GB2123153A

GB2123153A - Leak detector

Info

Publication number: GB2123153A
Application number: GB08218939A
Authority: GB
Inventors: Nigel Thomas Metcalfe Dennis
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1982-06-30
Filing date: 1982-06-30
Publication date: 1984-01-25
Also published as: DE3323631A1

Abstract

The article being tested is surrounded by a search gas such as helium and evacuated by a pumping system including a cryopump working at 20K, or some other selective pump, which removes most permanent gas components (such as oxygen, nitrogen and carbon dioxide) but does not remove the search gas thereby increasing the concentration of the search gas entering a detector such as a mass spectrometer. The gases enter the cryopump through a baffle 18 (fig. 1, not shown) and then contact first a radiation shield 2 (fig. 1) and then, successively, the outer and inner surfaces of cryopanel 3; both the shield and panel 3 being cooled by a two-stage cryogenerator (14 (fig. 1), and 16). The uncondensed gases then enter a mass spectrometer through inlet 5 which is arranged so that all gas entering it has contacted the panel 3 at least twice. <IMAGE>

Description

SPECIFICATION Improved leak detector The present invention relates to leak detectors and in particularto leak detectors incorporating mass spectrometers.

Leak detectors are known which operate by exhausting theinteriorofan article to be tested, surrounding the article with a search gas such as helium and testingforthe presence ofthe search gas by means of a mass spectrometer in those gases being exhausted from the interior ofthe article.

Such a known leak detector usually comprises at least one rotary pump to pre-exhaustthe article being leak tested and to backa diffusion pump which exhauststhroughaliquid nitrogen trap. A mass spectrometer head is attached to the body ofthe liquid nitrogen trap and for nonnal operations the mass spectrometer is tuned to helium which is used as the search gas surrounding the article to be leak tested.

One purpose of the liquid nitrogen trap is to protect the mass spectrometer from organic contamination which mayarisefrorn the vacuum pump. It performs the additional function of trapping any water vapour or volatile contaminants emitted from the article undertest.

Leak detectors of the type described above do suffer from the limitation that all permanent gases are pumped by a diffusion pumpatapproximatelythe same speeci {within a factor of 2). Although it is possible to remove water vapour by a well designed liquid nitrogen trap from the gases entering the mass spectrometer head, the search gas usually helium is still diluted by permanent gases.

It is an aim of the present invention to provide a leak detector which removes those permanent gases which heretofore have diluted the search gas.

According to its broadest aspect, the present investion provides apparatus for leaktesting an article including a pumping system for exhausting the interior of the article of gas which may include a search gas introduced th rough leaks, a detector for detecting the presence of search gas within the interiortogetherwith selective pump means for selectively removing one or more components of the gas in the interior whereby to increase the proportion of search gas to be detected.

Preferably, the detector is a mass spectrometer and the selective pump is a cryopump having at least one surface and normallytwo at a cryogenic temperature so arranged that substantially all the gases undertest within the article are intercepted by the cryosurface before entering the mass spectrometer.

Thecryogenictemperature mayfallwithinthe range 4 Kto 25"K. In most but not all cryopumps, the temperature ofthe cryosurface may fall within the range of 1 5 K to 250K, but is preferably 20"K.

An embodiment ofthe invention will now be described byway of example, reference being made to thefigures of the accompanying diagramatic drawings in which: Figure lisa diagramatic cross section through a leak detector according to the present invention; Figures 2 is a cross section through the leak detector of Figure 1 at right anglesto the cross section of Figure 7; Figure 3 is a detail in perspective ofthe leak detector of Figure 1.

As shown, a leak detector 10 includes a selective pump in the form of a cryopump 12 and a detector in the form of a mass spectrometerthe inlet portS only of which is shown.

Thecryopump 12comprisesa cryogenerator8 communicating with the interior of a vacuum enclosure 20 via a vacuum tube 7. The cryogenerator6 has a firststage 14which supportsandcools,within the enclosure 20, a radiation shield 2. The cryogenerator 6 also has a second stage 16which supports and cools a second stage cryopanel 3. The cryopanel 3 is surrounded bythe radiation shield 2 and as shown has an inverted dished or cupped shaped configuration. The cryopanel 3 in its turn surrounds the inlet portS of the mass spectrometer.

