as to hold said difference signal on a predetermined value and exploiting the action of the biasing member as a leak indication. Thereby, a negative feedback loop is established, where the biasing member controllably counteracts a change of force monitored due to leakage, so that in extreme no change of force will occur due to the fact that the biasing member maintains by appropriate action a constant reaction force. 
     In a most preferred embodiment biasing the container is not established by relatively moving external surfaces onto the wall of the container, but in that a pressure difference is installed between the inside of the container and its surrounding. Thereby, the pressure difference is in a most preferred embodiment established by evacuating the surrounding of the container. The flexible wall area of the container has then the tendency of bowing outwards, and if this bowing outwards is barred by stationary surfaces outside the container, the container will act with a respective force on such surfaces. This force is monitored. 
     So as to avoid that due to the inventively exploited biasing, an existing leak in a container is clogged by the wall area with such leak being urged onto an external surface, it is proposed to provide surface areas contacted by the wall of the container, as it is biased, with a structure. Such a structure may be realised by interposing a mesh- or grid-like member between wall area of the container and such an external surface or, and preferably, by roughening such surface as by etching or machining. 
     In a further preferred embodiment the first force measuring signal is stored and the difference signal is generated in dependency of the stored first force measuring signal and the second measuring signal 
     In a further preferred mode of operation, already in the first point in time there is generated the difference signal namely from the first force measuring signal stored, and the first force measuring signal unstored. The resulting difference signal, as a zero offset signal, is stored and zero offset of latter generated difference signal is compensated by the stored zero offset signal. 
     So as to early detect large leaks, then smaller leaks, it is further proposed to compare the biasing force monitored with at least one predetermined threshold value, at the latest when sampling at said first point in time, which leads to identifying very large leaks and further preferably to compare the difference signal with at least one predetermined threshold value. 
     The leak testing apparatus according to the present invention comprises a biasing arrangement for compressing or expanding a container under test, further a force detector applicable to the wall of the container under test and generating an electric output signal. The output of the force detector is operationally connected to a storing unit, the output of the storing unit operationally connected to a comparator unit. The second input of the comparator unit is operationally connected to the output of the force detector. 
     The invention is especially suited for leak testing so-called pouches, all around flexible wall containers, filled e.g. with pasty material. 
     Further preferred forms of realising the inventive method and the inventive apparatus will be become apparent to the skilled artisan reading the following detailed description as well as the claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     By way of examples the following figures show: 
     FIG. 1 schematically, a first embodiment of an inventive apparatus operating according to the inventive method, whereat a container under test is biased by compression, biasing member and force detector being arranged on opposite sides of the container; 
     FIG. 2 in a representation according to FIG. 1, an embodiment whereat the container resides on a support and biasing member as well as force detector are arranged on the opposite side of such support; 
     FIG. 3 in a schematic representation according to those of FIGS. 1 and 2, a further and preferred embodiment of the inventive apparatus and method, whereat biasing the container is realised by evacuating the surrounding of the container under test; 
     FIG. 4 a qualitative force versus time diagram explaining the inventive method as performed by an inventive apparatus; 
     FIG. 5 by means of a schematic and simplified functional block/signal-flow diagram, an embodiment of the inventive apparatus operating according to the inventive method; 
     FIG. 6 schematically and in a simplified form a preferred realisation of storing and comparing units as preferably used in the inventive apparatus; 
     FIGS. 7 and 8 schematically and in a perspective view, a test chamber for realising the invention as shown in FIG.  3  and for testing pouches; 
     FIGS. 9 and 10 schematically, further preferred features at a test cavity operated according to FIG. 3; 
     FIGS. 11 a  to  11   c  force-signal versus time diagrams showing a preferred realisation form of the inventive method by an inventive apparatus; 
     FIG. 12 by means of a signal flow/functional block diagram the embodiment of the inventive apparatus performing measurements as explained with the help of the FIGS. 11 a  to  11   c;    
     FIG. 13 a force signal versus time diagram showing the statistic distribution of biasing force reached after a predetermined time of biasing at unleaky containers of the same type due e.g. to manufacturing tolerances; 
     FIG. 14 a simplified functional block/signal-flow diagram showing a further preferred feature of the inventive apparatus and method for generating an adaptive threshold value at the embodiment according to FIG. 12; 
     FIG. 15 over time, qualitatively time courses of adaptively varied threshold values of the inventive apparatus and method as realised by the embodiment of FIGS. 14 and 16, where 
     FIG. 16 shows an embodiment for adaptively adjusting a further reference or threshold value for the inventive method and as realised at the preferred apparatus, and 
     FIG. 17 schematically shows an inline plant for inline assembling and testing containers. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows schematically one principle according to the present invention. A container to be leak tested,  1 , has an area of its wall  3  which is flexible. The principle of the present invention resides in the fact that for leak testing container  1  a biasing member  5  is moved by means of a drive  7  towards and onto the wall of the container  1  and a force detector  9  monitors the reaction force F and generates an electrical signal F el  according to that force F. As shown in FIG. 2 in a preferred mode the force detector  9  is directly coupled to the biasing member  5  and both are driven relative to and onto the flexible area  3  of the wall of the container  1 , which latter resides, e.g., on a base plate  11 . 
