Abstract:
Methods and apparatus for large leak testing are provided. The apparatus includes a test line to receive a sample containing a trace gas, a mass spectrometer to detect the trace gas and having an inlet for receiving the trace gas, a first vacuum pump characterized by a relatively high reverse flow rate for light gases and a relatively low reverse flow rate for heavy gases, the first vacuum pump having a pump inlet and a foreline, the pump inlet being coupled to the inlet of the mass spectrometer, and a second vacuum pump configured to back the first vacuum pump. The apparatus further includes a trace gas permeable member coupled between the test line and the pump inlet, the foreline, or, in the case where the first vacuum pump is a turbomolecular pump, a midstage line.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to detection of leaks in articles and, more particularly, to methods and apparatus for helium leak detection over a large range of leak rates, including large leaks.  
       BACKGROUND OF THE INVENTION  
       [0002]     Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a tracer gas which passes through the smallest of leaks in a sealed test piece. After passing through a leak, a test sample containing helium is drawn into a leak detection instrument and is measured. An important component of the instrument is a mass spectrometer tube which detects and measures the helium. The input test sample is ionized and mass analyzed by the spectrometer tube in order to separate the helium component. In one approach, a test piece is pressurized with helium. A sniffer probe connected to the test port of the leak detector is moved around the exterior of the test piece. Helium passes through leaks in the test piece, is drawn into the probe and is measured by the leak detector. In another approach, the interior of the test piece is coupled to the test port of the leak detector and is evacuated. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector.  
         [0003]     One of the difficulties associated with helium mass spectrometer leak detection is that the inlet of the mass spectrometer tube must be maintained at a relatively low pressure, typically 2×10 −4  Torr. In a so-called conventional leak detector, the test port, which is connected to the test piece or to the sniffer probe, must be maintained at relatively low pressure. Thus, the vacuum pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, it may be difficult or impossible to reach the required pressure level. If the required pressure level can be reached, the pumping cycle is lengthy.  
         [0004]     Techniques have been proposed in the prior art to overcome this difficulty. A counterflow leak detector disclosed in U.S. Pat. No. 3,690,151, issued Sept. 12, 1972 to Briggs, utilizes a technique of reverse flow of helium through a diffusion pump to the mass spectrometer. The leak detector test port can be operated at the pressure of the diffusion pump foreline. A similar approach utilizes reverse flow of helium through a turbomolecular pump. A technique for gross leak detection is disclosed in U.S. Pat. No. 4,735,084 issued Apr. 5, 1988 to Fruzzetti. The tracer gas is passed in reverse direction through one or two stages of a mechanical vacuum pump. These techniques have permitted the test port pressure to be higher than for conventional leak detectors. Nonetheless, reaching the higher test port pressure can be difficult when testing large volumes, dirty parts or parts with large leaks.  
         [0005]     A simplified schematic diagram of a prior art leak detector for large leak testing is shown in  FIG. 1 . A test piece  10  is attached to an inlet flange  12 . Inlet flange  12  defines a test port of the leak detector and is connected through a test valve  32  and a differential pressure aperture  30  to a test line  14 . The leak detector includes a forepump  16 , a roughing pump  18 , a turbomolecular pump (turbopump)  20  and a mass spectrometer  22 . A foreline  24  of turbopump  20  is connected to test line  14 , and mass spectrometer  22  is connected to the inlet of turbopump  20 . A midstage line  26  of turbopump  20  may be connected to test line  14 . Forepump  16  rough pumps test line  14  and test piece  10  from ambient pressure and also backs turbopump  20 . Helium that enters the test port from test piece  10  flows in contraflow or reverse direction through turbopump  20  and into mass spectrometer  22 . The mass spectrometer detects the helium and indicates a helium leak rate. An alternate prior art non-contraflow configuration uses a direct connection  34  between test line  14  and the inlet of mass spectrometer  22 .  
         [0006]     For large leak testing, where the test port pressure may be greater than the allowable foreline pressure of the turbopump  20 , roughing pump  18  is utilized in prior art leak detectors with a roughing line  28  and a roughing valve  36 . Aperture  30  operates such that most of the gas flows to roughing pump  18  while a fraction of the gas flows to the forepump  16 , with helium passing in reverse direction through mass spectrometer  22 . A bypass valve  38  is used to bypass aperture  30 . Testing with two pumps and a differential pressure aperture is inherently unreliable since, for example, the aperture can become partially plugged by contamination, resulting in erroneous readings. Furthermore, the cost of the roughing pump and associated hardware significantly increases cost.  