Achevron baffle 18forms partofthe enclosure 20 containing the radiation shield 2 and the enclosure shields the cryopanel 3 from external radiation whilst permitting the entrythere into of gases pumped from the interiorofan article to betested.

The second stage 16 conveniently maintains the cryopanel 3 at a temperature within the range 1 5 Kto 25"K but preferably at 20 K or lower although a temperature of as low as 4"K may be achieved. The first stage 14 of the cryogenerator 6 maintains the radiation shield at a temperature of approximately 80"K In use, the article to betested is connected to the leak detector such that gases pumped from its interior passthrough the baffle 18 and make initial contact with the relativelywarm surfaces of the radiation shield 2.The radiation shield has a temperature of approximately 800which will remove water vapour and any other gases condensable atthis temperature.

The residual gases then pass on and contactthe cold outerandfinally innersurface ofthe cryopanel 3 which is maintained at a temperature of approximately 20"K. At this temperature only helium, hydrogen and neon will avoid being condensated and thus only these gases will find theirwayto the inletS ofthe mass spectrometer. The mass spectrometer inlet 5 is so designed that all the permanent gases must have collided at leasttwice with thecryopanel 3 and have been intercepted by the cryopanel before entering the mass spectrometer head so ensuring completecondensation of all the gases except helium, hydrogen and neon.

Although notshown,a smallvapourorturbo- molecular pump backed by a conventional mechanical pump is connected to the vacuum tube 7 to remove when necessary those gases helium, hydrogen and neon which are not pumped bythe cryopump.

The advantage ofthe leakdetector described above is that it will remove those permanent gaseswhich normally dilute the search gas helium and ther.eW increase the proportion of the search gas being detected. The cryopump with a surface of 20"K or lower is capable of removing permanent components such as oxygen, nitrogen, carbon dioxide and the like ofthe gas in the article undertest and the only gases which are not satisfactorily condensed at this temperature are helium, hydrogen and neon. The lasttwo gases form a very small proportion of any permanent gas in the articles being tested and will not normally affectthe performance of the mass spectrometer. In this manner, the sensitivity of the leak detector described above is greatly improved overthat of known mass spectrometer leak detectors especially in those cases where the vessel to be tested is at a relatively high pressure compared to the normal running pressure of between l0-4and 1 O-smbar of the mass spectrometer head.

Claims

1. Apparatusforleaktesting an article includes a pumping system for exhausting the interior of the article of gas which may include a search gas introduced through leaks, a detectorfordetecting the presence of search gas within the interior together with selective pump meansforselectively removing one or more components ofthe gas in the interior whereby to increase the proportion of search gas to be detected.

2. Apparatus as claimed in Claim 1, wherein the detector is a mass spectrometer.

3. Apparatus as claimed in Claim 1 or Claim 2, wherein the selective pump means is a cryopump having at least one cryosurface positioned to intercept gas entering the detector and to selectively remove the gas component or components.

4. Apparatus as claimed in Claim 3 or Claim 4, wherein the cryosurface is a dish arranged substantiallyto surround the detector inlet.

5. Apparatus as claimed in Claims 3 , 4,wherein the cryosurface comprises the second stage of the cryopump.

6. Apparatus as claimed in any one of Claims 3to 5, wherein the gas is air and the search gas is helium and the cryosurface isa arranged to operate ata temperature within the range of 4 K to 25"K.

7. Apparatus as claimed in Claim 6, wherein the cryosurface is arranged to operate at a temperature within the range 1 to 25"K.

8. Apparatus as claimed in Claim 6 or Claim 7, wherein the cryosurface is arranged to operate at a temperature of 20"K.

9. Apparatus as claimed in any preceding claim, wherein the pumping system comprises a vapour or turbo-molecular pump backed by a mechanical pump.

10. Apparatus substantially as herein described with referenceto and adaptedto operate substantially as herein described with reference to the accompanying drawings.