     In a still further preferred embodiment and as shown in FIG. 3 the drive  7 , which moves one of the biasing member  5 , of force detector  9  or of a combined force detector and biasing member  5 / 9  arrangement with respect to the flexible area  3  of the wall of container  1 , is in fact realised as a pneumatic drive. Force detector  9  and biasing member  5  are kept stationary in a test chamber  13 . 
     By means of an evacuation pump  15  the test chamber  13  is evacuated, thereby generating a pressure difference Δp between the surrounding of the container  1  and its interior, which is directed from the inside to the outside of the container. Thereby, the flexible wall portion  3  is bent outwards and moved towards and onto the force detector  9 , which here and as a preferred embodiment simultaneously acts as biasing member and as force detector. As shown in dotted lines, it is also possible to pressurize the container  1  e.g. with a source  16  of pressurised gas, and dependent on the wall structure of container  1 , to have area  3  bowing outwards. 
     Irrespective of the technique, which is inventively exploited and with respect to where the biasing member  5 , where the force detector  9  are arranged and how the drive  7  is realised, as a mechanical drive as shown in FIG. 1 or  2  or by a pressure difference applied as shown in FIG. 3, biasing container  1  by relatively moving biasing member  5  towards and onto container  1  leads to force detector  9  detecting a rising force F as the container  1  is urged together in the embodiments according to FIG. 1 or  2  or is expanded according to the preferred embodiment of FIG.  3 . According to FIG. 4, as soon as biasing member  5  contacts the wall t 0  of container  1 , the reaction force F rises as biasing member  5  is further urged onto the wall of container  1 . After a predetermined time t 1  the relative movement of container wall  1  and biasing member  5  is stopped. This leads to a constant reaction force F O , if the container is unleaky and its wall does not further react up to achieving equilibrium of shape. 
     If the container under stress has a large leak LL according to course (b), then the biasing movement of the biasing member will not lead to a reaction force F achieving F o  at all, but after the time span according to t 1 −t 0  a considerably smaller force F LL  will be measured or monitored by the force detector  9 . 
     Thus, a large leak LL is inventively already detected if the biasing member is moved at a predetermined rate or speed towards and onto the container wall and after a predetermined time span as of t 1−t   0  a predetermined force, as of F O , is not reached. 
     Preferably such a behaviour of the container is already detected after a time span which is shorter than t 1−t   0 , so as to become able to stop biasing of the container early enough and before pressing or suctioning a product contained in the container to and into its surrounding. Thus, preferably, there is installed a shorter time span t LL−t   0  and after this time span of increased biasing it is checked whether a predetermined threshold force, according to FIG. 4 as of F LL , is reached or not. If it is not reached according to the biasing course (b) further biasing is stopped and the heavily leaky container is freed of any bias as quickly as possible. 
     If the container  1  is not heavily leaky, the reaction force monitored, F, will reach after the predetermined time span of increasing biasing, t 1−t   0 , the threshold value as of F O  as required and leakage behaviour of the container will only be detected afterwards. 
     After having checked for large leaks LL and having disabled further biasing of the container as at time t 1 , preferably a predetermined time span t 2 −t 1  up to t 2  is installed, during which the system consisting of container  1 , biasing member  5  and force detector  9  is left for attaining equilibrium e.g. of the shape of the container. 
     Thereby, in a preferred mode, t 2  is set on the maximum value according to t max , thus there is valid t 2 =t max . This is especially done if the container under test does not experience e.g. a volume change under the stress of the bias, which leads to a decrease of reaction force in a transient phase which decrease is not due to leakage. 
