Measuring and testing – With fluid pressure – Leakage
Reexamination Certificate
1998-10-28
2001-05-08
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Leakage
C073S601000, C250S573000
Reexamination Certificate
active
06227036
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention relates to the problem of gas leaks. In particular, the invention pertains to a method and system for detection and location of leaks from various pressurized components.
Many industrial and domestic machines use or convey pressurized gases or liquids. Leaks from these machines may be costly to consumers as well as manufacturers, and moreover, depending on the composition of the gas or liquid, the leaks could be harmful to the environment.
Heretofore, several methods of leak testing have been employed. Methods such as bubble visualization, sniffing, pressure decay, evacuating the part to create a vacuum and surrounding the part with a gas and checking for any intrusion of the gas into the part, pressurization/immersion, pressurization/soaping, and pressurization/ammonia sensitive painting are some of the various techniques that have been used with varying degrees of success. Most of these techniques and their drawbacks are described in U.S. Pat. No. 5,161,408 to McRae, et al. The McRae et al. patent discusses a photo-acoustic leak detection system and method that employs a single microphone and narrowband signal processing. Photo-acoustics is the excitation of acoustic waves by unsteady heat addition from a light source. This phenomena was discovered more than a century ago. Photo-acoustics has been used primarily for spectroscopy and most measurements have been made in closed acoustically resonant cells.
Despite the need for a method and system for photoacoustic detection and localization of leaks, especially small leaks, without the necessity of synchronization and with the use of broadband matched-field signal processing, none was known. Thus, there existed the need for a method and system for photoacoustic detection and localization of leaks without synchronization and with the use of broadband matched-field signal processing.
The disclosed method and system of this invention is applicable to a wide range of leak problems, including but not limited to consumer or industrial products such as automobile components, refrigerators, air conditioners and electrical equipment that includes parts that contain liquids or pressurized gases.
The present method and system does not require synchronization, and uses broadband matched-field signal processing across all signal frequencies. The method and system are described below.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and system for photoacoustic leak detection and localization.
It is an object of the invention to use Matched Field Processing (MFP) for signal processing which technique was originally developed for use in underwater acoustics.
The present system and method for photoacoustic leak detection and localization has several innovative features. The first innovation is the use of multiple ultra-sensitive microphones for detecting the photoacoustic sound emitted by the tracer gas when excited by the collimated light. While a single microphone has been employed in the past, the present system uses multiple ultra-sensitive microphones for sound detection. The second innovation is the detection of the sound and combination of signals from the sound across a broad bandwidth, the broadband allowing for multiple frequencies across all signal frequencies. The third innovation is non-synchronous processing of the signal wherein the signal processing is not connected to the beam scanning means. Matched Field Processing is employed to determine the actual leak location. In the prior art, particularly, the McRae et al. Patent, synchronous processing with regard to the scan positioning signal output was required.
Leaks from pressurized containers cause problems with the environment, consumers and manufacturers. The invention is a leak detection technique employing photoacoustic sounds produced by the interaction of a carbon dioxide (CO2) laser turned to 10.6 microns with a photoactive tracer gas, sulfur hexafluoride (SF
6
), emitted by calibrated leak sources. Sulfur hexafluoride (SF
6
) gas is inert, non-toxic and safe for use in occupied human environments.
The part being tested is filled with the gas and if a leak exists, a cloud of the gas will form near the leak. Each time the laser scans the part and encounters a cloud of tracer gas, the gas is heated and expands and launches a photoacoustic sound pulse. The sound pulses generated by the scanning process are recorded by multiple ultra-sensitive microphones. The human ear can hear the photoacoustic sound from large leaks, even though the a majority of the sound produced occurs at frequencies beyond the audible range. However, photoacoustic sound from small leaks is not detectable by the human ear, requiring the ultra-sensitive microphones.
After the photoacoustic pulses are recorded, the magnitude is compared with background noise measurement at several frequencies for a determination if a leak is present. If the presence of a leak is found, then the location of the leak is determined. The recorded sound is processed by using Matched Field Processing (MFP) as the signal processing technique. Knowledge of the acoustic characteristics of the surroundings is necessary for use of the MFP technique. The MFP technique is capable of handling the output of any number of microphones across many signal frequencies.
The location of the microphones with respect to the test part, the speed of sound, and the location and approximate shape of objects that can cause echoes must be known. The photoacoustic sounds recorded at the microphones are reversed in time through computer simulation and cause the sounds to converge to the apparent point of origin.
Measured acoustic signals are compared to predicted acoustic signals to produce an ambiguity surface based on a computational model of the acoustic environment with a variable test source location. When there is a good match between the predicted and measured acoustic signals, the ambiguity surface achieves a maximum and the test source is presumed to be at or near the location of the actual acoustic source. In this sense, the MFP ambiguity surface can be thought of as a spatial correlation, which when normalized properly gives a probability map for the source location. The accuracy of the MFP-determined sound-source location is strongly dependent on the accuracy of the acoustic model of the environment and the signal-to-noise ratio of the measurements. Hence, source localization to better than a wavelength may be possible with a good environmental model at high signal-to-noise ratios. Generally MFP has been developed as a narrowband technique, but broadband source localization by incoherently combining MFP results from several frequencies is a straight forward extension (Baggeroer et al., 1988).
Two particular implementations of MFP do not require a priori information about background noise: the Bartlett processor (Bucker, 1976) which is technically equivalent to back-propagation or time-inversion of the received signals (Jackson and Dowling 1991), and the minimum variance (MV) distortionless processor (described in Jensen et al. 1994) which typically provides better side lobe control than the Bartlett processor when the signal-to-noise ratio comfortably exceeds 0 dB. Although more sophisticated MFP schemes exist (Collins and Kuperman 1991, Collins et al. 1994, Fialkowski et al. 1997). The two implementations herein described are relatively easy to implement and provide important baseline results for photoacoustic leak localization. But other MFP techniques can be used.
A carbon dioxide laser tuned to 10.6 microns and the tracer gas sulfur hexafluoride (SF
6
) are used to produce photoacoustic sounds. The laser beam is rapidly scanned across a flat plate (or parallel to the part) with a leak of known-rate present or absent along the scan. Acoustic measurements are made with two or four microphones and the recorded signal is Fourier analyzed and compared to a previously measured background sound level at the harmonics of the scan rate (the signal frequencies).
Dowling David R.
Yonak Serdar H.
Burns Barbara M.
Politzer Jay L.
The Regents of the University of Michigan
Williams Hezron
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