Abstract:
The invention relates to a method for operating a film leak indicator ( 1 ) comprising two films ( 5, 6 ), which are each tensioned inside a frame ( 3, 4 ) and which delimit a testing space ( 80 ) when a test piece ( 79 ) is introduced. In addition, said film leak indicator is equipped with two vacuum pumps ( 13, 20 ). According to the invention, the testing space ( 80 ) is firstly evacuated and the film leak indicator is switched over to a leak search mode after the evacuation is completed. In order to prevent the search for leaks from being impaired by fluctuating helium concentrations in the ambient air, the invention provides that, before starting the leak search process, the helium concentration of the ambient air is measured and this measurement is taken into account during subsequent leakage measurements.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a method for operating a film leak indicator comprising two films, which are each tensioned inside a frame and which delimit a testing space when a test piece is introduced. In addition, said film leak indicator is equipped with two vacuum pumps. According to the present invention, the testing space is firstly evacuated and the film leak indicator is switched over to a leak search mode after the evacuation is completed and where furthermore during the first evacuation phase of the testing space, only one of the two vacuum pumps and where in the leak search mode only the second vacuum pump is utilized (in accordance with patent application 198 46 799.0). Moreover, the present invention relates to a film leak indicator suited for implementing said method.  
           [0002]    In the instance of integral leak detection in accordance with the principal patent application, the test piece is already filled with the test gas, helium in this case, before the measurement. The helium escaping through a present leak is detected in the testing chamber. The natural ambient air already contains a small amount of helium, which, however, is mostly negligible. Owing to filling processes the share of helium may increase in certain cases. Then the detection system of the leak indicator can now no longer discern unambiguously whether the measured helium originates from the test piece or the ambient air trapped in the testing chamber. The measured results become more uncertain; the detection limit of the leak indicator is degraded.  
           [0003]    It is the task of the present invention to operate and implement a film leak indicator in accordance with the principal patent application in such a manner that increased helium concentrations in the ambient air will not give rise to incorrect measurements.  
           [0004]    This task is solved through the measures contained in the patent claims.  
           [0005]    Further advantages and details of the present invention shall be explained with reference to the design examples depicted in the drawing figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic of a film leak indicator in accordance with the present Invention, including its circuit diagram; and  
         [0007]    [0007]FIG. 2 is a partial section through the frames in which the films are tensioned And further details are depicted. 
     
    
     DETAILED DESCRIPTION  
       [0008]    Drawing FIG. 1 schematically depicts the film leak indicator  1  with its frames  3  and  4  joined via articulation  2 , as well as the foils  5  and  6  tensioned in said frames. The frames  3  and  4  are circular in shape. The bottom frame is supported by the rim  7  of the plate-shaped bottom  8 —preferably made of steel. Fleece sections  9  located on the films  5 ,  6  ensure the formation of a coherent testing space when the test piece has been introduced. The principal design of a film leak indicator of this kind is, moreover, known from DE-A-196 42 099.  
         [0009]    The bottom frame  4  is equipped with several bore holes not depicted in detail which open out to the inside of the testing space which is formed by the two films  5 ,  6  when a testing piece is introduced. Connected to these bore holes are line sections  10  which are linked via the common line  11  to the valve  12  with the fore vacuum pump  13 . With reference to the valve  12  upstream, also pressure gauge  14  and venting valve  15  are connected to the line  11 .  
         [0010]    The bottom film  6  is equipped approximately at its center with a line connection  18 , to which the line section  19 , the filter  20  and the line  21  with valve  22  are adjoined. Also, the line  21  is equipped with a pressure gauge  23  and with a venting valve  24 .  
         [0011]    The section of the line  21  which is located between the connection for the pressure gauge  23  and the valve  22 , is connected via two lines  26  and  27  parallel with respect to each other with a line  28  extending between the test gas detector  29  and a second fore-vacuum pump  30 . Located in the line  26  is a throttle valve  32 . The line  27  is equipped with a valve  33 .  