     At or after reaching t 2  the monitored reaction force F, then prevailing, F 2 , is sampled and stored. After lapse of a further time span t 3 −t 2  up to t 3  again the monitored reaction force F is sampled as F 3  and is compared with reaction force F 2  as was stored. Thus, the difference ΔF of F 3  and F 2  is principally evaluated as leak indicative signal. 
     As further shown in FIG. 4 it is also possible to sample and store force F 2  on the rising slope of biasing the container  1  and to wait for the force F monitored to re-reach in the falling slope of F, after having stopped further biasing—t 1 —the value according to F 2 , thereby indicating that the system has in fact stabilized. In this case time moment t 2  will be defined by the force F as monitored re-reaching the preset and stored value F 2 . 
     In FIG. 5 the inventive apparatus in its principle form which performs the procedure as explained with the help of FIG. 4 is schematically shown. Thereby, the same reference numbers are used as in the previous figures with respect to features already described. In the test chamber  13 , which is vacuum tight, the container  1  to be tested is deposited. The vacuum pump  15  is operated controlled by a timing unit  17 . Pump  15  evacuates chamber  13  preferably at a constant and adjustable rate. 
     Combined biasing member and force detector  9 / 5  is rigidly mounted within chamber  13  and preferably opposite and adjacent to the area  3  of flexible wall of container  1 . The force detector  9  generates electrical signal S(F) as a function of the force acting between area  3  and contact area of the biasing/force detector assembly  9 / 5 , which is as schematically shown provided with a surface structure  19  to prevent that surface shutting a leak of area  3  incidentally just happening to be located there, where area  3  is or is going to contact the assembly  9 / 5 . The same structuring  19   a  is preferably provided at the bottom surface of chamber  13 . 
     The signal S(F) is fed at a time t LL , controlled by timing unit  17  as schematically shown and by switch unit SW 1 , to a comparator unit  21 , where at time moment t LL  the output signal S(F) is compared with the large leak indicative threshold value S 0 (F LL ) as preset at unit  23 . 
     Whenever at moment t LL , S 0 (F LL ) is not reached by the force is signal S(F), switching unit SW 2 , the input thereof being connected to S(F), is opened disabling via a control unit  25  further biasing e.g. by pump  15 . If the threshold value S(F LL ) is at least reached by S(F) at the moment t LL , then signal S(F) is led to a further switching unit SW 3 , where, controlled from timing unit  17  at moment t 2 , the prevailing signal is in fact sampled and stored in storing unit  27 . Thus, in unit  27  there is stored a value according to force F 2  of FIG.  4 . The output of the storing unit  27  is fed to a comparing unit  28 , to which, again controlled from timing unit  17 , at moment t 3  signal S(F) is additionally fed according to the then prevailing value F 3 . Thus, comparing unit  28  compares the force value at moment t 2  with the value of that force prevailing at moment t 3 . The output ΔF of comparator unit  28  is indicative of leak behaviour of container  1  under test beside of a large leak prevailing, which has been previously detected. 
     Instead of evaluating directly the output signal of comparator unit  28  it is possible to control biasing as a function of the output signal of comparator unit  28 . Thereby, a negative feedback control loop is installed (not shown), wherein the comparator unit  28  compares a rated value according to the stored signal in storing unit  27  with an instantaneously prevailing signal, S(F) and as an adjusting unit in the negative feedback control loop a biasing member is operated to minimize the output signal of comparator unit  28 . Thereby, the control signal of such biasing member  15  is exploited as leak indicative signal. 
     In FIG. 6 a most preferred realisation of storing unit  27  and comparator unit  28 , schematically shown in FIG. 5, is depicted. 
     The output signal of the force detector  9  in assembly  9 / 5  is input to a conversion unit  121 , which comprises, as an input stage, an analogue to digital converter  121   a , followed by a digital to analogue converter  121   b . The output of the converter stage  121  is fed to a difference amplifier unit  123 , which additionally receives directly the output signal from force detector  9 . The output of the difference amplifier  123  according to the comparator unit  28  of FIG. 5, acts on a further amplifier unit  125 , the output of which being superimposed at  128  to its input via storage unit  127 . The input of the storage unit  127  is fed from the output of unit  125 . A timer unit  129 , as timer unit  17  of FIG. 5, controls the arrangement. For storing the signal according to the force value F 2  as of FIG. 5, at time t 2  the timer unit  129  enables a conversion cycle at unit  121 , so that a reconverted analogue output signal el O (F 2 ) appears at the analogue output. 
