Abstract:
A leak detector comprises a cell provided with a tracer gas inlet preferably permeable to a tracer gas. In the cell, the tracer gas is caused to assume an energetically higher metastable state. By means of laser spectroscopy the absorption spectrum of the metastable tracer gas is sampled in an optical measuring section, whereby the presence of tracer gas is detected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention refers to a leak detector with optical tracer gas detection. 
     2. Description of the Prior Art 
     Leak detectors typically include a mass spectrometer or a similar gas analysis apparatus to examine a gas drawn in for the presence of a tracer gas. Tracer gases of choice are helium and other noble gases as well as hydrogen. The use of a mass spectrometer requires the generation of a high vacuum which in turn demands for bulky vacuum pumps. Further, leak detectors are known that have a cell which comprises gas-tight walls and is closed with a membrane selectively permeable to the tracer gas, where the membrane forms a tracer gas inlet. Within the cell, the partial pressure of the tracer gas rises if the tracer gas is present at the membrane outside the cell. Since the cell holds no other gases than the tracer gas, the partial pressure of the tracer gas can be measured in the cell by means of a total pressure measurement. This gives information about the tracer gas partial pressure in the ambience. Thus, it is not only possible to detect the presence of tracer gas in the environment, but quantitative measurements are also possible. Pressure measurement inside the cell requires an intricate measuring device and a pumping function for the removal of the tracer gas. Penning or magnetron cells are suitable cells for this purpose. 
     DE 198 53 049 C2 describes another type of leak detector wherein a carrier gas is pumped through the object under test and wherein it is detected whether a tracer gas is present in the outflowing carrier gas. If this is the case, a leak in the object under test has been determined. The gas pumped from the object under test is passed through a discharge cell and caused to assume a metastable state. The carrier gas or tracer gas of choice for generating the gas discharge is helium. It is the purpose of the gas discharge to cause the tracer gas to assume the metastable state. The discharge cell includes an optical measurement path formed by a laser and a photo detector receiving the beam from the laser. The excited atoms of the tracer gas or of the tracer gas component to be detected are measured in the discharge cell by means of laser absorption spectrometry. This measuring principle requires that the carrier gas necessary for the excitation of the tracer gas be passed through the object under test. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a leak detector with a high sensitivity, a short response time and a simple structure. 
     The present leak detector is defined in claim  1 . The leak detector comprises a cell with a gas inlet that is preferably permeable to a tracer gas. Ideally, the gas inlet is selectively or exclusively permeable to the tracer gas. This means that the membrane blocks the outside or atmospheric pressure, while it allows the tracer gas and possibly also singular other gases to pass. For instance, the membrane may comprise a thin layer of quartz or silicon oxide (SiO 2 ). The same is selectively permeable to light gases such as hydrogen or helium, especially when it is heated. The membrane keeps heavier gases and water vapor as well as anything else from the inside of the cell that could disturb the metastable state of the tracer gas. Thus, an “absolute selectivity” of the membrane to the tracer gas is not required. Rather, it is sufficient that the membrane allows the tracer gas to pass, while other gas components may also be entrained. 
     The invention allows for different leak test methods. For instance, the leak detector may be a sniffer leak detector comprising a probe passed along the outside of an object under test and detecting the escape of a tracer gas. On the other hand, the leak detector may also comprise a suction device drawing gas from an object under test, where a region containing the tracer gas is created outside the object under test. 
     Although it is not necessary to evacuate the cell, a preferred embodiment of the invention provides that the cell is connected with a vacuum pump device. 
     The excitation of the metastable state can be achieved by particle collisions of a buffer gas in a gas plasma or in a gas discharge. Another possibility provides for a direct electron impact, wherein the electrons coming from an electron source (cathode) hit the tracer gas and take it to a higher energetic level. Here, no buffer gas is needed. Further possible ways of excitation are excitation by X-rays, multi-photon excitation, Raman-type population, and a collision with neutral atoms/molecules, e.g. in an ultrasound beam. 
