Abstract:
In leak detection, the signal generated by a test gas is superposed by interferences which fade as the vacuum generation in a container proceeds. Depending on the negative slope of the volume signal (MS), a lower indication limit (AG) is calculated. Upon activation of a zero function, the volume signal (MS) is not reduced to zero but only to the level of the indication limit (AG). Any exceeding of the indication limit is identified as a leak. Thus, the maximum sensitivity of the leak detection is guaranteed at any time.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention refers to a method for leak detection, wherein gas is drawn off and the presence of test gas is detected in the drawn-off gas. 
       BACKGROUND OF THE INVENTION 
       [0002]    Minute leak rates of a container can be reliably determined by the vacuum method. The smaller the leak rate, the higher the demands regarding purity and final vacuum. In local leak detection, a vacuum pump evacuates the container until the test pressure required for the leak detector is reached. Suspected leaks are then sprayed with a fine test gas jet from outside. Test gas entering the container is pumped off by the vacuum pump and detected by a mass spectrometer. 
         [0003]    A test gas frequently used for leak detectors is helium. There is a problem with the restricted selectivity of the mass spectrometer. Water precipitates both on the outside and on the inside of containers used for leakage measuring. The H 2  component of water also includes parts whose presence susceptibly interfere with the measuring of helium. At the beginning of the pumping, the amount signal, which is supposed to represent only the amount of test gas, is superposed by a noise value generated by the presence of water or other contaminations. The noise value fades with increasing pumping time to asymptotically approximate a horizontal line. However, this line never reaches a value of zero, since an absolute vacuum can not be achieved, just as well as an absolute absence of leakage can not be achieved. It is thus a matter of the respective application which portion of the fading volume flow characteristic is selected for leakage measuring. 
         [0004]    Since the curve of the volume signal fades as the pumping time increases, the sudden occurrence of test gas that causes a rise in the curve of the volume signal is superimposed by the fading background signal. When the background signal decreases to a greater extent than the detection signal rises, no detection signal is determined at all. The corresponding leak is not detected by the leak detector; it remains invisible. 
         [0005]    It has been suggested to provide a leak detector with a zero function. Here, the device is equipped with a zero key that may be pressed by the user to subtract the previous signal from the current signal. Thus, the signal amplitude is set to zero. If then the still fading background signal decreases to a larger extent than the volume signal rises due to the leak, a negative signal results in which the signal rise caused by the leak is neither detectable nor measurable. 
         [0006]    In the commonly used methods, the user can press the zero key at any time, the background signal being set to zero. As a consequence, a seemingly low leak rate is indicated, whereas the actual leak rate is higher. Such a mode of operation may have fatal consequences. It is important that no leak rate remains unnoticed. On the other hand, one leak rate indicated too much is less problematic. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide a method for leak detection, wherein the safety of leak detection is increased. 
         [0008]    According to the invention, signal dithering or signal turbulence of the a measuring signal is determined and evaluated. The signal dithering is the variation of the signal background per unit time. In a first variant of the method, the volume signal is not set to zero by the generation of the command signal usually referred to as the zero signal, but it is merely reduced down to a lower indication limit so that the signal value obtained is still positive. The lower indication limit indicates to what extent a leak can be detected. The function of the zero signal is not blocked. If the leak rate determined is above the lower indication limit, it is indicated; otherwise, it is not indicated. The method does not operate with a zero level. It is determined up to which signal dithering which leak rate is still sufficiently well measurable. Thus, it is automatically indicated which sensitivity the device has at the time the zero signal has been generated by pressing the zero key. 
         [0009]    In a second variant of the present method, the signal dithering of the volume signal is also determined. The user sets a so-called “trigger value” that indicates the desired sensitivity of the leak detection; for example, a leak rate value of 10 −10  mbar l/s (millibar times liter per second). The zero function is released only if the signal dithering of the volume signal is smaller than the trigger value. As long as the instability of the volume signal is greater than the trigger value, the zero function is blocked and no leak indication is given. Thus, the user has to wait until the volume signal has quieted so far that the desired sensitivity set by the trigger value is reached. 
         [0010]    According to the invention, the zero function is blocked for the user. The zero function is only enabled when, due to the smoothing of the signal, the device can reliably measure leak rates corresponding to the input trigger value. 
         [0011]    The invention further refers to corresponding leak detectors for the first and the second variants of the method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    The following is a detailed description of embodiments of the invention with reference to the drawings, in which: 
           [0013]      FIG. 1  is a unifilar drawing of a leak detector operating according to the present method, 
           [0014]      FIG. 2  is a block diagram of a first variant of the method, 
           [0015]      FIG. 3  is a time chart of the volume signal according to the first variant, 
           [0016]      FIG. 4  is a block diagram according to a second variant, and 
           [0017]      FIG. 5  is a time chart of the volume signal according to the second variant. 
       