     Simultaneously the substantially same signal S(F) from force detector  9  is applied as a signal el (F 2 ) to the second input of unit  123 . Thus, at the output unit  125 , a zero signal should appear. Nevertheless, in general a zero offset signal will appear at the output of unit  125 , which signal is stored in the storing unit  127 , enabled by the timing unit  129 , according to unit  17  of FIG.  5 . At time t 3  (FIG. 5) no conversion is triggered at the unit  121 , so that there appears at the input of amplifier  123  directly from force detector  9  the signal according to the force value F 3  prevailing at t 3 , and from stage  121  the stored signal according to force value F 2 , which was prevailing at t 2 . Further, the zero offset signal, which was stored to unit  127 , is now superimposed as an offset compensating signal to the output of unit  123 , so that the resulting signal at the output of amplifier unit  125  is zero offset compensated. This allows a very accurate measurement of the force difference ΔF as of FIG.  4 . 
     When looking at either of the FIG. 1,  2  or  3  it becomes clear that even if vacuum is used to bias the container&#39;s wall towards and onto the force detector, the volume of the test chamber  13  is not very critical with respect to the volume of the container to be tested. Whereas in evaluating a pressure as is done in prior art leak testers, here inventively a force is evaluated. When evaluating a pressure as e.g. the pressure prevailing in the surrounding of a container to be tested, then accuracy of measurement is largely dependent on the remaining volume between the wall of the test chamber and that of the container, because leakage will affect the pressure in that intermediate volume the more the smaller than said intermediated volume is selected. According to the present invention by providing biasing a wall portion of the container a wall portion of the container is urged against the force detector. Leakage to the surrounding will affect such force irrespective of the surrounding volume and thus of the relative volume of the test chamber with respect to the container to be tested. 
     Nevertheless, under the aspect of shortening testing cycles it is recommended to provide testing chambers which are minimum in volume with respect to the containers to be tested therein, if biasing is performed by vacuumising according to FIG.  3 . 
     By selecting the established biasing according to F O  of FIG.  4  and thereby the force and thus signal S(F), the level of measuring is set and may be selected. As the flexible wall portion in its bowing action will reside along a successively larger contact area on the force detector and/or the biasing member with an eye on the embodiment of FIG. 3, establishing a larger biasing pressure difference Δp will lead to an overproportionally rising biasing force F. This accords to an amplification of the signal ΔF according to FIG. 4 to be exploited. This again significantly improves accuracy of the overall measuring system and makes it easy to establish the range of evaluation signals 
     In a preferred embodiment operating according to FIG. 3 pouches filled with a product are tested. In FIGS. 7 and 8 there is shown, in a simplified representation, two halves of a test chamber or test cavity according to chamber  13  of FIG. 3, tailored for testing pouches. 
     According to FIG. 7 there is provided in a basis  30  a recess  32  substantially shaped according to pouch  34  (dashed lines) to be tested therein. For instance in the base plate  30  there is applied one or more than one suctioning line  36  to be connected to an evacuating pump according to pump  15 . 
     The top plate  37  as of FIG. 8, which is conceived similarly to the bottom plate  30 , has a recess  38 , which, once the top plate  37  is deposited upon the base plate  30 , defines with recess  32  the test chamber or test cavity. The bottom surface  40   b  and the top surface  40   a  of the two plates  30  and  37  do snugly and vacuum tightly fit and are thereby, if necessary, provided with respective sealing members all around the recesses  32 / 38 . In one (or possibly in both) of the plates  30 ,  37  there is installed a force detector arrangement  42  with a large detection surface  44  fitted to the shape of the test cavity. The force detector arrangement  42  preferably operates on the principle of resistance gauge, i.e. pressurising the surface  44  will generate a force according to pressure multiplied by contact surface, which will slightly bend the resistance gauge element, thereby generating the electric signal S(F) according to FIG.  5 . 
     Nevertheless, other force detectors operating on different physical principles may clearly be used, thereby preferably force detectors, which operate on minimum mechanical movement. Thus, e.g. a piezo force detector may be used. 