     The optical detection of metastable helium may be effected by absorption spectroscopy or fluorescence spectroscopy. For absorption spectroscopy, the laser source may be subjected to modulation that covers the absorption spectrum of higher excitation states. 
     According to a special embodiment provides that the cell additionally comprises a pump connection consisting of a membrane that is preferably permeable to the tracer gas, wherein the pump connection connects the cell to a chamber which is in turn connected to a vacuum pump device or an atmosphere free of tracer gas. The pump connection allows the removal of the helium from the cell, either into the ambience or by pumping action. It is the purpose of the pump connection to vent the tracer gas from inside the cell to the outside after the tracer gas has been removed from in front of the membrane. 
     In another embodiment of the invention, the cell is hermetically sealed except for the tracer gas inlet, in which way a partial pressure of the tracer gas develops inside the cell that is equal to the partial pressure of the tracer gas in the ambience. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention including the best mode thereof, enabling one of ordinary skill in the art to carry out the invention, is set forth in greater detail in the following description, including reference to the accompanying drawings in which 
         FIG. 1  is a schematic illustration of the principle of the invention, 
         FIG. 2  illustrates a concrete first embodiment in which the metastable excitation is achieved with a gas plasma from a buffer gas, 
         FIG. 3  illustrates a second embodiment comprising an additional pump connection at the cell for drawing tracer gas from the cell so as to achieve short response times, 
         FIG. 4  illustrates an embodiment in which the cell comprises a vacuum connection, and 
         FIG. 5  illustrates another embodiment in which the cell is sealed hermetically except for the tracer gas inlet so that a partial pressure develops inside the cell that is equal to the partial pressure in the ambience. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The general idea of the invention is illustrated in  FIG. 1 . The heart of the present leak detector is a cell  10  with an inner space  11  that is closed with a tracer gas inlet  12 . The tracer gas inlet  12  comprises a membrane  13  that is preferably permeable to the tracer gas, e.g. helium. Preferably, it is a membrane selectively permeable only to helium. The membrane  13  is permeable in both directions. Thus, a pressure develops in the inner space  11  of the cell  10  that is equal to the partial pressure of the tracer gas outside the cell. The cell  10  has been evacuated before, so that it cannot hold any other gas but the tracer gas. The cell  10  contains an excitation device (not illustrated) by which the tracer gas is taken to a higher state of excitation. 
     The optical detection of the metastable tracer gas is effected with a measuring section  14  comprising a laser  15  and a photo detector  16  receiving the laser beam  17 . The wavelength of the laser beam  17  emitted by the laser  15  is set according to an absorption line of the tracer gas (e.g. helium). For instance, the wavelength of the laser beam is set to 1083.034 nm so as to achieve a higher energy level 2 3 P 2  starting from the metastable level 2 3 S 1 . With a laser frequency of 1083.025 nm, the energy level 2 3 P 1  would be achieved, and the energy level 2 3 P 0  would be reached with a wavelength of 1082,908 nm. When the tracer gas in the metastable state is illuminated by the wavelength mentioned, this wavelength is absorbed. Reference may be made to DE 198 53 049 C2 for details. 
     The radiation of the laser beam  17  is modulated so that a region covering the basic wavelength is detected. Absorption spectroscopy allows the detection of the absorption wavelengths. This principle is the same for all embodiments described hereinafter. The cell  10  could also be referred to as a spectrometer cell. Preferably, it is made of glass. The laser  15  and the photo detector  16  may be arranged in the cell  10  or outside the same. 
     In the embodiment of  FIG. 2 , the cell  10  is provided with the measuring section  14 . The cell is closed with a membrane  13  that is selectively permeable to the tracer gas (helium) or is at least preferably pervious to this gas. The membrane  13  has a porous supporting body  20  that is permeable to gas and a thin filter layer  21  of SiO 2  or quartz of a few nm thickness. A heating device  22 , which is preferably external to the absorption section, serves to heat the filter layer. 