         [0007]     European Patent Application No. 0 352 371 published Jan. 31, 1990 discloses a helium leak detector including an ion pump connected to a probe in the form of a silica glass capillary tube. The silica glass tube is heated to a temperature between 300° C. and 900° C. and thereby becomes permeable to helium. U.S. Pat. No. 5,325,708 issued Jul. 5, 1994 to De Simon discloses a helium detecting unit using a quartz capillary membrane, a filament for heating the membrane and an ion pump. U.S. Pat. No. 5,661,229 issued Aug. 26, 1997 to Bohm et al. discloses a leak detector with a polymer or heated quartz window for selectively passing helium to a gas-consuming vacuum gauge.  
         [0008]     All of the prior art helium leak detectors have had one or more drawbacks, including limited pressure ranges, susceptibility to contaminants and/or high cost. Accordingly, there is a need for improved methods and apparatus for leak detection.  
       SUMMARY OF THE INVENTION  
       [0009]     According to a first aspect of the invention, apparatus for leak detection is provided. The apparatus comprises a test line configured to receive a sample containing a trace gas, a mass spectrometer configured to detect the trace gas and having an inlet for receiving the trace gas, a first vacuum pump characterized by a relatively high reverse flow rate for light gases and a relatively low reverse flow rate for heavy gases, the first vacuum pump having a pump inlet and a foreline, the pump inlet being coupled to the inlet of the mass spectrometer, and a foreline valve coupled between the foreline of the first vacuum pump and the test line. The apparatus further comprises a trace gas permeable member coupled between the test line and the inlet of the mass spectrometer, and a second vacuum pump having an inlet coupled to the test line.  
         [0010]     The permeable member may be permeable to helium, and the trace gas permeability of the permeable member may be controllable. In some embodiments, the permeable member comprises a quartz member. The apparatus may further comprise a heating element in thermal contact with the quartz member and a controller configured to control the heating element. In other embodiments, the permeable member comprises a polymer member.  
         [0011]     The apparatus may further include a controller configured to increase the permeability of the permeable member and to close the foreline valve at relatively high pressures in the test line and configured to decrease the permeability of the permeable member and to open the foreline valve at relatively low pressures in the test line.  
         [0012]     According to a second aspect of the invention, a method for leak detection is provided. The method comprises pumping gas from a test volume through a test line, at relatively high pressures in the test line, passing a first portion of the pumped gas through a trace gas permeable member to a mass spectrometer, and, at relatively low pressures in the test line, passing a second portion of the pump gas in reverse direction through a vacuum pump to the mass spectrometer. The vacuum pump is characterized by a relatively high reverse flow rate for light gases and a relatively low reverse flow rate for heavy gases.  
         [0013]     According to a third aspect of the invention, apparatus for leak detection is provided. The apparatus comprises a test line configured to receive a sample containing a trace gas; a mass spectrometer configured to detect the trace gas and having an inlet for receiving the trace gas; a first vacuum pump characterized by a relatively high reverse flow rate for light gases and a relatively low reverse flow rate for heavy gases, the first vacuum pump having a pump inlet and a foreline, the pump inlet being coupled to the inlet of the mass spectrometer; a second vacuum pump configured to back the first vacuum pump; and a trace gas permeable member coupled between the test line and at least one of the pump inlet and the foreline.  
         [0014]     According to a fourth aspect of the invention, apparatus for leak detection is provided. The apparatus comprises a test line configured to receive a sample containing a trace gas; a mass spectrometer configured to detect the trace gas and having an inlet for receiving the trace gas; a turbomolecular vacuum pump having a pump inlet, a midstage line and a foreline, the pump inlet being coupled to the inlet of the mass spectrometer; a forepump configured to back the turbomolecular vacuum pump; and a trace gas permeable member coupled between the test line and the midstage line of the turbomolecular vacuum pump. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0016]      FIG. 1  is a block diagram of a prior art dual pump leak detector for large leak testing;  
         [0017]      FIG. 2  is a simplified block diagram of apparatus for leak detection in accordance with a first embodiment of the invention;  
         [0018]      FIG. 2A  is a simplified partial cross-sectional diagram of the apparatus of  FIG. 2 , showing the permeable member;  
         [0019]      FIG. 3  is a simplified flow chart of a method for leak detection in the apparatus of  FIG. 2 ;  
         [0020]      FIG. 4  is a simplified block diagram of apparatus for leak detection in accordance with a second embodiment of the invention;  
         [0021]      FIG. 5A  is a simplified block diagram of apparatus for leak detection in accordance with a third embodiment of the invention;  
         [0022]      FIG. 5B  is a simplified block diagram of apparatus for leak detection in accordance with a fourth embodiment of the invention; and  
         [0023]      FIG. 5C  is a simplified block diagram of apparatus for leak detection in accordance with a fifth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     Apparatus for leak detection in accordance with a first embodiment of the invention is shown in  FIG. 2 . A test piece  110  having a test volume  111  is attached to an inlet flange  112 . Inlet flange  112  defines a test port of a leak detector and is connected through a test valve  136  to a test line  114 . A forepump  116  has an inlet coupled to test line  114  for pumping of test line  114  and test volume  111 . The leak detector further includes a turbopump  120 , a mass spectrometer  122 , a foreline valve  124 , a midstage valve  138 , a trace gas permeable member  130 , a controller  132  and associated conduits. Mass spectrometer  122  has an inlet  162  coupled to an inlet of vacuum pump  120 . A foreline  140  of turbopump  120  is coupled through foreline valve  124  to test line  114 . An optional midstage line  142  of turbopump  120  is coupled through midstage valve  138  to test line  114 .  