    
    
     DETAILED DESCRIPTION  
       [0018]    The leak detector  10  of  FIG. 1  comprises an inlet flange  11  to which the container to be tested is connected. A conduit  12  leads from the inlet flange  11  to the vacuum pump device  13 . The vacuum pump device  13  is comprised of a turbo molecular pump  14  and a downstream pre-vacuum pump  15 . The conduit  12  is connected to a side inlet of the turbo molecular pump  14 , the inlet side of which is connected to a mass spectrometer  16 . In the turbo molecular pump  14 , a test gas, e.g., helium, contained in the drawn-off gas reaches the mass spectrometer  16  in the counter flow to the feed direction, where it is identified. The mass spectrometer  16  supplies the volume signal representing the volume of test gas detected to a microcomputer  17 , which executes the treatment described in the following. The microcomputer  17  is connected with a control unit  18  comprising a display device  19 , e.g., a monitor, an input device  20  with various keys and a zero key  21 . 
         [0019]    The container  25  to be examined is connected to the inlet flange  11 , the container having a (undesired) leak  26 . The leak  26  is sprayed with test gas, e.g. helium, from a spray gun  27 . The test gas entering the container  25  reaches the mass spectrometer  16  via the turbo molecular pump  14 . The volume of test gas is displayed as the volume signal MS on the display device  19  in the form of a curve and/or as a numerical value. 
         [0020]    As illustrated in  FIG. 3 , at the beginning of the suction operation, the volume signal MS has a relatively high value. During the suction operation, the volume signal MS fades asymptotically. The high value of the volume signal MS is due to water and other contamination, as well as residual amounts of helium contained in the gas drawn off. Thus, the volume signal MS has a drift that is influenced by external influences. This drift may greatly exceed the measuring signal. The representation of the volume signal MS along the coordinate in  FIG. 3  is established logarithmically in decimal powers. The volume signal MS illustrated in the initial part of the curve is obtained in the absence of a leak. It forms the signal background before which a leak is still to be detected. 
         [0021]      FIG. 2  illustrates the structure of an embodiment of the evaluation circuit or of the treatment in the microcomputer  17 , respectively. The volume signal MS intended to indicate the leak rate Q is supplied to one input of a maximum value selector  30  via a subtractor. At its subtraction input, the subtractor  31  receives a signal from a memory  32 , connected to the output of the subtractor  31 . The memory is activated by a “zero” signal on line  33  such that it initiates a subtracting operation wherein the output signal of the subtractor  31  is subtracted from the volume signal MS. This yields the reduced volume signal MS z  which is supplied to the one input of the maximum value selector  30 . The entire value of the volume signal is subtracted in the subtractor  31 , whereby the value MS z  of zero is obtained. 
         [0022]    The volume signal MS is further supplied to a differential circuit  34  which, from the leak rate Q, forms a signal 
         [0000]    
       
         
           
             
                
               Q 
             
             
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               t 
             
           
         
       
     
         [0000]    representing the signal dithering. This signal is a time derivation of the leak rate. The stronger the signal, the steeper the decline of the volume signal MS is ( FIG. 3 ). The signal from the differential circuit  34  is multiplied by a constant of 1/k. From this, the lower indication limit AG is determined. The value of AG is supplied to the second input of the maximum value selector  30 . The maximum value selector selects the highest value among the two input values MS z  and AG. This value will be displayed on the display device as the display signal AS. 
         [0023]      FIG. 3  illustrates the course of the display signal AS, i.e. the condition with the zero key pressed. It is assumed in  FIG. 3  that the zero key  21  is pressed at the time t 1  to prepare the detection of a leak. Shortly thereafter, test gas is sprayed against the container  25  using a spray gun  27 . The spraying happens at the time t 2  and ends at the time t 3 . 
         [0024]    It is apparent that the display signal AS generated by the maximum value selector  30  drops to the value of the indication limit AG at the time t 1  because, from the time t 1  on, the indication limit AG is greater than the volume signal MS z  then generated by the subtractor  31 . If the leak is sprayed at time t 2 , the measuring signal rises beyond the indication limit AG by the spraying, so that a pulse  37  is generated which, however, fades already during the spraying in accordance with the general drift and eventually ends on the curve of the indication limit AG. The pulse  37  is clearly identifiable within the curve of the display signal AS and is thus detectable as a leak. 
         [0025]    After some time, the process of the actuation of the zero key can be repeated, whereupon the container is sprayed again. The lower indication limit AG is fixed and displayed on the display device. A leak may be determined by the variation of the displayed value of AG to a higher value corresponding to the pulse  37 . Thus, the display device always indicates the lower indication limit AG at which a leak rate is sufficiently well displayable. A user can thus perform a leak detection with the previously displayed sensitivity while a desired indication limit AG is displayed. 
         [0026]      FIG. 4  illustrates an embodiment of the second variant of the invention. Again, the measured leak rate Q is supplied to a subtractor  31  as a volume signal MS. The output of the subtractor is connected with the subtraction input of the subtractor via a memory  32 . The signal output from the memory  32  is caused by a signal on line  33 . The zero signal inputted by a zero key is supplied to a disabling means  40  generating the signal for line  33 . The disabling means  40  is enabled by a signal on line  41 . 
         [0027]    At a trigger input  42 , which may be the input device  20  of  FIG. 1 , the user will input a trigger value T in mbar*l/s. The trigger value first represents a limit value, the exceeding of which indicates “too big” a leak. 
         [0028]    The value Q z  which the measured leak rate ahs assumed after the enabling of the zero function, is supplied to one input of a comparator  43 . The other input of the comparator  43  generates an output signal if Q z &gt;T. This output signal activates the trigger alarm  44  that indicates that the volume signal is greater than the inputted trigger value and thus exceeds the limit value. This means the detection of a leak. The size of the leak is indicated at the leak rate display  45  which receives the signal Q z . 
         [0029]    The volume signal MS which represents the leak rate Q is supplied to a differential circuit  50  forming the differential quotient 
         [0000]    
       