     This heating device is arranged at a distance from the filter layer  21  so that tracer gas can pass through the filter layer. Details on the structure of the membrane  13  are described in EP 0 831 964 B1 (=U.S. Pat. No. 6,277,177 B1). 
     The cell  10  comprises an excitation device  25  by which helium that has entered the cell is caused to assume a metastable state. In this instance, the excitation device comprises a cathode  26  which is part of a gas discharge section which produces a plasma from a buffer gas in the inner space  11 . The buffer gas is an inert gas, preferably a noble gas, except for helium which is used as the tracer gas. 
     The cell  10  may
         a) be exposed either directly or indirectly to ambient air (sniffer leak detection), or   b) be connected to a test chamber  28  which contains an object under test filled with tracer gas (integral leak test), or   c) be connected with an evacuated object under test that is sprayed with tracer gas from the outside (vacuum leak test).       

     The tracer gas will then pass the membrane  13  and reach the inner space  11  of the cell  10 . A test leak  30  can be provided in front of the inlet, from which tracer gas escapes in a defined volume flow. The test leak  30  is used to calibrate the leak detector so as to obtain quantitative information about the tracer gas concentration. 
     The cell  10  is further provided with a buffer gas inlet  32  through which a buffer gas is fed that is ionized by the excitation device  25 . Moreover, the cell  10  is connected with a vacuum pump device  34  via a connection  33 . The vacuum pump device draws the mixture of buffer gas and tracer gas from the cell  10 . 
     The embodiment of  FIG. 3  differs from the preceding embodiment in that the cell  10  is not connected to a vacuum pump device. The cell  10  comprises the tracer gas inlet  12  connected with the test chamber  28 . The cell further includes the measuring section  14  as well as an excitation device  25  formed by electrodes. 
     The cell  10  of  FIG. 3  is provided with a pump connection  35  connected with a suction chamber  36 . The pump chamber  36  has a buffer gas connection  37  and a pump connection  38  connected with a vacuum pump device (not illustrated). It is the function of the pump connection  35  to selectively conduct only tracer gas from the cell  10  into the suction pump  36 . It is configured in the same manner as the tracer gas inlet  12 . Together with the pump chamber  36 , the pump connection  35  forms a pump or a one-way valve for removing the tracer gas from the cell  10 . 
     The embodiment of  FIG. 4  is mostly similar to that of  FIG. 2 , however with the difference that a test chamber  28  ( FIG. 2 ) is omitted. The tracer gas inlet  12  is exposed to ambient air either directly or by drawing in ambient air through a sniffer conduit and feeding it to the inlet  12 . When a tracer gas cloud  40  reaches the tracer gas inlet  12 , it is sucked in and the tracer gas gets into the cell  10  where it is caused to take a metastable state and is then detected. The cell further comprises a buffer gas connection  41  and a pump connection  38  that may be connected to a vacuum pump device as illustrated in  FIG. 2 . The embodiment of  FIG. 4  is suited for use as a sniffer leak detector. 
       FIG. 5  illustrates another embodiment of a leak detector comprising a cell  10  with a tracer gas inlet  12 , as well as a measuring section  14  of the kind described before. The cell  10  has no pump connection. When tracer gas is present in front of the tracer gas inlet  12 , a pressure develops inside the cell  10  that is equal to the partial pressure of the tracer gas (helium) in front of the membrane. If the tracer gas is removed from in front of the membrane, the tracer gas flows from the cell  10  through the tracer gas inlet  12  back into the atmosphere. 
     The embodiment of  FIG. 5  also comprises an excitation device (not illustrated) that causes the helium in the cell  10  to assume a metastable state so that it is optically detectable. When the metastable helium contacts the wall of the cell or the tracer gas inlet or another device, it looses energy and is thus restored to its ground state. Therefore, the helium escaping from the cell  10  is no longer in a metastable state. 
     Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the true scope of the invention as defined by the claims that follow. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.