Abstract:
To achieve a large measurement range from small up to larger leakage rates, a switchover from normal operation to gross operation occurs. In gross operation, the sucked-in gas flow is separated by different throttles, wherein the throttle that leads to the test gas sensor has a low flow rate. This manner of operation prevents a too large quantity of test gas from reaching the sensor surface and contaminating the sensor. In another alternative, in gross operation the test gas flows only across a part of the sensor surface. The other part is flushed.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The disclosure is directed to a sniffing leak detector comprising a test gas sensor, and in particular to a sniffing leak detector with a large measurement range of leakage rates. 
         [0003]    2. Discussion of the Background Art 
         [0004]    Patent Application DE 10 2005 021 909 (not pre-published) describes a sniffing leak detector comprising, for example, a test gas sensor, wherein a sniffing probe is connected to the test gas sensor via a sniffing line. A vacuum pump creates a vacuum in a suction chamber provided in front of the quartz window of the test gas sensor. This sniffing leak detector allows different modes of operation, namely normal, standby, protection against contamination and simulation of a gross leak. In the mode for protection against contamination, a venting valve is temporarily connected to the suction chamber of the test gas sensor, thereby causing a flushing effect. 
         [0005]    The conventional analysis apparatus for sniffing leak detection with a high detection sensitivity for helium employs mass-spectroscopy methods for detection. These require high vacuum conditions of p&lt;10 −4  mbar. Such pressure conditions are obtained using a pump system requiring a turbomolecular pump. This kind of pump is complex. Further there are sudden failures because of fused filaments of the mass spectrometer. 
         [0006]    The Patent Application 10 2005 021 909 mentioned above describes a sniffing leak detector with a test gas sensor adapted to detect helium leakage rates in a range from 10 −7  mbar l/s to 10 −3  mbar l/s. For smaller leakage rates, the detection range is restricted due to the limited signal stability or the limited sensitivity. For higher leakage rates, the limitation is defined by a possible contamination of the sensor. At a measured leakage rate of 10 −3  mbar l/s, the helium partial pressure in front of the sensor is about 0.05 mbar. The sensor must be protected against helium partial pressures above this limit. This is achieved by changing over the gas guide so that, upon exceeding the fixed signal intensity, the apparatus temporarily switches off the detection of helium. 
         [0007]    It is an object of the disclosure to provide a sniffing leak detector having a widened leakage rate measurement range. 
       SUMMARY 
       [0008]    A first variant of the sniffing leak detector according to the present disclosure comprises a flow divider allows to simultaneously connect the sniffing line to the suction chamber of the test gas sensor via a suction line and to the vacuum line via the open valve. 
         [0009]    The disclosure allows to detect leakage rates by conducting a part of the sucked-in gas to the gas feed pump and conducting a smaller part to the sensor. 
         [0010]    A second variant provides that the sniffing line is connected with a valve means selectively connecting the sniffing line to one of a plurality of inlets of the suction chamber, these inlets causing flow paths of different lengths along the sensor surface of the test gas sensor. This variant may be referred to as a “partial surface variant”. 
         [0011]    In both variants it is advantageous for the suction chamber arranged downstream of the test gas sensor to be connected to a venting valve through which ambient air can be sucked in. In this way, helium can be flushed away from the sensor surface. Preferably, the venting valve is connected to the gas guide in series with a throttle. 
         [0012]    A desired working pressure of about 250 mbar can be generated and maintained in front of the sensor surface by a suitable valve/throttle system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    The following is a detailed description of embodiments of the disclosure with reference to the drawings. 
           [0014]    In the Figures: 
           [0015]      FIG. 1  a diagram of a first embodiment referred to as the flow division variant, and 
           [0016]      FIG. 2  a diagram of a second embodiment referred to as the partial surface variant. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    The flow division variant illustrated in  FIG. 1  comprises a basic unit  10  connected to a sniffing probe  12  via a valve V 2 . The sniffing probe  12  may be guided manually to check a test object for leaks from which a test gas escapes. 
         [0018]    The basic unit  10  includes a vacuum pump  13  which, in the present case, is a two-stage pump with the pump stages  13   a  and  13   b  designed as diaphragm pumps. The vacuum pump generates a final pressure of about 3 mbar. 
         [0019]    A vacuum line  14  leads from the vacuum pump  13  to the suction chamber  15 . The suction chamber  15  is formed in front of the test gas sensor  16 . The walls of the suction chamber  15  abut the housing of the test gas sensor  16 . The sensor surface  17  of the test gas sensor  16  is enclosed by the suction chamber  15 . Within the suction chamber  15  a gas guide plate  18  is provided that is located at a distance opposite the sensor surface  17  and in parallel therewith. The sensor surface  17  and the gas guide plate  18  define the gas guide chamber  19 . The sniffing line  11  ends in the gas guide chamber  19 . The same has lateral openings  20  at opposite ends thereof through which gas can enter into the suction chamber  15 . The gas guide chamber  19  causes a spreading of the gas in front of the sensor surface  17 . 