     Especially when the test cavity as formed by the two recesses  32  and  38  of FIGS. 7 and 8 for testing pouches is made to snugly fit the shape of a container  1  (a pouch) to be tested therein, it is possible to get additional information especially about large leaks by measuring the electric impedance outside the container under test, which is changed whenever e.g. a liquid content of a leaky container is urged or suctioned out of such container. As shown only in the bottom plate  30  of FIG.  7  and not in the top plate  37  of FIG. 8, the inner surface of the test cavity may be subdivided in electrically conductive electrodes  44 . Every second electrode  44  is connected to one input connector  46  to an impedance measuring unit  48 , every electrode in-between to input connector  49 . Impedance measuring unit  48  may measure AC and/or DC impedance, preferably DC impedance. Thus, whenever the container, as pouch  34 , is biased and a liquid or pasty content is pressed into the test cavity, irrespective of large leak measurement according to FIGS. 4 and 5 as was discussed above, a change in impedance measured at the unit  48  will indicate such a leak, and the output signal of the impedance measuring unit  48  will stop further biasing of the container. 
     For cleaning a test cavity e.g. in case content of a leaky container has been pouring out into the test cavity, further (not shown) lines or pipes may be provided abutting in the test cavity and connected to liquid and/or gaseous cleaning media as to a source of air or, and preferably, nitrogen and/or of pressurised liquid flushing medium and further (not shown) a heater may be incorporated into the walls of the test cavity to dry and additionally clean a spoiled test cavity. 
     A most important feature, which is preferably provided irrespective whether the system operates according to FIG. 1 or  2  or according to FIG. 3, shall now be described with the help of FIGS. 9 and 10. 
     Whenever a container  1  to be tested is biased, be it according to the teaching of the FIG. 1 or  2 , to which FIG. 10 is directed or according to FIG. 3, to which FIG. 9 is directed, at least two wall portions of the container, which are disposed one opposite the other, denoted in the FIGS. 9 and 10 as  51   a  and  51   b , will be firmly pressed onto the biasing member/force detector arrangement or more generically to surfaces. Whenever there happens a leak to be in such an area of container&#39;s wall, such a leak might be clogged by such a surface. Therefore and as schematically shown in FIGS. 9 and 10 there is provided at all surface areas to which, during biasing the container, a wall area thereof is pressed, a surface structure, so that such a surface does only contact the wall of the container  1  at distinct contact areas, leaving substantial parts of such wall portion uncontacted. This may be realised by providing a mesh- or grid-like member between the respective surfaces and wall portions of container  1  or by roughening such surfaces by machining such as by etching or sand-blasting. Mechanical abutments  53  as schematically shown in the FIGS. 9 and 10, which contact respective distinct areas of container&#39;s wall are formed by such microstructuring of the respective surface. With an eye on the embodiment according to FIGS. 7 and 9 it is therefore recommended to have the surface of the respective plates  30  and  37 , which form the recesses  32  and  38 , mechanically machined to have a roughened microstructure. Thereby, it is prevented that any leak in the wall of the container may be clogged by the wall area of the container having such leak being urged onto a surface of the system, be it the biasing member surface, the force detector surface or another part of the test cavity&#39;s surface. 
     Force versus time courses as measured according to the inventive method and with an inventive apparatus in preferred mode are shown in FIG. 11 a  for very large and large leaks VGL, in FIG. 11 b  for small leaks and for unleaky containers in FIG. 11 c . These figs. shall be discussed in connection with FIG. 12, which shows a preferred monitoring and control unit. 
     According to FIG. 11 a  the timing unit  201  of FIG. 12 initiates at time t 1O  biasing of a container  1  under test, be it according to the embodiment of FIG. 1 or  2  or  3 . According to the embodiment of FIG. 3, thus the timing unit  201  initiates evacuation of the test cavity  13 . 
     This is shown in FIG. 12 by the biasing start signal BIST/t 10 . 
     After a fixed predetermined amount of time ΔT the output signal of the force detector S(P) becomes compared with a first reference signal preset at a presetting source  107 , RFVGL. To this target, comparator unit  102  is enabled by timer unit  201  at t 10 +ΔT. 
     If after time span ΔT the actual monitored force according to the electric signal S(F) of FIG. 12 has not reached the value of RFVGL according to course I of FIG. 11 a , this means that a very large leak VGL is present. This is detected at comparator  109  generating the output signal VGL. If according to the characteristics shown in the block  109  of FIG. 12 the output signal of this comparator unit enabled at t 11 =t 10 +ΔT is e.g. still at a high value, indicating presence of a VGL, this is output at the VGL output. If the biasing force F has reached and crossed the reference level RFVGL according to course II of FIG. 11 a  the VGL output signal is not generated. 