         [0025]     Turbopump  120  may be replaced with a diffusion pump, a so-called hybrid turbopump or a molecular drag pump. A midstage connection is not utilized on a diffusion pump. In a hybrid turbopump, one or more of the axial pumping stages are replaced with disks which rotate at high speed and function as molecular drag stages. This configuration is disclosed in U.S. Pat. No. 5,238,362 issued Aug. 24, 1993 to Casaro et al. 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  140  to mass spectrometer  122  and other gases are substantially blocked.  
         [0026]     Trace gas permeable member  130  is coupled through a conduit  148  between test line  114  and the inlet  162  of mass spectrometer  122 . Permeable member  130  is a material that is permeable to the trace gas used in the leak detector, typically helium, under specified conditions. Permeable member  130  substantially passes, or permeates, the trace gas while substantially blocking other gases, liquids and particles. The permeable member  130  thus acts as a trace gas window in the sense of allowing the trace gas to pass while blocking other gases, liquids and particles.  
         [0027]     As shown in  FIG. 2A , permeable member  130  may be sealed in a fixture  150  connected to conduit  148  such that gas passing through conduit  148  to mass spectrometer  122  passes through permeable member  130 . Permeable member  130  may have the shape of a disk, for example.  
         [0028]     Quartz, or silica glass, is an example of a material that is permeable to helium. In particular, the helium permeability of quartz varies with temperature. At elevated temperatures in the range of 300° C. to 900° C., quartz has a relatively high helium permeability. At room temperature, quartz has a relatively low helium permeability. As further shown in  FIG. 2A , the leak detector may be provided with a heating element  154  in thermal contact with quartz permeable member  130 . Heating element  154  may be energized by controller  132  to increase the helium permeability of quartz permeable member  130 . By controlling the temperature of permeable member  130 , a helium window is provided. At a relatively high temperature (e.g. 300° C. to 900° C.), helium permeability is high and the helium window is open. At a relatively low temperature (e.g. room temperature), helium permeability is low and the helium window is closed. Other suitable trace gas permeable member materials include polymers such as tetrafluoroethylene, known under the trade name Teflon.  
         [0029]     Operation of the leak detector shown in  FIG. 2  is described with reference to the flow chart of  FIG. 3 . In step  200 , test piece  110  ( FIG. 2 ) is mounted on the test port of the leak detector. More particularly, test piece  110  is mounted on inlet flange  112 . In step  202 , test valve  136  is opened and test piece  110  is pumped with forepump  116 , typically beginning from atmospheric pressure. In step  204 , the heating element  154  is energized, and foreline valve  124  and midstage valve  138  are closed. This effectively blocks helium from reaching mass spectrometer  122  through turbopump  120  and increases the helium permeability of permeable member  130 . Thus, helium can flow from test line  114  through permeable member  130  to mass spectrometer  122 . The permeable member  130  prevents other gases and contaminants from entering the mass spectrometer  122 . Thus, leak testing can begin as soon as forepump  116  begins pumping the test volume. In prior art leak detectors, leak testing could not begin until a sufficiently low test line pressure was achieved for proper turbopump and mass spectrometer operation. This time advantage is important for many applications. If the pressure level in mass spectrometer  122  becomes too high, foreline valve  124  automatically opens and a test valve  136  closes for a brief time period to reconnect forepump  116  and re-establish the desired pressure level. In step  206 , a determination is made as to whether the test piece  110  has a large leak, based on the amount of helium received through permeable member  130 . If a large leak is detected, the test piece  110  is classified as having failed the test, and the test is terminated.  
