Abstract:
Trace gas leak detectors and methods for trace gas leak detection of large leaks at relatively high test pressures are provided. A trace gas leak detector includes a test port to receive a sample containing a trace gas, the test port connected to a test line, a mass spectrometer to detect the trace gas, a high vacuum pump having an inlet port coupled to the inlet of the mass spectrometer, and a forepump having a main inlet, at least one intermediate inlet and an exhaust. The main inlet of the forepump is coupled to the exhaust port of the high vacuum pump. The intermediate inlet is controllably connected to the test line. The forepump is selected from the group consisting of a scroll vacuum pump and a screw vacuum pump.

Description:
FIELD OF THE INVENTION 
     This invention relates to detection of leaks in sealed articles and, more particularly, to systems and methods for trace gas leak detection of large leaks at relatively high test port pressures. The trace gas is typically helium but is not limited to helium. 
     BACKGROUND OF THE INVENTION 
     Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a trace gas, which passes through the smallest of leaks in a sealed test piece. The helium is then drawn into a leak detection instrument and is measured. The quantity of helium corresponds to the leak rate. An important component of the instrument is a mass spectrometer, which detects and measures the helium. The input gas is ionized and mass analyzed by the spectrometer in order to separate the helium component, which is then measured. In one approach, the interior of a test piece is coupled to the test port of the leak detector. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector. 
     One of the requirements of the mass spectrometer is that the inlet through which the helium and other gases are received be maintained at a relatively low pressure, typically below 2×10 −4  Torr. In a conventional leak detector, a vacuum pumping arrangement is utilized to maintain the input of the mass spectrometer at the required pressure. However, since the test port must be maintained at a relatively low pressure during a leak test, the rough pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, which results in a high test port pressure, it may be difficult or impossible to achieve the required pressure level. 
     In a counterflow leak detector disclosed in the U.S. Pat. No. 3,690,151, issued Sep. 12, 1972 to Briggs, the mass spectrometer tube is connected to the inlet of a diffusion pump and the helium trace gas is introduced through the foreline, or exhaust port, of the diffusion pump. The diffusion pump exhibits a high pressure ratio for heavier gases, but a low pressure ratio for lighter gases such as helium. Therefore, helium diffuses at an acceptable rate in a reverse direction through the diffusion pump to the mass spectrometer and is measured. Heavier gases in the sample are to a large degree blocked by the diffusion pump and prevented from reaching the mass spectrometer. Due to the method of reverse flow in the diffusion pump, the leak detector test port can be operated at a much higher operating pressure, typically 100 millitorr. 
     A test port pressure of 100 millitorr is satisfactory for many leak test applications. However, it is desirable in some applications to perform leak tests on very large or leaky parts where this test port pressure cannot be attained. In another prior art arrangement, a flow restrictor is positioned between the test port and the foreline of the high vacuum pump. Separate roughing pumps are used to pump the test port and the foreline of the high vacuum pump. This approach permits a higher test port pressure but is more complex and expensive because of the need for two roughing pumps. 
     Another prior art approach is disclosed in the U.S. Pat. No. 4,735,084, issued Apr. 5, 1998, to Fruzzetti. The trace gas passes in reverse direction through one or two stages of a mechanical vacuum pump, thereby achieving a high test port pressure. 
     A counterflow leak detector with high and low sensitivity operating modes is disclosed in the U.S. Pat. No. 4,845,360, issued Jul. 4, 1989, to Landfors. A diffusion pump includes a conventional foreline and a second foreline provided with an ejector stage. The leak detector has high and low sensitivity operating modes. 
     A leak detector which utilizes a turbomolecular vacuum pump having an inlet connected to a gas sensor, an outlet connected to a forepump and an intermediate inlet connected to the test port is disclosed in the U.S. Pat. No. 4,472,962, issued Sep. 25, 1984 to Mennenga. 
