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One transducer is used as the transmitter to which an electrical signal is applied to generate sound pressure waves inside the cavity, whereas the second transducer is used as a receiver to detect the standing waves, the so-called cavity resonances. A series of equidistant in frequency resonance interference peaks can be observed if the excitation signal is a periodic oscillatory waveform that is swept in frequency to cover a wide frequency range.

The spacing between any two adjacent peaks is directly proportional to the sound speed in the liquid at the median frequency of the two peaks. The full-width-at-half-maximum of any resonance peak is related to the sound attenuation in the liquid at the peak frequency. Thus, by using a swept-sine wave excitation, both sound speed and sound attenuation for a fluid liquid, mixture, emulsion, suspension, etc.

The transducers need not be placed in contact with the fluid, the transducer system being located near the outside of the container wall, and measurements are made from the outside of the container. The fluid under investigation is contained in the space between the two walls, and the observed interference standing wave pattern has two superimposed components: in addition to the standing waves generated inside the liquid cavity, one observes standing wave patterns in the wall thickness of the container.

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Air-coupled, high-power ultrasonic transducers for generating pressure waves, and for detecting these waves, were disposed at opposite sides of a container in which the fluid under investigation resides, and facing each other and the container. Containers may be fabricated from glass, metal, plastics, etc. The frequency range of the transducers was between 0. This permits a straight path for the sound through the container since, at these high frequencies, sound waves travel in a tightly confined beam.

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For example, the wavelength in air for 1 MHz sound wave is 0. The beam should impinge on the surface of a container in particular, for flat surfaces such as a Starna cell almost perpendicularly thereto. Sound is transmitted directly through the container and into the liquid. There are no Lamb wave modes generated in the container walls since such waves would become severely attenuated by the liquid inside. Lamb waves also have a much smaller sound speed in container walls a plate than in the bulk form of the same material; as a result, if Lamb waves were generated there would be a delayed signal which can easily be identified with respect to those waves that travel directly through the wall.

Such signals have not been observed from the embodiments of the apparatus of the present invention utilized. Additionally, there are no shear waves or Lamb wave modes possible in the liquid, since liquids only sustain longitudinal waves pressure waves.


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If the container has at least two parallel walls, and pressure waves are made to impinge on one of these walls, the opposite wall being used to detect waves generated in the fluid under investigation, strong resonances may be set up in the fluid, and resonances may be directly observed. By contrast, for non-parallel, but regular walls, resonances still are generated in the fluid, but their amplitudes are much smaller.

Further, as a result of the large impedance mismatch between air and the container wall, a large portion of the excitation sound energy is reflected, and a small fraction thereof enters the container wall. These waves propagate through the wall and into the fluid under investigation.


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A small amount of energy enters the opposite wall and is reradiated and detected by the detecting transducer which also suffers from losses due to impedance mismatch. Therefore, means for extracting small signal-to-noise ratio signals from the detected signal are provided.

In the present SFAI procedure, as will be described in more detail hereinbelow, measurements at each frequency between chosen start and stop frequencies are made through a narrow band as an example, between about Hz and Hz tracking filter restricting the received noise to a narrow, sliding frequency window throughout the entire frequency sweep range.

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In what follows, the term tracking filter will be used as representative of a filter which is effective at the various sine-wave frequencies employed and for which one embodiment is described in FIG. There are many ways to implement such a tracking filter in practice and the electronics to do so are commercially available, such as a vector network analyzer. The swept frequency technique permits a 5 V excitation signal to generate high-quality measurements. Although the frequency spectrum produced in many situations contained much noise after the large signal-to-noise ratio enhancement described hereinabove, upon transformation of the data into the time domain using a Fast Fourier Transform, the quality of the data was much improved.

The time-domain data provided high-quality sound pulse transmission information from which time-of-flight of the acoustic waves through the fluid could readily be determined. Since the size of the container is easily measured, this led to an accurate determination of the speed of sound in the liquid.

Further, the quality of the data was sufficiently good that multiple passes of the waves through the liquid could be measured, thereby providing sound attenuation information. Pulse echo technology cannot provide adequate signal-to-noise ratio for such measurements. In accordance with the teachings of the present invention, however, it is possible to obtain the same time-domain information but from a frequency domain technique such that the noise bandwidth can be significantly reduced orders of magnitude. Another advantage of using a time-domain procedure derived from a frequency sweep is the ability to achieve much higher signal-to-noise ratio information using much lower excitation voltages.

The total improvement can be as high as 80 db.

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Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Turning now to FIG.

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Transmitting transducer, 12 , and receiving transducer, 14 , are gas matrix air-coupled transducers obtained from Ultran Laboratories in Pennsylvania. Transducers 12 and 14 are broadband transducers frequency range: kHz Shown also is container cap, 19 , adapted for rendering container 18 fluid-tight.