     The VGL signal preferably stops the biasing cycle, because this would lead just to pressing content of the container under test into the surrounding. 
     As shown by the course II of FIG. 11 a  as VGL does not occur, biasing of the container under test continues up to a further moment of time t 13 . At the time t 13  the timer unit  201  disables biasing drive, be it the mechanical drive  7  according to the embodiments of the FIGS. 1 and 2, or the evacuation pump  15  as of the embodiment of FIG.  3 . 
     Further, position of timer unit  201  enables comparator unit  111 , to which a further reference value RFGL is led, generated by a reference Signal source  113 . If at time t 13  the force detected by the force detector has not reached RFGL, then comparator unit  111  generates an output signal GL indicating that the container under test has a large leak GL. Here again. some reactions are taken with respect to further operation of the testing system. 
     If either of the signals VGL or GL are initiated by the respective comparators  109 ,  111  the timer unit  201  is principally reset because the testing has been completed and the quality of the instantaneously tested container  1  established has been identified. This is schematically shown in FIG. 12 by the signal RS 201 . If not reset shortly after t 13  the value S(F) (t 13 ) of the force detected by the force detector is stored in a holding or storing unit  117 . The output of the holding or storing unit  117  is led to one input of the difference forming unit  119 , whereas the second input of this unit  119  is connected to the output S(F) of the force detector. After a presettable test cycle time T T  starting at t 13  or at the moment of storing data in storing unit  117 , as schematically shown by unit  121  of FIG. 12 the force difference ΔF-signal is fed to a further comparator unit  125  enabled at the lapse of testing time T T . 
     By means of a further reference value source  127  the reference value ΔFREF is fed to the comparator unit  125 . As will be explained later the value of ΔFREF may controllably be varied in time and/or a reference value φ R , to which ΔFREF is referred to, may also controllably be varied in time. 
     If the ΔF-signal at time t 13 +T T  is larger than the reference value ΔFREF, then a signal FL is generated at unit  125 , indicating presence of a fine leak FL in the container  1  under test. This according to the situation as shown in FIG. 11 b . If the ΔF-signal does not reach ΔFREF then the container is considered unleaky, as none of the signals VGL, GL and FL has been generated. This accords with FIG. 11 c.    
     If the VGL signal is generated according to FIG. 12 irrespective of the embodiment according to one of the FIGS. 1,  2  and  3 , further biasing is immediately stopped. In the embodiment of FIG. 3 making use of an evacuation pump  15  as a biasing drive, the evacuation pump  15  is immediately disconnected from the respective testing chamber  13 . This because by a very large leak the vacuum pump  15  could become contaminated by leaking content of the container  1 . 
     In a multiple chamber inline testing system making use of the embodiment of FIG. 3 with a multitude of testing chambers occurrence of the signal GL—indicating a large leak—and possibly even the occurrence of the signal FL—indicating for a fine leak—leads preferably to disabling or “bypassing” that chamber from further being supplied with containers to be tested, whereas the other chambers are still operating and performing tests on newly supplied containers. 
     This bypass of a testing chamber  13 , whereat a container has been identified as heavily or even slightly leaking, is performed so as not to influence further testing results at that chamber and especially not to spoil the vacuum pump  15  connected thereto due to content of the leaky container being suctioned towards and into such pump. This bypass chamber is reconditioned during further testing cycles at the other chambers after the leaky container having been removed. 
     Reconditioning may be done by heating that chamber  13 , flushing it by a liquid and/or a gas, preferably nitrogen, especially by a heated gas. 
     When looking to the FIGS. 11 a  and  11   b  it may be recognised that setting the reference value RFGL and especially setting of the reference force difference value ΔFREF may be very critical and may largely influence accuracy of the system. Thereby, influences as surrounding temperature, tolerances of container manufacturing etc. may influence the prevailing force course and lead to false results if these critical reference levels and especially ΔFREF are set for utmost accuracy. 
     In FIG. 13 there is qualitatively shown the biasing force course according to the courses of FIGS. 11 a  to  11   b , but measured at containers of the same type which have been proven as unleaky. This may have been done by long-term experiments and/or leak detecting systems, which are standard and of utmost accuracy, but slow and/or very expensive. 