     The U.S. Pat. No. 5,542,828, issued Aug. 6, 1996, to Grenci et al. discloses a system for vacuum pumping a mass spectrometer, which uses a scroll vacuum pump in combination with a high vacuum pump. 
     None of the prior art arrangements for leak detection of large volume and/or leaky parts at relatively high pressures has been entirely satisfactory. Accordingly, there is a need for new and improved systems and methods for trace gas leak detection of large leaks at relatively high test pressures. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, a trace gas leak detector comprises a test port to receive a sample containing a trace gas, the test port connected to a test line, a mass spectrometer to detect the trace gas, the mass spectrometer having an inlet, a high vacuum pump having an inlet port coupled to the inlet of the mass spectrometer, the high vacuum pump having an exhaust port, and a forepump having a main inlet, at least one intermediate inlet and an exhaust, the main inlet coupled to the exhaust port of the high vacuum pump, and the intermediate inlet controllably connected to the test line. The forepump is selected from the group consisting of a scroll vacuum pump and a screw vacuum pump. 
     According to another aspect of the invention, a method for trace gas leak detection comprises providing a mass spectrometer to detect the trace gas, the mass spectrometer having an inlet, and a high vacuum pump having an inlet port coupled to the inlet of the mass spectrometer, the high vacuum pump having an exhaust port, providing a forepump having a main inlet, at least one intermediate inlet and an exhaust, the forepump selected from the group consisting of a scroll vacuum pump and a screw vacuum pump, coupling the main inlet of the forepump to the exhaust port of the high vacuum pump, and supplying a sample containing the trace gas to the intermediate inlet, wherein the trace gas moves in reverse direction through the forepump and the high vacuum pump and is detected by the mass spectrometer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  is a simplified block diagram of a trace gas leak detector in accordance with an embodiment of the invention; 
         FIG. 2  is a simplified cross-sectional view of a scroll vacuum pump; 
         FIG. 3  is a simplified cross-sectional view of a screw vacuum pump; and 
         FIG. 4  is a flow chart that illustrates operation of the trace gas leak detector of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A trace gas leak detector in accordance with an embodiment of the invention is shown in  FIG. 1 . A test piece  10  having a test volume  12  is attached to an inlet flange  14 . Inlet flange  14  defines a test port of the leak detector and is connected through a test valve  16  to a test line  20 . Test line  20  is coupled through a roughing valve  22  to a main inlet  24  of a forepump  30 . A forepump exhaust  32  may exhaust into atmosphere or into an exhaust conduit. The leak detector further includes a high vacuum pump  40  and a mass spectrometer  42 . Mass spectrometer  42  has an inlet  44  coupled to an inlet of high vacuum pump  40 . A foreline  46 , or exhaust port, of high vacuum pump  40  is coupled to the main inlet  24  of forepump  30  and is coupled through roughing valve  22  to test line  20 . An electronic controller  50  controls mass spectrometer  42 , high vacuum pump  40 , forepump  30  and all valves in the leak detector during operation. 
     In accordance with an embodiment of the invention, forepump  30  is a scroll vacuum pump or a screw vacuum pump having at least one intermediate inlet  60 . Each of these pump types has a working volume that extends from the main inlet to the exhaust. The pressure along the working volume varies more or less continuously from the main inlet to the exhaust. Intermediate inlet  60  is connected to the working volume of forepump  30  at an intermediate location between main inlet  24  and exhaust  32 . As a result, intermediate inlet  60  operates at an intermediate pressure between the pressure of main inlet  24  and the pressure of exhaust  32 . Thus, in general, intermediate inlet  60  operates at a higher pressure than main inlet  24 . Intermediate inlet  60  is coupled through an intermediate valve  62  to test line  20 . 
     Forepump  30  may have one intermediate inlet  60  or may have more than one intermediate inlets to different locations in the working volume between main inlet  24  and exhaust  32 . In the embodiment of  FIG. 1 , forepump  30  optionally includes a second intermediate inlet  66 , which is coupled through a second intermediate valve  68  to test line  20 . 
