Abstract:
A sniffer leak detector comprising a detector with a quartz window, the window being selectively permeable to a tracer gas. The quartz window is located in a suction chamber that is connected to a vacuum pump. To reduce the response time, a gas guidance plate is situated in the suction chamber, the guidance plate and quartz window together delimiting a narrow gas guidance chamber.

Description:
BACKGROUND 
     1. Field of the Disclosure 
     The disclosure refers to a sniffer leak detector comprising a detector with a quartz window, and in particular a sniffer leak detector that can do with a relatively simple detection means for the tracer gas. 
     2. Discussion of the Background Art 
     For leak testing containers and pipe systems, helium is typically used as the tracer gas, and the escape of helium is detected using mass-spectroscopic methods. For this purpose, the gas is ionized and the individual constituents of the gas or their fragments are then separated using a combination of electric and magnetic fields. After separation, the helium ions are detected using a detector and the signal is subsequently amplified and evaluated. Helium may also be detected by gas chromatographic methods, where, however, the detection sensitivity is lower. 
     During sniffer leak detection, a constant defined volume of gas is drawn through a sniffer conduit. Depending on the gas throughput through the sniffer conduit and the leakage rate of a helium leak (in leak tests, test specimens are filled with helium and closed), a certain tracer gas concentration occurs in the sniffed gas flow. A small portion of this sniffed gas mixture is introduced into the detection system (mass spectrometer) via a flow divider. For a successful separation of masses in the analyser unit, the pressure in the standard detection system must not exceed several 1×10 −4  mbar. Thus, the helium partial pressure to be detected is merely 10 −10  mbar for a helium concentration of 1 ppm. 
     Generating a total pressure on the order of 1×10 −4  mbar requires the use of a turbomolecular pump. The use of such a pump is expensive and increases the susceptance to failure and the maintenance effort of the system. It is desirable to allow for the detection of a tracer gas under atmospheric conditions so that the turbo molecular pump and the flow divider can be omitted. 
     U.S. Pat. No. 6,277,177 B1 (Leybold Vakuum GmbH) and DE 100 31 882 A1 (Leybold Vakuum GmbH) describe detectors for helium or hydrogen that comprise a quartz window membrane closing a vacuum-tight housing. The quartz window membrane forms a selective passage for the gas to be detected. The tracer gas detector arranged in the housing is a gas pressure sensor reacting to the total pressure of the gas having entered the housing. Since the quartz membrane is gas selective, the gas pressure detector determines the pressure of the tracer gas in the housing. The pressure sensor arranged in the housing preferably is a Penning pressure sensor having two mutually spaced parallel cathode plates and an anode ring provided between the cathode plates. The electric circuit including the cathode plates and the anode ring includes a current measuring device for measuring the cathode or anode current. A magnetic field is generated by a permanent magnet situated outside the closed housing. Such a Penning detector has the advantage that it is much simpler and more economic than a sniffer leak detector comprising a mass spectrometer. In quartz window technology, the entire gas flow is directed past the detector. Also during operation, the pressure at the sensor may correspond to atmospheric pressure. Thus, the helium partial pressure is 10 −3  mbar for a He concentration of 1 ppm, i.e. it is seven orders of magnitude higher than the pressure of the standard method. Separating helium and other gases is effected by means of a thin quartz membrane exclusively allowing helium to get into the detection volume. 
     It is the object of the disclosure to provide a sniffer leak detector that is of a simple and economic structure and has a high sensitivity. 
     SUMMARY OF THE DISCLOSURE 
     It is a sniffer leak detector comprising a quartz window selectively permeable to a tracer gas. The gas to be analyzed is supplied to the quartz window via a sniffer probe. The gas is passed through a suction chamber delimited by the quartz window such that it flows along the quartz window. 
     A quartz window sensor has the advantage of being operable at a pressure of several 100 mbar. In contrast to the “single-spot” gas inlet in the shape of the flow divider of a mass spectrometer system, the gas inlet of a quartz window detector is distributed over the entire surface of the quartz window covering an area of several square centimeters. The effect of the suction chamber is that the tracer gas supplied at a single spot by the sniffer conduit spreads out uniformly in front of the quartz window before being sucked away by the vacuum pump. Due to the low partial pressure of the tracer gas inside the quartz window detector, a portion of the tracer gas permeates through the quartz membrane into the housing despite the effect of the vacuum pump. By the tracer gas flow being distributed uniformly in front of the quartz window, a high responsiveness is achieved. 
     According to a preferred embodiment, the suction chamber includes a gas guidance plate that defines a gas guidance chamber together with the quartz window, the sniffer conduit terminating in the gas guidance chamber. The gas guidance chamber is substantially narrower than the suction chamber. It defines the volume formed in front of the quartz window. The response time of the sniffer leak detector is essentially determined by the volume of gas that has to be exchanged in front of the sensor. The time τ required to exchange the gas volume in front of the detector membrane can be calculated as follows: 
     
       
         
           
             τ 
             = 
             
               pV 
               
                 q 
                 pV 
               
             
           
         
       
         
         
           
             p: pressure in front of the membrane; V: volume in front of the membrane; q pV : flow. 
           