         [0012]    The fore-vacuum pump  13  is preferably of a single-stage design, the fore-vacuum pump  30  of a two-stage design. The pump  30  is equipped with a gas ballast device. With valve  31  open, air (or also inert gas) flows into the pump  30 .  
         [0013]    Located in the test gas detector  29  is a turbomolecular vacuum pump  35 , the outlet of which is connected to line  28 . Connected to the inlet of the turbomolecular vacuum pump is a mass spectrometer  36 . Moreover, a pressure gauge  37  is a component of the leak indicator  29 , said gauge measuring the pressure in line  28 .  
         [0014]    The two lines  11  and  28  are linked down stream with respect to the valves  12  and  22  via a line  38  into which also the line  21  opens out. In this line  38  there are located between the opening of the line  21  and the line  28 , the valves  41  and  42 . A line  44 , which is linked to a coupling  45 , opens out into the section of the line  38  located between valves  41  and  42 . The coupling  45  serves the purpose of providing the connection of the line  44  to the sniffer line  47  of a sniffer  48 . The sniffer probe designated as  51  of the sniffer  48  comprises handle  52  and sniffer tip  53 .  
         [0015]    A holster  54  is provided for depositing the sniffer probe. Said holster is either affixed at the film leak indicator  1  or designed by way of a separate stand. It has a hollow space  55  which accepts the sniffer tip when depositing the probe  51 . Moreover, a seal  56  is provided which seals off the hollow space  55  against the handle  52  when the probe  51  is deposited. Connected to the hollow space  55  is a line  57 , which is linked via a coupling  58  to a low-pressure switch  59 , accommodated in the casing of the film leak indicator  1 . A capillary (about 0.5 mm in diameter) serves as the sniffer line  47  in a basically known manner. This also applies to the line  57  so that the low pressure generated by the sniffer tip  53  in the space  55  will quickly affect the low-pressure switch  59 .  
         [0016]    In the film leak indicator in accordance with drawing FIG. 1, the leak searching cycle is performed automatically. The control center, depicted only as block  61 , is provided to control the process. Linked to it are all measuring instruments and all components which are to be controlled. This also applied to a switch, which is actuated upon closing the testing space. In the presented example of an embodiment, said switch is a proximity switch comprising a metal pin  62  provided in frame  3  and a sensor  63  arranged at frame  4 . The sensor  62  is linked to the control center  61 . Also, other switches, be they electrically, mechanically or optically operated, can be employed to this end.  
         [0017]    The multitude of electrical connecting lines between the control center  61  and the components is not depicted for clarity. Linked to the control center are also two signaling lights  64 ,  65  of which one lights up in green when the test object is sensed as being leak-tight and where the other lights up in red when the test object is sensed as being leaky.  
         [0018]    In the instance of an operationally ready film leak indicator  1  in accordance with the principal patent application, all valves—except valve  41 —are closed when the testing space—as depicted in drawing FIG. 1—is open or when the top frame  3  has been lifted off the bottom frame  4 . After placement of a test piece on the bottom film  6 , and after having placed the top frame  3  on the bottom frame  4 , the proximity switch  62 ,  63  initiates the leak searching process. To this end, a check is performed first whether there is present in the system an increased helium background that might falsify the leakage measurements. This is conducted with the aid of the mass spectrometer  36 . If it indicates too high a background, the valve  31  is opened and the pump  30  is operated with its gas ballast active for such a period of time, until the helium background has attained a non-detrimental value.  
         [0019]    Generally an increased helium background will not be present so that upon closing the testing space also the actual leakage searching cycle commences. Initially the valves  12  and  22  are open. This results in an extremely rapid evacuation of the space between the films  5 ,  6 . Outside of the area in which the fleece sections  9  are present, the films  5 , 6  indirect contact with each other form the seal for the testing space.  