     High vacuum pump  40  may be a turbomolecular pump, a so-called hybrid turbopump, a molecular drag pump or a diffusion pump. In a hybrid turbopump, one or more of the axial pumping stages of the turbomolecular pump are replaced with disks which rotate at high speed and which function as molecular drag stages. This configuration is disclosed in the U.S. Pat. No. 5,238,362, issued Aug. 24, 1993 to Casaro et al. The hybrid turbopump may include additional pumping stages, such as regenerative stages, as described in the U.S. Pat. No. 5,538,373, issued Oct. 25, 1994 to Hablanian. In each case, the vacuum pump is characterized by a relatively high reverse flow rate for light gases, such as helium, and a relatively low reverse flow rate for heavy gases, so that helium passes through the vacuum pump in a reverse direction from foreline  46  to mass spectrometer  42  and other gases are substantially blocked. The reverse flow rate refers to the flow rate in a reverse direction from the foreline of the pump to its inlet. 
     A simplified cross-sectional view of a scroll-type vacuum pump, or a scroll pump, suitable for use as forepump  30  in the leak detector of  FIG. 1  is shown in  FIG. 2 . Gas is evacuated from a vacuum chamber or other equipment, such as a leak detector, connected to main inlet  24  of the pump. The pump further includes exhaust  32  for discharge of the gas being pumped. The scroll pump includes a set of intermeshed, spiral-shaped scroll blades. The scroll pump of  FIG. 2  includes a stationary scroll blade  100  extending from a stationary scroll plate  102  and an orbiting scroll blade  104  extending from an orbiting scroll plate (not shown). Scroll blades  100  and  104  extend axially toward each other and are intermeshed together to form interblade pockets  110 . Orbiting motion of scroll blade  104  relative to scroll blade  100  produces a scroll-type pumping action of gas entering into the interblade pockets  110  between the scroll blades. The interblade pockets  110  move from main inlet  24  toward exhaust  32 , thereby pumping gas in the interblade pockets. The interblade pockets  110  constitute the working volume of the scroll vacuum pump. The construction and operation of scroll vacuum pumps is generally known to those skilled in the art. 
     As further shown in  FIG. 2 , the scroll vacuum pump is provided with intermediate inlet  60 . Intermediate inlet  60  may be implemented as a hole through stationary scroll plate  102  to access the interblade pockets  110 . Intermediate inlet  60  may be located at any position between main inlet  24  and exhaust  32  in accordance with the expected pressure in test line  20  during a leak test. Furthermore, optional second intermediate inlet  66  may be positioned as desired along the spiral path between main inlet  24  and exhaust  32 . Because of the continuous nature of the pumping path between main inlet  24  and exhaust  32  of the scroll pump, each intermediate inlet can be located over a range of positions, with the position selected according to the expected test line pressure in a particular application. 
     A simplified cross-sectional view of a screw vacuum pump suitable for use as forepump  30  in the leak detector of  FIG. 1  is shown in  FIG. 3 . An enclosed pump housing  130  is provided with main inlet  24  and exhaust  32 . A first screw  132  and a second screw  134  are mounted within housing  130  by suitable bearings (not shown) for rotation about parallel axes  136  and  138 , respectively. Screw  132  includes threads  142 , and screw  134  includes threads  144 . Screws  132  and  134  are positioned in side-by-side relationship within housing  130  such that threads  142  and  144  intermesh. Threads  142  and  144  are spaced slightly from an inside wall of housing  130  to permit unhindered rotation, while minimizing leakage between threads  142 ,  144  and housing  130 . Typical spacings are on the order of a few thousandths of an inch. Intermeshed threads  142  and  144 , and housing  130  define a plurality of enclosed cavities  150 ,  152 ,  154 , etc. 
