Measuring and testing – With fluid pressure – Leakage
Patent
1996-02-01
1997-08-26
Williams, Hezron E.
Measuring and testing
With fluid pressure
Leakage
31511191, G01M 304
Patent
active
056612290
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a test gas detector, preferably for leak detection instruments, with a selective, preferably test-gas-permeable inlet system, and with a device recording the presence of test gas. Moreover, the invention relates to a method for operating of a test gas detector of this kind.
In the case of high-sensitivity leak detection, chiefly only helium is considered as the test gas. Therefore, in the following text, mainly helium is referred too as the test gas.
In helium leak detection instruments, mass spectrometers are commonly employed as detectors which are set to the mass of helium. Operation of a mass spectrometer requires a high vacuum pump system (high vacuum pump, backing pump etc.) through which the helium entering in the case of a positive test, is removed (refer to DE-A-34 21 533, for example).
From EU-A-352 371 a helium leak detector of the kind affected here, is known. A getter ion pump is employed as the vacuum pump. Even relatively small getter ion pumps are heavy in weight and thus unhandy.
It is the task of the present to significantly reduce the complexity in the generation of the vacuum for test gas detectors, particularly helium detectors for leak detection instruments.
According to the present invention this task is solved for a test gas detector of the aforementioned kind, in that the test gas recording apparatus comprises a gas consuming vacuum gauge. Ionization vacuum gauges, preferably cold cathode ionization gauges, possess a gas consuming property. Vacuum gauges of this kind exhibit only a relatively low pumping speed for helium. Therefore, if helium enters into the vacuum gauge through the inlet of the test gas detector, this will make itself felt by way of a pressure increase. The rate at which the pressure increases is a measure of the magnitude of the helium partial pressure, and is thus--when employing a detector of this kind for the purpose of leak detection--a measure of the leak rate.
A special advantage of the present invention is, that involved evacuation systems for the test gas detector are no longer necessary. Since the ionization vacuum gauge itself has a gas consuming effect, it is in a position to maintain the operating vacuum in the measurement space. Additionally, the measurement space may contain a getter material, which supports the pumping effect of the vacuum gauge.
Preferably the getter material has the property to pump all gases--except the test gas. A getter material of this kind is SAES Getters Type ST707, for example. The sensitivity of the measurements is not impaired by a getter material of this kind.
A further advantage offered by the present invention isthat an inadmissibly high pressure increase in the measurement space which limits the service life of the test gas detector is not to be expected. On the one hand, the ionization vacuum gauge pumps, to a slight extent, also the helium entering the measurement space; on the other hand there is the possibility of being able to remove helium entering the measurement space via the gas inlet again. For this--during the standby mode, for example--a vacuum having a low helium partial pressure is generated and maintained ahead of the open selective inlet system. Thus helium in the measurement space diffuses back through the selective inlet into the pumping line.
The current signal supplied by the vacuum gauge is preferably applied to a high-sensitivity current-to-voltage converter. This is, within the scope of the present invention, preferably designed as a differentiating stage, the amplification of which increases with rising frequency. In the case of such a differentiating stage, a pressure increase will result in a constant output signal which is proportional to the leak rate.
Separating walls made of solid bodies, the permeability of which for helium exceed by many decades the permeability for other gases, are employed as the selective inlet. These solid bodies may, for example, be quartz glass, silica which are supported by a sintered metal, if required. Through heating, the permeability
REFERENCES:
patent: 3591827 (1971-07-01), Hall
patent: 5193380 (1993-03-01), Tallon
patent: 5325708 (1994-07-01), De Simon
patent: 5386717 (1995-02-01), Toda
patent: 5390533 (1995-02-01), Schulte et al.
Technisches Messen, Vo. 49, No. 5, May 1982, Muenchen, DE pp. 193-199, R. Larson, Present State of the Art of Helium Mass Spectrometer Leak.
Bohm Thomas
Dobler Ulrich
Leybold Aktiengesellschaft
Politzer Jay L.
Williams Hezron E.
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