Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1997-07-11
1999-06-29
Le, Que T.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250205, 73 405A, G01N 1506
Patent
active
059171936
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method and a device for checking the leak tightness of containers, housings, and similar test objects, of the type described in the preamble of patent claims 1 and 13.
BACKGROUND OF THE INVENTION
In a known method for checking the. leak tightness of containers (U.S. Pat. No.5,161,408), the test object is subjected to a pressurized test gas, and the test gas escaping from test test object in the instance of a leak is subjected in an analyzing volume to an intensity-modulated electromagnetic wave field which is emitted by a laser, the wave field containing frequency components which are absorbed by the test gas, producing a photo-acoustic signal which is indicative of a leak. When the laser light is radiated into the analyzing volume with a frequency which is tuned to the absorption frequency of the test gas, a portion of the molecules of the test gas are excited to a higher state of energy by the absorption of the electromagnetic waves. Through collisions with other molecules in the analyzing volume the excited molecules may impart some or all of their excitation energy and transform the energy into translational, rotational or vibrational energy of the collision partners. The rise in translational energy of the molecules in the analyzing volume corresponds to a temperature rise and therefore a rise in pressure. When the intensity of the irradiation wave field is periodically varied, periodic pressure changes result, which can be detected by means of a pressure or acoustic sensor. As a test gas sulfur hexafluoride (SF.sub.6) can for instance be used, and the laser can be a CO.sub.2 -laser. In the known method the photoacoustic effect is used to localize a leak at the surface of the test object. It is not possible, though, to quantatively determine the size of the leak.
Based on this, it is the object of the invention to develop a method and a device for cheking the leak tightness of the type described above, with which a quantitative leak check may be performed with little expenditure.
For the solution of this object the combinations of features stated in patent claims 1 and 13 are proposed. Further developments and embodiments of the invention result from the dependent claims.
SUMMARY OF THE INVENTION
The solution according to the invention is based on the idea that even small changes of the laser temperature may lead to a sudden change of the mode structure, which make a quantitative analysis of photo-acoustic signals with respect to the present test gas concentration difficult or impossible. In order to avoid this disadvantage it is proposed according to the invention that the temperature of the laser is governed to a temperature set value and that the excitation power of the laser is held constant over a time average, if needed, so that the photo-acoustic signal is a measure for the test gas concentration in the test volume and/or for the size of a leak. This makes it possible for the first time to quantitatively analyze the photo-acoustic signals even when using longitudinal multi-mode lasers having a number of frequencies which can be absorbed by the test gas. It is therefore not necessary to rely on the technically much more complex and larger frequency-stabilized mono-mode lasers.
The intensity of the radiation field may be modulated by chopping a laser beam emitted by the laser or by pulsing the excitation power.
With the method according to the invention it is also easily possible to vary the mode structure and therefore the emitted frequency band of the laser by adjusting the temperature set value. This makes it possible to eliminate the influence of other laser light-absorbing gases on the measurement of the test gas concentration.
According to a preferred embodiment of the invention it is provided that an additional reference volume having a given test gas concentration is subjected to the intensity-modulated radiation field emitted by the laser, and that the photo-acoustic signal generated in the reference volume is measured and analysed
REFERENCES:
patent: 4457162 (1984-07-01), Rush et al.
patent: 4979820 (1990-12-01), Shakkottai et al.
Schroff Gerhart
Stetter Michael
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