Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Content or effect of a constituent of a liquid mixture
Reexamination Certificate
1999-06-01
2001-06-12
Chapman, John E. (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Content or effect of a constituent of a liquid mixture
C073S024020, C073S061490, C356S432000
Reexamination Certificate
active
06244101
ABSTRACT:
FIELD OF THE INVENTION
The present invention is a multi-frequency acoustic transducer.
BACKGROUND OF THE INVENTION
Photoacoustic detection uses piezoelectric transducers to measure pressure waves from the radiationless decay of a laser excited state, generated by pulsed irradiation. The conversion of energy to heat generates a pressure wave that can be detected with a piezoelectric transducer. Presently, photoacoustic detection is a very sensitive technique but it is not very selective.
Traditionally, two types of dual element transducers are commercially available. First, a dual element longitudinal transducer in a single housing wherein both elements are matched to have the same frequency, for example 1 MHz and 1 MHz, or 5 MHz and 5 MHz, have been used for thickness gauging and/or flaw characterization. One element is used for transmitting a signal and the second is used for receiving the returned signal. The two matched elements are angled toward one another to create a crossed beam sound path to yield better near surface resolution. In making these dual element longitudinal transducers, the frequencies are carefully matched in order to assure an optimum transmitter/receiver beam overlap. The second type of commercially available photoacoustic transducer is a dual element longitudinal/shear wave transducer in a single housing. This is used to make both longitudinal and shear wave measurements with a single transducer. For example a longitudinal element may have a frequency of 1 MHz and the shear element may have a frequency of 2.5 MHz. Each element is specific for either shear waves or longitudinal waves. This second type is used for specific applications where the transducer may be fixed in order to obtain both the longitudinal and shear wave signals. Couplants used for acoustic scanning do not transmit shear wave signals.
In both types of transducers, the amplitude of the observed photoacoustic signal provides the sensitivity, i.e. measurement of the quantity of absorbing species. However, the selectivity, i.e. identifying the absorbing species in a mixture may be possible by seeking one or more wavelengths that is/are not absorbed by certain species of the mixture. Because of overlap of absorption spectra, this technique is limited to mixtures of species with limited absorption spectra overlap.
Thus, there is a need in the art for a photoacoustic transducer that provides greater selectivity without relying upon limited absorption spectra overlap.
SUMMARY OF THE INVENTION
The present invention is a transducer for photoacoustic detection having at least two piezoelectric elements wherein at least a first piezoelectric element has a first frequency and at least a second piezoelectric element has a second frequency. The improvement according to the present invention is that at least two piezoelectric elements are longitudinal elements for detecting pressure waves; and
the first frequency is different from said second frequency. The at least two piezoelectric elements are used as receivers only. In other words, the invention is a multi-frequency longitudinal transducer for photoacoustic detection.
It is an object of the present invention to provide a transducer for photoacoustic detection that is a multi-frequency longitudinal wave transducer.
It is another object of the present invention to provide a method of identifying and measuring chemical species in a mixture.
An important advantage of the present invention is increased selectivity with no reduction in sensitivity.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.
REFERENCES:
patent: 4051371 (1977-09-01), Dewey, Jr. et al.
patent: 4303343 (1981-12-01), Patel et al.
patent: 5616826 (1997-04-01), Pellaux et al.
patent: 5933245 (1999-08-01), Wood et al.
patent: 6160255 (2000-12-01), Sausa
D Lynch et al., “A Pulsed Photoacoustic Microcalorimeter for the Detection of Upper Excited-State Processes and Intersystem Crossing Yields”,Applied Spectroscopy, vol. 43, No. 5, 1989, pp. 826-833.
Amonette James E.
Autrey S. Thomas
Foster-Mills Nancy S.
Posakony Gerald J.
Battelle (Memorial Institute)
Chapman John E.
Zimmerman Paul W.
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