         [0030]     If a large leak is not detected in step  206 , the heating element  154  is de-energized in step  208  and the leak detector is configured for medium or small leak detection. Midstage valve  138  is opened for medium leak detection, and foreline valve  124  is opened for small leak detection. In some cases, midstage valve  138  and foreline valve  124  can both be opened. In this mode, helium in test line  114  passes through turbopump  120  in reverse direction from foreline  140  and/or midstage line  142  to mass spectrometer  122 . This mode permits test line  114  to operate at the foreline pressure of turbopump  120 . In step  210 , a determination is made as to whether test piece  110  has a medium or small leak. The detection of a medium or small leak is based on the amount of helium that passes from test line  114  through turbopump  120  to mass spectrometer  122 . If a medium or small leak is detected in step  210 , the test piece is classified as having a leak and fails the leak test. If a leak is not detected in step  210 , the test piece passes the leak test.  
         [0031]     The permeable member  130  can be made of any suitable material that is permeable to the trace gas, typically helium, and may have any shape or dimension. Examples of suitable materials include quartz and permeable polymers. When quartz is utilized, a heating element heats the quartz material to increase helium permeability while selectively blocking most other gases, water vapor and particles. The quartz has a constant permeability for a given temperature. The temperature can be adjusted to control the permeability and therefore the sensitivity. A heating element is not required in the case of a permeable polymer. The permeable member can be mounted at the inlet of the mass spectrometer. The helium which permeates through the permeable member is detected by the mass spectrometer, and the signal is converted to a leak measurement. The permeable member can operate at vacuum, at atmospheric pressure or at a pressure slightly higher than atmospheric pressure. The permeable member can operate in an atmosphere that contains gases, particles and in wet environments. The permeable member permits large leak detection in a helium leak detector with a single backing pump.  
         [0032]     Apparatus for leak detection in accordance with a second embodiment of the invention is shown in  FIG. 4 . Like elements in  FIGS. 2 and 4  have the same reference numerals. In the embodiment of  FIG. 4 , trace gas permeable member  130  is coupled between test line  114  and foreline  140  of turbopump  120 . A bypass valve  160  is coupled between foreline  140  and test line  114 . Permeable member  130  is bypassed when valve  160  is open. In operation, helium in test line  114  passes through permeable member  130  to the foreline  140  of turbopump  120 . The helium then passes in reverse direction through turbopump  120  to inlet  162  of mass spectrometer  122  and is measured by mass spectrometer  122 . In other embodiments, a combination of trace gas permeable member  130  and bypass valve  160  may be connected to midstage line  142  ( FIG. 2 ) of turbopump  120  or may be connected directly to inlet  162  of mass spectrometer  122 .  
         [0033]     Apparatus for leak detection in accordance with a third embodiment of the invention is shown in  FIG. 5A . Like elements in  FIGS. 2, 4  and  5 A have the same reference numerals. In the embodiment of  FIG. 5A , foreline  140  of turbopump  120  is connected to forepump  116 , and test line  114  is connected to a separate roughing pump  170 . Trace gas permeable member  130  is coupled between inlet  162  of mass spectrometer  122  and test line  114 . Bypass valve  160  is coupled in parallel with permeable member  130 .  
         [0034]     Apparatus for leak detection in accordance with a fourth embodiment of the invention is shown in  FIG. 5B . Like elements in  FIGS. 2, 4 ,  5 A and  5 B have the same reference numerals. In the embodiment of  FIG. 5B , trace gas permeable member  130  is coupled between test line  114  and foreline  140  of turbopump  120 . Bypass valve  160  is coupled in parallel with permeable member  130 .  
         [0035]     Apparatus for leak detection in accordance with a fifth embodiment of the invention is shown in  FIG. 5C . Like elements in  FIGS. 2, 4 ,  5 A and  5 C have the same reference numerals. In the embodiment of  FIG. 5C , trace gas permeable member  130  is coupled between test line  114  and midstage line  142  of turbopump  120 . Bypass valve  160  is coupled in parallel with permeable member  130 .  
         [0036]     The foregoing embodiments illustrate that permeable member  130  may be utilized at different points in the leak detection apparatus to achieve different leak detection sensitivities. An optional bypass valve may be coupled in parallel with permeable member  130 . The leak detection apparatus may utilize a single forepump or a forepump and a separate roughing pump. In further embodiments, permeable members may be utilized at more than one point in the leak detection apparatus.  
         [0037]     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.