         [0020]    The test gas sensor  16  is configured like the sensor described in DE 100 31 882 A1. The sensor surface  17  is a membrane selectively permeable to helium. Moreover, the test gas sensor  16  includes a Penning pressure sensor or another pressure sensor generating an electric signal indicating the pressure in the housing closed by a quartz membrane. From this pressure the signal for the detected amount of test gas is derived. 
         [0021]    The vacuum line  14  includes a first throttle D 1  between the vacuum pump  13  and the suction chamber  15 , which throttle determines the throughput for the normal mode of operation. The first throttle D 1  is shunted by a bypass line  26  including a valve V 1 . 
         [0022]    A throttle D 3  is provided in an air inlet line. The valve V 3  connects either the inlet E 1  or the inlet E 2  with the outlet A. The -inlet E 1  is connected to a flow divider  30  connected to the inlet of the test gas sensor  16  through a line  31 . The line  31  includes a throttle D 4 . 
         [0023]    Another duct leads from the flow divider  30  via a valve V 4  and a throttle D 2  to the vacuum line  14 . The throttles D 2  and D 4  are matched such that the flow through D 2  is substantially larger than the flow through D 4 . The flow through D 2  is at least 10 times that through D 4  and in particular at least 50 times larger. Preferably, the flow through D 2  is about a hundred times the flow through D 4 . 
         [0024]    A pressure gauge  32  is connected to the suction chamber  15 . 
         [0025]    In normal operation of the flow division mode, the gas drawn in through the sniffing line  11  via the open valve V 2  is guided through the valve V 3  to the test gas detector  16 . The valve V 4  is closed. Using the valve/throttle system V 1 , D 1 , a working pressure of about 250 mbar is maintained in front of the sensor surface  17 . 
         [0026]    When the concentration of the test gas in the sniffed gas exceeds a predetermined limit, the system automatically switches to the gross mode. Here, the sniffed gas is guided through the valve V 2  to the line  31  via the flow divider  30 . At the branch point  30 , a first partial flow and a second partial flow are formed at the same time, the first partial flow being formed via the throttle D 4  and the second partial flow being formed via the valve V 4  and the throttle D 2 . The smaller flow that reaches the test gas sensor  16  via the throttle D 4  is guided along the sensor surface  17  to the vacuum pump  13 . 
         [0027]    During the gross operation mode, the valve V 3  is switched to the inlet E 2 , whereas the inlet E 1  is blocked. Air is drawn in via the inlet E 2 . Thereby, a fast exchange of gas is achieved in front of the sensor surface  17  using the air as a flushing gas. The throttles D 2 , D 3  and D 4  are dimensioned such that the desired flow ratio is realized, e.g. a ratio of 1:100. 
         [0028]    The additional flush gas flow through the throttle D 3  causes a complete exchange of the gas in front of the sensor surface  17  within a short time. This fast exchange would not be achieved with a flow exclusively passing through the throttle D 4 . 
         [0029]    The embodiment in  FIG. 2  corresponds to the partial surface variant, where in the gross mode of operation the test gas flows only across a fraction of the sensor surface  17 . Those components of the second variant that are also present in the first variant will not be explained again hereunder so that the following description is restricted to the differences. 
         [0030]    According to  FIG. 2 , at a position behind the valve V 2 , the sniffing line  11  is connected with the inlet E of a valve V 5  having two outlets A 1  and A 2 . The outlets A 1 , A 2  are connected to different inlets E 1 , E 2  of the gas guiding space  19 . The first inlet E 1  is located at the end of the sensor surface  17  averted from the opening  20 , whereas the inlet E 2  is closer to the opening  20 . In the present embodiment at least one opening  20  is provided at only one end of the test gas sensor  16  so that the inflowing gas has to travel paths of different lengths to the opening  20 , depending on the position of the respective inlet. The venting valve V 3  is connected to the inlet E 1  corresponding to the longer flow path along the sensor surface  17 . 
         [0031]    In normal operation of the partial surface mode in  FIG. 2  the gas sucked in by the sniffing probe  12  is guided to the test gas sensor  16  via the valve V 2  and then via the valve V 5 . The valve V 5  is set to the position E-A 1  so that the gas is supplied to the inlet E 1  of the test gas sensor and passes across the entire sensor surface  17 . In front of the sensor  16  a working pressure of about 250 mbar is maintained. 
         [0032]    In the gross operation mode the valve V 5  is in the position E-A 2  and gas is supplied to the right inlet E 2  of the test gas sensor  16 . From there, the gas flows only across a part of the sensor surface  17 . Thereafter, the gas directly reaches the pump system. With the inlet above the throttle D 3  and the valve V 3 , an additional airflow is created along the sensor surface  17 , thereby preventing helium from accumulating in front of the sensor or that high test gas concentrations dwell there. 
         [0033]    It is also possible to combine both variants described.