     At t 13  the force values measured at the tight containers are slightly different and define a statistic distribution as shown in FIG.  13 . There results an average value (RFGL) m . The value of RFGL as used at the comparator  111  of FIG. 12 or as used according to the FIGS. 11 a  to  11   c  is found in that an offset value ΔRFGL is subtracted from (RFGL) m . During ongoing operation on large series of equal containers, temperatures and manufacturing tolerances of such containers may vary. Such parameters may slowly change and may vary (RFGL) m . 
     Every time during multiple successive testing at the respective times t 13  up to which the respective container has been identified as not heavily leaky, the actual output signal of the force detector is entered into an averaging unit  130  as shown in FIG. 14, wherein the last m values of actual force of not heavily leaky containers are averaged. The output average result signal accords with (RFGL) m  of FIG. 13, now varies in time e.g. due to varying manufacturing parameters of one and the same type of containers. To the output average result {overscore (S(F))} and according to FIG. 13 the off set ΔRFGL is subtracted, the result of this operation is a dynamically varying reference value RFGL, which is applied to comparator unit  111  of FIG.  12 . This dynamically varying reference value RFGL is shown in FIG. 15 qualitatively, starting from an initial setting as e.g found as was explained with the help of measurements at unleaky test containers. 
     As may clearly be seen from FIG. 15 the average force value {overscore (S(F))}(t 13 ) is now the basis for also referring ΔFREF to. Therefore, and as is shown in FIG. 12, the force difference reference value ΔFREF is not referred to an absolute static value as φ R , but is referred to {overscore (S(F))}. 
     An even further improvement of accuracy is reached, which may be realised separately or additionally to realising a dynamic RFGL and based thereon a dynamic upper limit of ΔFREF. Thereby and according to FIG. 16 at the end of the time span T T  the actual force difference ΔF-signal is led to an averaging unit  135  whenever the output signal FL indicates that the container under test is unleaky. The output signal of unit  135 , which accords to an average force difference signal {overscore (ΔF)} averaged over the last m test cycles is, offset by an amount ΔΔF, the result thereof being used as time varying ΔFREF-signal applied at unit  127  of FIG.  12 . 
     Looking back on FIG. 15 whereat a constant ΔFREF signal was applied, the technique of averaging ΔF results as schematically shown with the course (ΔFREF) t  in a dynamically varying value ΔFREF, varying according to variations of disturbing parameters influencing such force difference. It is clear that provision of a dynamically varying (ΔFREF ) t  signal according to that representation in FIG. 15 could be realised without providing a dynamically varying base value {overscore (S(F))} in referring (ΔFREF) t  to a stable constant value φ R  as shown in FIG. 12 in dashed representation instead of referring to a dynamically varying {overscore (S(F))} value. 
     It is evident that preferably the evaluation of the output signal S(F) of the one or more than one force detectors is performed digitally. 
     In FIG. 17 there is shown an inline plant, wherein generically assembling and testing of containers is done inline. As an example pouches are first welded at a welding station  60  in a base plate  30  as shown in FIG. 7 used as carrier and support for assembling. with the same carrier, namely base plate  30 , after a pouch has been assembled therein by welding, the carrier formed by plate  30  is moved to an applicator station, where the top plate  37  as of FIG.  8  is assembled upon the bottom plate  30  Thereafter, the thus sealingly closed test cavity is moved and applied to a test station  64 , where the inventive test is performed. The system of welder  60  and/or applicator  62  and/or tester  64  may thereby be stationary with respect to a conveyor  66  for base plate  30 . Nevertheless, and depending on time requested for a certain operation, especially tester  64  may be moved together with conveyor  66  for a predetermined time, so as to become independent of speed of conveyor  66 . 
     With the inventive method and apparatus there is provided a leak testing technique which is much less critical in achieving the same accuracy as with leak testing techniques evaluating pressure measurements. Biasing containers according to the present invention is much simpler than establishing a perfect vacuum around such container and measuring a biasing force considerably easier than accurately measuring the time development of a vacuum pressure surrounding the container. In vacuum measurement much more unknown and uncontrollable parameters may affect the measured entities, namely vacuum pressure, than in the here inventively exploited force measurement. Whereas setting of the measuring level in vacuum measurement technique greatly influences the expenditure for vacuum pumps, varying and setting a bias force is of much less effort. 
     The inventive method and apparatus are especially suited for testing pouches, but clearly may be used for testing all kinds of containers up to big tanks as long as a wall portion thereof is flexibly bendable. The present invention may be realised at inline plants with a multitude of testing stations, e.g. arranged on a carousel with a very high throughput.