         
           
             
               
                  
                 Q 
               
               
                  
                 t 
               
             
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         [0000]    The output signal D of the differential circuit  50  indicates the signal dithering of the leak rate Q, i.e. the (negative) slope of the volume signal. The signal D of the differential circuit  50  indicates the signal dithering of the leak rate Q, i.e. the (negative) slope of the volume signal. The signal D is supplied to two logic circuits  51  and  52 . The first logic circuit  51  supplies an output signal if the following condition is fulfilled: 
         [0000]      ( D&lt; 0)         (| D|&gt;c*T ). 
         [0030]    The second logic circuit  52  generates an output signal if the following condition is fulfilled: 
         [0000]      ( D&gt; 0)         (| D|&lt;k*T ). 
         [0031]    Herein
       D: indicates the signal dithering in mbar (millibar)*l/s (liters per second) per minute,   T: is the set trigger value in mbar*l/s, and   c, k: are constant values, where c&gt;k.       
 
         [0035]    Through the selection of the constant values c and k, the minimum duration can be predefined, during which a leak of the size T is visible to a user before it becomes “invisible” again by the negative drift of the leak rate signal. Here, c&gt;k is necessary to obtain a hysteresis between the “disabling” and the “enabling” of the zero function. This function prevents an erroneous operation of the zero function and guarantees that leaks with the size of the set trigger value are detected by the user. 
         [0036]    The output signals of the logic circuits  51  and  52  control a flip flop  53  to the output of which the line  41  is connected that controls the disabling means  40 . The signal from the logic circuit  51  controls the setting input S and the signal from the logic circuit  52  controls the resetting input R of the flip flop  53 . The output of the flip flop is connected with an indicating device  54  having two lights  55 ,  56  of different colors. The light  55  is lit when the zero function is enabled, and the light  56  is lit when the zero function is disabled. 
         [0037]      FIG. 5  illustrates the course in time of the leak rate Q, forming the volume signal MS. In  FIG. 5 , the representation of the volume signal MS is also based on decimal powers. From the leak rate Q, the signal dithering D is determined, which is also plotted in  FIG. 5  scaled as a curve 
         [0000]    
       
         
           
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                 D 
                 k 
               
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         [0000]    The curve 
         [0000]    
       
         
           
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               D 
               k 
             
           
         
       
     
         [0000]    intersects the trigger value T at the point P. This means, that the zero function is enabled by the disabling means  40  in  FIG. 4 . The indicating light  55  is lit. The user may now generate the zero signal manually. From the time t 1  to the time t 2 , the user sprays test gas against the leak using a spray gun  27 . This causes a positive rise  60  of the volume signal. From the time t 2 , the zero function is disabled. As soon as the signal D passes the set trigger value T from the top down again, the zero function is enabled again from the time t 3  on. In  FIG. 5 , the hysteresis caused by the constants c and k is not illustrated for the sake of clarity. 
         [0038]    The signal rise  60  indicates a detected leak. 
         [0039]    The variant of  FIGS. 4 and 5  is based on the determination of whether the leak rate is still measurable with the value of the desired trigger value T. If it is measurable, the zero function is enabled; if it is not measurable, the zero function is disabled. 
         [0040]    In the above described embodiments, gas is drawn from a container to check this gas for test gas. In a variation, the invention is also applicable in sniffing leak detection, where a leak is detected by a suctioning probe drawing in ambient air at the site to be checked.