Transmitted and radiated signals received by receiving transducer 14 , are directed through narrow-bandpass filter circuitry, 24 , to digital signal processor electronics, 26 , where it is processed, including conversion of the frequency spectrum to the time domain by FFT Fast Fourier Transform , stored, and displayed by display apparatus DSP 26 may also carry out various math operations to process the data further, if desired.

To recover information in a frequency sweep measurement one needs to restrict the noise to a narrow bandwidth. Further, amplitude and phase measurements must be made within this narrow bandwidth. A complex Fourier transform using both amplitude and phase values is then made to recover the time-domain data with very high signal-to-noise ratio. In principle, the measurements are made through a narrow band-pass filter that effectively tracks the frequency sweep. The received signal has traveled a certain distance in the container walls and the fluid which generates a time delay or phase delay relative to the impressed signal.

Additionally, the signal may also lose some energy during this propagation and so its amplitude may vary. The received signal is multiplied with signals in quadrature and the resultant signals passed through a low pass filters to remove extraneous higher frequency signals.

The cutoff frequency of the low-pass filter is chosen at a desired fixed frequency, such as Hz for all sweep frequencies, the system generating in effect a tracking narrow band-pass filter having a desired bandwidth, but using fixed, narrow low band-pass filters. As stated hereinabove, the sweep measurement requires the simultaneous determination of both phase and amplitude of the received signal within a narrow frequency band.

This measurement needs to be made at every frequency as the frequency is varied between selected limits low and high values. The circuitry used for this is described in FIG. Two double-balanced mixers, 42 and 44 , and two low-pass filters, 46 and 48 , respectively, mix the signals from DDS 30 with the output from amplifier 38 and filter the high frequencies.

The filtered signals are directed to analog-to-digital converters ADC , 50 and 52 , respectively and stored in memory, DSP 26 processes the stored signals which may be displayed by display Microcontroller 22 is programmed to communicate with DDS 30 to generate two sine waves having the same frequency and a phase difference between the two outputs. This signal goes through power amplifier, 62 , to drive transmitter T transducer The signal from receiver R transducer 14 is amplified by a variable gain user selectable gain amplifier 38 and directed to simultaneously to mixers 42 and Mixers 42 and 44 also receive in quadrature sine wave signals 32 and 34 , respectively, and the resulting outputs are low-pass filtered, 46 and 48 , respectively.

The cut-off frequency of low pass filters 46 and 48 for example, Hz determines the narrow-frequency bandwidth of the measurement at a given frequency.

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The output of low-pass filters 46 and 48 are then digitized by ADCs 50 and 52 , respectively, converted to gain and phase values by DSP 26 , and stored in memory. The resulting frequency spectrum may be displayed on display device As stated, DSP 26 may perform various math operations to process the data further, as desired. Typically, the frequency sweeps are performed with between and frequency steps between the two chosen frequency limits depending on the transducers used and the sample being tested.

The described apparatus has the capability to produce and process a frequency range of Hz to 10 MHz, and the number of frequency steps can be as large as 64, steps.


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Typical sweep times vary from 2 s to 10 s for a given measurement, but the electronics is not limited to that. The cut-off frequency of the low-pass filters can be increased to obtain higher speed, if desired. The excitation voltage for the transmitter transducer is typically less than 10 V this low voltage protects the transducers from degradation over time , although the apparatus has the capability of generating higher voltages.

This means only the signal within that Hz bandwidth this value can be different than Hz, and the exact value is not critical is detected, thereby improving the signal-to-noise ratio immensely. The processing gain achieved by this frequency sweep approach is better than 80 dB. The electronics described permits measurement of both the amplitude and the phase of the received signal as compared to the transmitted signal as a function of frequency. However, in a significant number of situations, signal processing with the improvement in signal-to-noise ratio by 80 dB, the measured spectrum of the transmitted sound through the container as a function of frequency may appear like a noise spectrum in cases where the container walls are not parallel.

However, this noisy spectrum contains information regarding how the sound propagates through the fluid inside the container and how each frequency behaves as it interacts with the fluid medium. Transforming this frequency spectrum to the time domain using complex real and imaginary Fast Fourier Transform of the data, permits multiple time domain echoes to be clearly seen, thereby permitting the determination of sound attenuation as the sound waves traverse the container.

Contact SFAI has demonstrated that sound speed and sound attenuation can be used to identify and characterize various fluids and chemicals which is shown to be possible using the non-contact transducers in accordance with the present invention. The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Effective date : Year of fee payment : 4. Apparatus and method for non-contact stand-off ultrasonic determination of certain characteristics of fluids in containers or pipes are described.