     A synchronizing gear  160  is connected by a shaft  162  to screw  132 , and a synchronizing gear  164  is connected by a shaft  166  to screw  134 . The synchronizing gears  160  and  164  are intermeshed to provide synchronized rotation of screws  132  and  134  about axes  136  and  138 , respectively. Shaft  166  is connected to a motor  170 . When the motor is energized, screws  132  and  134  rotate in synchronism, so that the threads  142  and  144  remain intermeshed. Motor  60 , synchronizing gears  160  and  164 , and the connecting shafts constitute a drive mechanism for the vacuum pump. 
     As further shown in  FIG. 3 , the screw vacuum pump is provided with intermediate inlet  60 . Intermediate inlet  60  may be implemented as a hole through housing  130  to access one of cavities  150 ,  152 ,  154 , etc. Intermediate inlet  60  may be located at any position between main inlet  24  and exhaust  32  in accordance with the expected pressure in test line  20  during a leak test. Furthermore, optional second intermediate inlet  66  may be positioned as desired along the path between main inlet  24  and exhaust  32 . Because of the continuous nature of the pumping path between main inlet  24  and exhaust  32  of the screw pump, each intermediate inlet can be located over a range of positions, with the positions selected according to the expected test line pressure in a particular application. 
     In operation, motor  170  causes screws  132  and  134  to rotate about axes  136  and  138 , respectively, so that the enclosed cavities  150 ,  152 ,  154 , etc. move from main inlet  24  toward exhaust  32 . Gas enters the vacuum pump through main inlet  24  and is carried in the enclosed cavities to exhaust  32 , thereby performing gas pumping. 
     Operation of the leak detector shown in  FIG. 1  is described with reference to the flow chart of  FIG. 4 . In step  200 , test piece  10  ( FIG. 1 ) is mounted on the test port of the leak detector. More particularly, test piece  10  is mounted on inlet flange  14 . In step  202 , test valve  16  and roughing valve  22  are opened, and test piece  10  is vacuum pumped with forepump  30 , typically beginning from atmospheric pressure. In step  204 , roughing valve  22  is closed and intermediate valve  62  is opened, thus introducing a test sample from test piece  10  into the intermediate inlet  60  of forepump  30 . Helium in the test sample diffuses in a reverse direction from intermediate inlet  60  to main inlet  24  of forepump  30  and then in a reverse direction through high vacuum pump  40  to mass spectrometer  42 . This mode permits test line  20  to operate at the pressure of intermediate inlet  60  of forepump  30 . This pressure is higher than the pressure at foreline  46  of high vacuum pump  40 . In step  206 , a determination is made by mass spectrometer  42  as to whether test piece  10  has a large leak, based on the amount of helium received through intermediate inlet  60 . If a large leak is detected, the test piece  10  is classified as having failed the test and the test is terminated. 
     As discussed above, the forepump  30  may include more than one intermediate inlet. In one embodiment, the intermediate inlet that is best matched to the pressure of the test line  20  is selected for the large leak test. In another embodiment, the intermediate inlets are used in succession to perform a large leak test and one or more intermediate leak tests. In each embodiment, the intermediate valve coupled to the selected intermediate inlet is opened to perform a leak test. 
     If a large leak is not detected in step  206 , the leak detector is configured for small leak detection. In step  208 , intermediate valve  62  is closed and roughing valve  22  is opened for small leak detection. In this mode, helium in test line  20  passes through high vacuum pump  40  in the reverse direction from foreline  46  to mass spectrometer  42 . This mode permits the test line  20  to operate at the foreline pressure of high vacuum pump  40 . In step  210 , a determination is made as to whether test piece  10  has a small leak. The detection of a small leak is based on the amount of helium that passes from test line  20  through high vacuum pump  40  to mass spectrometer  42 . If a small leak is detected in step  210 , the test piece is classified as having a leak and fails the test. If a leak is not detected in step  210 , the test piece passes the leak test. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.