         [0020]    In the instance of the leak searching method in accordance with the principal patent application, throttle valve  32  is opened at a pressure of a few 100 mbar (100 to 300 mbar). It is so rated that the necessary fore-vacuum pressure of the turbomolecular pump  35  does not attain inadmissibly high values. Gross leak detection commences upon opening the throttle valve  32 . If helium flows through the throttle valve  32  it will, in a counter-flow through the turbomolecular pump  35 , flow to the mass spectrometer  36 . If helium is recorded, the test piece has a leak; the leak searching cycle is then cancelled.  
         [0021]    In case helium is not yet recorded, the evacuation process is continued. If the pressure measured with instrument  23  attains a value, which lies in the order of magnitude of the fore-vacuum pressure of the turbomolecular pump  35 , then the valves  12 ,  22  and  32  are closed and the valve  33  is opened. Now the phase of leak searching at the highest sensitivity commences. This phase is cancelled when either test gas is recorded, i.e. when the test piece has a leak, or it is cancelled after a defined period of time. Either a fixed time span is defined, or testing continues until the pressure (measured with instrument  23 ) drops below a certain value. If no test gas is recorded within this time, such a result will lead to the conclusion as to a leak-tight test piece.  
         [0022]    During the high sensitivity leak-searching phase, only the lines  19 ,  21  and  27  provide the connection between the testing space and the leak indicator  29 . Bursting of the test piece is no longer to be expected. Moreover, there is located between the lines  19  and  21  the particle filter  20  which keeps any contaminants away from the leak indicator  29 .  
         [0023]    The leak detection cycle in accordance with the principal patent application is terminated such that all previously open valves—except for valve  41 —are closed, and valves  15 ,  24  are opened. The testing space is vented; the top frame  3  can be lifted off from the bottom frame  4 . It is expedient when the two frames  3 ,  4  which are joined in an articulated manner are exposed to the effect of a not depicted spring device in the area of the articulation  2 , where the force of said spring device acts in the opening direction. Its force should be so rated that the vacuum generated during leak searching keeps the testing space closed, and that the frame  3  reverts to its open position after the venting process.  
         [0024]    If during the leak test it is determined that a test piece is leaky, and then the user will be interested in acquiring the location of the leak. To this end, the film leak indicator in accordance with the present invention is equipped with a sniffer  48 . It is connected with its sniffer line  47  via coupling  45  to the line  44 .  
         [0025]    As long as the sniffer  48  is not utilized, it remains deposited in a holster  54 . This holster  54  is equipped with the means  55  to  59  already detailed, with which the control center  61  can determine whether or not the probe  51  has been deposited in the holster  54 . Also other means—be they mechanically electrically or inductively operated switches—may be employed to this end.  
         [0026]    As long as the probe  51  is placed in holster  54 , the valve  41  is maintained—as already mentioned—in the open state. Thus the sniffer is kept continuously in standby. Only after it has been removed from its holster  54 , will the valve  41  close and the valve  42  open. The gas flow passing through the sniffer  48  passes thus into the line  28  and the gas flow is maintained by pump  30 . In this state, sniffer leak detection will be possible. The helium taken up by the sniffer tip  53  passes in a counter-flow through the turbomolecular pump  35  into the mass spectrometer  36 .  
         [0027]    The control center  61  is programmed in such a manner that no switchover of the sniffer  48  from standby to the leak-searching mode (switchover of valves  41  and  42 ) can occur as long as the proximity switch  62 ,  63  is closed. The possibility of simultaneous leak searching via the film leak indicator  1  and via the sniffer  48  resulting in incorrect measurements is thus excluded.  
         [0028]    In the instance of a quantitative leak search, the searcher will generally be interested in the leak rate in mbar  1 /s of the leak found.  
         [0029]    In the analysis of test pieces manufactured in large numbers there is, however, also an interest in measuring the concentration of the test gas in the test piece. This may be implemented in that the sniffer probe  53  is made to prick into the test piece or in that the test piece is equipped with a defined leakage, and a leak searching cycle is performed in the film leak indicator  1 . The control center is, for this reason, so arranged that on a display not depicted, both the leak rate and also the concentration can be read off.  
