Measuring and testing – With fluid pressure – Leakage
Patent
1995-02-14
1996-12-17
Noland, Thomas P.
Measuring and testing
With fluid pressure
Leakage
G01M 302
Patent
active
055855482
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a counterflow leak-detector unit with an inlet to which a test specimen, or a chamber housing a test specimen, is connected and with a test-gas detector to which is connected a high-vacuum pump which produces the necessary pressure in the detector, the inlet to the leak-detector unit being connected to an intermediate line in the high-vacuum pump.
2. Discussion of the Prior Art
A leak-detector of this kind is known from DE-PS 31 24 205. The stage on the high-vacuum side with respect to the intermediate line is designed as a counterflow stage and has a relatively low compression for light gases, in particular for helium which is generally employed. The pump stage (pre-stage) on the forevacuum side with respect to the intermediate line serves--besides a forevacuum pump--to evacuate the specimen or the specimen chamber, so that relatively fast cycle times can be achieved. The pre-requisite for this is, that the pre-stage has a relatively high pumping speed. During the leak detection process the test gases are also admitted into the intermediate line of the high-vacuum pump. Due to the relatively high pumping speed of the pre-stage, sensitivity of the leak detection process is limited.
A counterflow leak-detector of the aforementioned kind is known from European Patent Publication 268 777, in which the inlet of the leak-detector unit is directly connectable to the test gas detector, i.e. to the inlet side of the high-vacuum pump. Due to this measure it is basically possible to increase sensitivity; but in practice this solution can hardly be employed. The test gas detector is commonly a highly sensitive mass spectrometer which is set to the mass of the test gas, and which requires an operating pressure of less then 10.sup.-4 mbar. Owing to the direct connection of the specimen to the test gas detector there exists the danger of an inadmissibly high pressure increase, which gives rise to faulty measurements or misinterpretations.
SUMMARY OF THE INVENTION
It is the task of the present invention to create a counterflow leak-detection unit of the aforementioned kind, where measurements can be made with high sensitivity, without the danger of an inadmissibly high pressure increase in the test gas detector.
This task is solved by the present invention, by providing a constriction between the two stages of the high-vacuum pump separated by the intermediate line and by connecting the inlet of the leak-detector unit to the outlet side of the high-vacuum pump stage at a point upstream of the constriction. In a leak-detector designed in this fashion, the relatively high pumping speed of the pre-stage is not yet effective where the leak-detector unit is connected to the high-vacuum pump, i.e. upstream ahead of the constriction, so that at a given helium gas flow, a high test gas resp. helium partial pressure is created, which determines the sensitivity together with the relatively low compression of the counterflow stage. The presence of the constriction thus effects a considerable improvement in the sensitivity of the leak-detector unit designed according to the present invention, in comparison to the leak-detector unit known from DE-PS 31 24 205. The danger of an inadmissibly high pressure increase in the test gas detector (European Patent Publication 268 777) does not exist, because of the presence of the counterflow stage in the connection between the specimen and the test gas detector.
Preferably the inlet of the leak-detector is additionally also connectable to the inlet side of the pre-stage, i.e. downstream after the constriction, and preferably also to the outlet side of the pump stage on the forevacuum side. In such an arrangement it is possible to conduct the leak detection process at three different levels of sensitivity.
This task can also be solved through measures in which a constriction is provided in the intermediate line separating the pump stages of the high-vacuum pump, which is controllable or can at least be switched bet
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Bley Werner Grosse
Bohm Thomas
Larkin Daniel S.
Leybold Aktiengesellschaft
Noland Thomas P.
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