Measuring and testing – With fluid pressure – Leakage
Patent
1999-03-19
2000-02-08
Larkin, Daniel S.
Measuring and testing
With fluid pressure
Leakage
G01M 304
Patent
active
060216638
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
In order to improve the quality of products, in particular series manufactured products with hollow spaces (vessels, housings, heat exchangers, condensers, etc.), the deployment of helium leak detection methods is becoming more and more popular so as to determine and sort out faulty products at an early stage. For reasons of time and cost, the cycle time (time required to test each test body) should be as short as possible.
In a vacuum leak detection process, a light gas, especially helium, is employed as the test gas. In the case of the presence of a leak, this enters into the evacuated test body and it is recorded with the aid of a mass spectrometer. The test body is evacuated with the aid of a fore-vacuum pump accommodated in the leak detector. A high-vacuum pump serves to evacuate the mass spectrometer, since the operating pressure of a mass spectrometer must not exceed a maximum of 10.sup.-4 mbar. One test body at a time is evacuated, and after the pressure (P.sub.u)(switchover pressure) has been attained, the instrument switches over from the evacuation mode to the measurement mode. P.sub.u is the pressure that prevails at the intermediate inlet of the high-vacuum pump. It must be selected to be adequately low, so that the pressure in the mass spectrometer will not exceed the maximum admissible operating pressure.
SUMMARY OF THE INVENTION
To be on the safe side, the pressure (P.sub.u) is selected to be very low in order to avoid endangering the operation of the mass spectrometer under all circumstances. This approach has the effect that the switchover from the evacuation mode to the measurement mode can only be effected after a relatively long evacuation time has elapsed. This applies in particular to the younger generation of leak detectors in which dry (oil-free) backing pumps like, for example, diaphragm pumps, scroll pumps, or the like are employed having at low pressures (e.g. >5 mbar) a relatively low pumping capacity.
It is the task of the present invention to shorten the evacuation time for the test bodies which chiefly affect the cycle time.
The present invention is based on the realization that, in the case of leak testing a plurality of similar test bodies, a higher switchover pressure (P.sub.u) may be selected as previously selected for the reason of being on the safe side. An earlier switchover from the evacuation mode to the measurement mode does not only offer the advantage of being able to commence with the leak detection process earlier; moreover, immediately after the switchover has occurred, the high pumping capacity of the high-vacuum stage following downstream of the intermediate inlet becomes effective so that the evacuation time itself is considerably shortened. Because of the similarity of the test bodies a learning process may be applied, through which an optimum switchover pressure (P.sub.u opt) may be determined.
DESCRIPTION OF THE INVENTION
Details of the present invention shall be explained by referring the drawing figure of a schematically depicted leak detector designed according to the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The depicted leak detector 1 is equipped with a connection for a test body 2 (test port 3). The test body is, for example, sprayed from the outside with helium (spray gun 4). If the test body has a leak, helium will enter into the leak detector 1 and will be recorded with the aid of the mass spectrometer 5. Instead of test body 2, a chamber in which several test bodies have been accommodated, may be connected to test port 3.
A high-vacuum pump 6 having two stages 7 and 8 as well as an intermediate inlet 9 is employed for the generation of the necessary operating pressure in the spectrometer 5. Two separate pumps may be employed; preferable, however, is the deployment of a two-stage friction pump, a turbomolecular pump, for example, which is equipped with an intermediate inlet 9.
The stage 7 of the high-vacuum pump 6 has a relatively low compression for lighter gases, so that any helium entering
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patent: 5900537 (1999-05-01), Bohm et al.
Patents Abstracts of Japan vol. 007, No. 237 (M-250), (Oct. 21, 1983) & JP 58 125328 A (Nissan Jidosha KK) (Jul. 26, 1983).
D.G. Mahoney: "Expanded-Range, Sealed Parts Leak Testing Technology for Helium Mass Spectrometer Leak Detection", Journal of Vaccuum Science and Technology, Part A., (Aug. 1994) pp. 1740-1743.
Larkin Daniel S.
Leybold Vakuum GmbH
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