Measuring and testing – With fluid pressure – Leakage
Patent
1997-07-03
1999-05-04
Williams, Hezron
Measuring and testing
With fluid pressure
Leakage
G01M 304
Patent
active
059005378
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a test gas leakage detection assembly.
A test gas leakage detection assembly having certain characteristics is known from DE-A-31 24 205. In a test gas leakage detector of that kind it is possible to perform a leakage detection process in two stages. As long as the pressure within the inlet area is still relatively high, the valve in the line leading to the intermediate connection remains closed. The gas mixture sucked in by the pre-vacuum pump arrives at the pre-vacuum side of the second stage of the friction pump. If there is test gas (helium, for example) already present in this gas owing to a relatively large leakage, it will then pass in a counter-flow through both stages of the friction pump into the detector and it is recorded. If the test sample does not have a large leakage, the valve in the line leading to the intermediate connection may be opened, so that the leakage detection process may be performed at a higher sensitivity.
In the described test gas leakage detector according to the state-of-the-art, the pre-vacuum pump is an oil-sealed pump so that there exists the danger of oil vapours entering the test sample connected at the inlet of the test gas leakage detector or the test chamber.
SUMMARY OF THE INVENTION
It is the initial task of the present invention to design a test gas leakage detector of the here affected kind in such a manner that oil-free leakage detection will be possible.
This task is solved through the present invention in that the pre-evacuation pump is designed as a diaphragm vacuum pump. Diaphragm vacuum pumps are capable of generating an entirely oil-free vacuum. The danger of oil vapours from the pre-vacuum pump entering into the inlet area of the leakage detector no longer exists.
Friction vacuum pumps can only be operated at a pre-vacuum pressure, the upper limit of which amounts to several mbar (5, for example). Diaphragm vacuum pumps have an ultimate pressure which also falls within this order of magnitude. Operation of a friction vacuum pump with a diaphragm vacuum pump as the pre-vacuum pump thus requires that the diaphragm vacuum pump be operated close to its ultimate pressure. However, as the pressure approaches the ultimate pressure, the pumping speed of a diaphragm vacuum pump approaches Zero. Since the response time of a test gas leakage detector of the here affected kind depends significantly on the pumping speed of the pre-vacuum pump, the response time will be very long when using diaphragm vacuum pumps as pre-vacuum pumps.
Furthermore it is the task of the present invention to reduce the response time of a test gas leakage detector of the presented kind equipped with a diaphragm vacuum pump as the pre-vacuum pump.
This task is solved through the present invention in that the connecting line between the inlet of the test gas leakage detector and the intermediate connection is controllable with respect to its conductance. The conductance of the connecting line must be controlled in such a manner that it is as high as possible; however, that pressure between the two friction pumps must not be exceeded which is required to maintain the operating pressure in the mass spectrometric detector. Thus there exists the possibility to link the inlet of the test gas leakage detector very early via the throttled connecting line to the intermediate connection, even when at that point of time the pressure at the inlet of the test gas leakage detector is still higher than the maximum permissible pressure between the two friction pump stages. The significant advantage of this scheme is that, compared to the diaphragm vacuum pump, the much higher pumping speed of the second friction pump stage is available very early which results in a significantly shortened response time.
BRIEF DESCRIPTION OF THE DRAWINGS
Further advantages and details of the present invention shall be explained through drawing FIGS. 1 and 2. Shown in
FIG. 1 is a design example of the test gas leakage detector according to the present invention; and
F
REFERENCES:
patent: 3699779 (1972-10-01), Schlichtig
patent: 4779449 (1988-10-01), Bley et al.
patent: 4893497 (1990-01-01), Danielson
patent: 5547338 (1996-08-01), Conrad et al.
Bohm Thomas
Widt Rudi
Leybold Aktiengesellschaft
Politzer Jay L.
Williams Hezron
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