         [0030]    From drawing FIG. 2, the design of the frames  3  and  4  is apparent. They each consist of an outer ring  71  and  72  respectively, and an inner ring  73 ,  74 . The films  5 ,  6  are affixed, preferably cemented, between the pair of rings. The inner rings in each instance are located within corresponding recesses  75 ,  76 . The recesses are arranged in such a manner that the outer rings  71 ,  72  face each other in their areas facing the testing space thereby defining the planes of the affixed films  5 ,  6 . In the outer area, the inner rings  73 ,  74  face each other directly. Located between these is a lop seal  77 . The films  5 ,  6  extend through the angle-shaped slot between the respective pairs of rings and are either tensioned or cemented.  
         [0031]    From drawing FIG. 2 it is furthermore apparent that the films  5 ,  6  form a testing space  80  in the presence of a test piece  79 . The fleece sections  9  ensure the formation of a coherent testing space  80 .  
         [0032]    The ring  72  of the bottom frame  4  is supported by the rim  7  of the plate-shaped bottom  8  and is cemented to it (adhesive layer  78 ). Assigned to the upper frame  3  is a carrier, which is formed by a steel profile  81  encompassing the frame  3  from the outside and partly from the top. Frame  3  is affixed in the axial direction in a floating manner to steel profile  81  so that after lowering, said frame rests uniformly against the entire circumference of the frame  4 . A pressing force is produced in addition by evacuating the testing space  80 . In the axial direction, the steel profile  81  extends over the frame  3  and down, and provides during the last phase of the lowering motion, a guide. The detector ring  85  serves the purpose of covering the steel ring  81 .  
         [0033]    Also depicted in drawing FIG. 2 is, that the depicted film leak indicator is equipped with a handle  82  running along circumference. Affixed to this is the holster  54  for the sniffer  48 . Located in the holster  54  are the means already detailed with reference to drawing FIG. 1, which are utilized for determining whether or not the probe  51  of the sniffer  48  has been deposited in holster  54 . Between the holster  54  and the housing  86  there extends the capillary hose  57 , which is inserted into coupling  58 .  
         [0034]    In the instance of the leak searching method in accordance with the principal patent application, measures are proposed allowing the avoidance of faulty measurements owing to internal contamination. However, faulty measurements can also occur if the ambient air around the leak indicator has an unknown and too high a helium concentration. Therefore it is also of interest to know at least the magnitude of this helium concentration in order, for example, to recognize a restricted detection limit or compensate it in a suitable manner.  
         [0035]    This is implemented through the object of the supplementary patent application in that the helium concentration of the air is measured before closing the testing chamber  80 . In order to permit this measurement at that time, the throttle valve  32 , which in the instance of the leak detection method in accordance with the principal patent application is open during gross leak searching, is modified so that it can also be open when the pressure in the line  21  is at atmospheric pressure. With chamber  80  open and with an open valve  32 , a small flow of air passes to the inlet of the detector  29 . The mass spectrometer  36  records the helium concentration in the entering air. The continuously measured values are stored for a short time (a few seconds will suffice) in a control device, for example.  
         [0036]    Through these means the helium concentration in the air can be taken into account during subsequent leak searching processes. If such a process is initiated by placing the frame  3  on the frame  4 , a value for the helium concentration in the air measured and saved just before closing the testing chamber can be taken into account in the evaluation of the leak measurement. If the test piece has a leak, then the leak search may be corrected automatically, for example. In the event of much too high helium concentrations in the air, a visual or acoustic warning may be initiated in addition.  
         [0037]    It is important that the throttle valve  32  be so designed (sufficiently small throttling aperture) that the fore-vacuum pressure of the turbomolecular pump  35  will also not attain inadmissibly high values when the line  21  is at atmospheric pressure. At least before the leak searching process commences, the throttle valve  32  must be open while the leak-searching chamber  80  is still open.