         
       
    
     The volume V of the gas guidance chamber is thus reduced by the gas guidance plate, whereby the response time is also reduced. On the other hand, the gas pressure p in front of the gas membrane can be chosen rather high, at several 100 mbar, to achieve a high sensitivity, since the volume has been made correspondingly small so that an overall exchange time clearly below 100 ms is obtained. The gas sniffed is guided along the gas guidance chamber past the quartz membrane. The cross section is dimensioned in view of a minimum volume and a sufficient flow conductance of the gas guidance chamber. It has to be sufficiently large for an unimpeded gas flow. After the gas has flown along the quartz membrane, it enters the suction chamber connected to the vacuum pump to be pumped away at a defined pumping speed. 
     The disclosure further refers to a protection of the sniffer leak detector against contamination. Following the presence of helium, a quartz window helium detector shows a signal decay occurring in two stages. There is a fast decay of the signal corresponding to the vacuum time constant of the sensor and a slow decay of the shifted offset current. If the sniffer leak detector is used to sniff air heavily charged with helium, a contamination of the sensor results in a shift of the offset. The degree of the shift depends on the amount of helium additionally taken in by the sensor. A shift in the offset has to be avoided for an exact and fast leak detection. This is achieved by lowering the total pressure in front of the detector membrane when a predetermined signal threshold is exceeded. To achieve this, a suction conduit from the vacuum pump to the suction chamber can be bridged by a non-throttled bypass conduit including a valve. The bypass conduit can be opened momentarily to increase the pumping speed at which the volume in front of the quartz membrane is pumped. Thereby, the total pressure, and thus the helium partial pressure in front of the detector membrane, is lowered. Thus, the amount of helium getting into the detector is reduced. 
     Upon exceeding an even higher signal threshold, a venting valve may be opened whereby the gas in front of the quartz membrane is replaced by air. In the meantime, the gas in the sniffer conduit heavily charged with tracer gas is supplied directly to the vacuum pump. 
     During its service life, the quartz window detector can take up only a limited amount of helium. In its natural state, air has a helium content of 5 ppm, so that the detector permanently takes up helium from the atmosphere during operation. The portion the detector takes up in the standby mode has to be reduced. To guarantee a quick restart with a high accuracy of measurement after a standby mode operation, however, the temperature profile on the detector membrane has to be maintained. During the standby mode, the pressure in front of the quartz membrane is reduced. The cooling efficiency at the heated detector membrane substantially depends on the gas flow (not the pressure) in front of the membrane. Lowering the pressure has only little effect on the flow so that the temperature profile on the detector membrane is maintained in the standby mode. Alternatively, in the stand-by mode, the gas flow may be sucked through an appropriate throttle instead of the sniffer conduit. This is advantageous in that the particle filter in the tip of the sniffer conduit is protected. 
     According to a preferred embodiment of the disclosure, a “gross leak simulation” mode can be run at certain intervals. Here, the operator trains the system for a fast signal decay. To do so, the sniffer leak detector sniffs at a location with a large helium leakage rate, e.g. 10 −3  mbar l/s, so that the system can “learn” again to decay quickly to the initial level after large amounts of helium. Generally, the test leak incorporated in a standard leak detector has a leakage rate of only 10 −5  mbar l/s. To simulate a high leakage rate, the pumping speed of the vacuum pump is reduced by means of a throttle such that the flow through the sniffer conduit decreases approx. by a factor of 100. Thus, the helium concentration increases by the same factor when sniffing at the built-in test leak. The system associates a correspondingly higher leakage rate to the increased concentration. In this manner, a leak of 10 −3  mbar l/s is simulated using a test leak of 10 −5  mbar l/s. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a detailed description of embodiments of the disclosure with reference to the drawings. 
         FIG. 1  is a block diagram of a first embodiment of the sniffer leak detector according to the disclosure, and 
         FIG. 2  is a block diagram of a second embodiment of the sniffer leak detector. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to  FIG. 1 , a basic device  10  is provided that is connected to a sniffer probe  12  via a flexible sniffer conduit  11 . The sniffer probe  12  can be guided manually to search the test object for leaks from which tracer gas escapes. 
     The basic device  10  includes a vacuum pump  13  which, in the present embodiment, is a two-stage pump with the pump stages  13   a  and  13   b  designed as diaphragm pumps. The vacuum pump generates a vacuum of about 50 mbar. 
     A suction conduit  14  leads from the vacuum pump  13  to the suction chamber  15 . The suction chamber  15  is formed in front of the quartz window detector  16 . The walls of the suction chamber  15  adjoin the housing of the quartz window detector  16 . The quartz window  17  of the quartz window detector  16  is enclosed by the suction chamber  15 . A gas guidance plate  18  is provided in the suction chamber  15 , which plate is arranged at a distance opposite and in parallel to the quartz window  17 . The quartz window  17  and the gas guidance plate  18  delimit the gas guidance chamber  19 . The sniffer conduit  11  terminates in the gas guidance chamber  19 . The same has lateral exit openings  20  through which the gas can enter the suction chamber  15 . The gas guidance chamber  19  causes a distribution of the gas in front of the quart window  17 . 
     The quartz window detector  16  is designed similar to the detector described in DE 100 31 882 A1. The quartz window  17  is formed by a membrane selectively permeable to helium. Moreover, the quartz window detector  16  includes a Penning pressure sensor or another pressure sensor generating an electric signal indicating the pressure in the housing sealed by the quartz membrane. The signal for the detected amount of tracer gas is derived from this pressure. 
     Between the vacuum pump  13  and the suction chamber  15 , the suction conduit  14  includes a first throttle D 1  determining the pumping speed for the “normal” mode. The first throttle D 1  is shunted by a shunt conduit  25  including a second throttle D 2  and a valve V 2  in series therewith. Further, the throttle D 1  is bridged by a bypass conduit  26  including a valve V 1 . 
     The suction chamber  15  is adapted to be connected to ambient air through a venting valve V 3 . 
     A contamination protection valve V 4  connects the sniffer conduit  11  to the suction inlet of the vacuum pump  13  without throttling. 
     The suction conduit  11  includes an additional valve V 5  situated immediately in front of the valve V 4  in the gas flow. The valve V 5  is a magnetic valve used to stop the flow during the standby mode and thus enabling to lower the pressure in front of the detector membrane of the quartz window detector  16  to the base pressure of the vacuum pumps. 
     The following table 1 indicates the different modes “Normal”, “Standby”, “Contamination protection” and “Gross leak simulation” together with the corresponding positions of the valves V 1 , V 2 , V 3  and V 4 . The table further indicates which element determines the pumping speed of the pumping device. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Pumping speed 
               
               
                 Mode 
                 V1 
                 V2 
                 V3 
                 V4 
                 determined by 
               
               
                   
               
             
             
               
                 Normal 
                 Closed 
                 Open 
                 Closed 
                 Closed 
                 Throttle 2 
               
               
                 Standby 
                 Open 
                 Open 
                 Open 
                 Closed 
                 Throttle 3 
               
               
                 Contamination 
                 Open 
                 Open 
                 Open 
                 Open 
                 Pump 
               
               
                 Protection 
                   
                   
                 (momentarily) 
               
               
                 Gross leak 
                 Closed 
                 Closed 
                 Closed 
                 Closed 
                 Throttle 1 
               
               
                 simulation 
               
               
                   
               
             
          
         
       
     
     The embodiment of  FIG. 2  differs from that of  FIG. 1  in that the venting valve is situated in the sniffer conduit  11 , thereby venting the gas guidance chamber  19 . The venting valve V 3  has a throttle D 3  provided at its inlet to restrict the venting flow. Also in this embodiment, the contamination protection valve V 4  is situated between the sniffer conduit  11  and the vacuum pump  13 . 
     The operation modes of the embodiment of  FIG. 2  are given in Table 2 below. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Pumping speed 
               
               
                 Mode 
                 V1 
                 V2 
                 V3 
                 V4 
                 determined by 
               
               
                   
               
             
             
               
                 Normal 
                 Closed 
                 Open 
                 Closed 
                 Closed 
                 Throttle 2 
               
               
                 Standby 
                 Open 
                 Open 
                 closed 
                 Closed 
                 Pump 
               
               
                 Contamination 
                 Open 
                 Open 
                 Open 
                 Open 
                 Pump 
               
               
                 Protection 
                   
                   
                 (momentarily) 
               
               
                 Gross leak 
                 Closed 
                 Closed 
                 Closed 
                 Closed 
                 Throttle 1 
               
               
                 simulation 
               
               
                   
               
             
          
         
       
     
     The two variants shown only differ in their operation in the standby mode. 
     In the first embodiment, the gas flow is still passing through the sniffer conduit  11  during standby. Since the sniffed gas passes no valve on its way from the sniffer probe  12  to the quartz window detector  16 , the system has a short response. 
     In the embodiment of  FIG. 2 , the gas flow is guided through the throttle D 3 . The flow conductance of the throttle D 3  is chosen to be the same as the flow conductance of the sniffer conduit  11 . Thus, it is guaranteed that the gas flow at the detector window  17  during standby mode is unchanged compared to normal operation. Moreover, during standby operation in this variant a particle filter provided in the sniffer conduit  11  is preserved.