Acoustic sensor

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

In this paper we approach the integrated optical sensors from the science/technological side. In that way it is avoided to discuss also all kinds of other competing sensing fields, in which the sensors are based on non-optical physical phenomena and are implemented as e.g. electrical, micromechanical or acoustical sensors. 

The acoustic sensor used for capturing process vibrations is often a standard accelerometer, covering a frequency range 0-50 kHz - or a so-called acoustic emission sensor (AE) often covering higher frequencies from 50 kHz up to several MHz. The 4396 and ATEX 5874 accelerometers from Briiel Kjaer are shown in Figure 9.2. The 4396 accelerometer is a standard accelerometer for industrial use and the ATEX 5874 is a certified accelerometer for use where explosion proof equipment is needed (EX certified). 

FFT spectra contain an abundance of potential information (as a function of frequency) related to the process/products characterized by the acoustic sensors. FFT spectra constitute fit for purpose input to any chemometric data analytical method deemed beneficial for making the final model relating the acoustic emissions (vibrations), X, to the salient functional properties (quality, quantity) of the products or process... 

Figure 9.20 shows loadings from a PCA of data from all four acoustic sensors A, B, C and D, which clearly shows that the progress in PC 1 in Figure 9.20 is caused by lump formation in chamber 1 measured by sensor B as indicated by the relatively high loading values for this sensor. 

For industrial fertilizer production reliable ammonia concentration data are essential. An experimental setup for acoustic chemometric prediction of ammonia concentration has been tested in a full-scale industrial plant. Figure 9.22 shows a bypass loop with the orifice plate. The acoustic sensor was again mounted onto the orifice plate [5]. To ensure constant differential pressure and temperature of the ammonia flow, two pressure transmitters and one temperature sensor were used. Reference samples were taken at the sample valve shown in Figure 9.22. 

Battlefield acoustic sensors are required to detect, localise, track, and classify or identify targets of interest, which typically include vehicles, aircraft and personnel73. 

In this part we will describe recent achievements in the development of biosensors based on DNA/RNA aptamers. These biosensors are usually prepared by immobilization of aptamer onto a solid support by various methods using chemisorption (aptamer is modified by thiol group) or by avidin-biotin technology (aptamer is modified by biotin) or by covalent attachment of amino group-labeled aptamer to a surface of self-assembly monolayer of 11-mercaptoundecanoic acid (11-MUA). Apart from the method of aptamer immobilization, the biosensors differ in the signal generation. To date, most extensively studied were the biosensors based on optical methods (fluorescence, SPR) and acoustic sensors based mostly on thickness shear mode (TSM) method. However, recently several investigators reported electrochemical sensors based on enzyme-labeled aptamers, electrochemical indicators and impedance spectroscopy methods of detection. 

Low-wave acoustic sensor [63] was used to detect interaction of thrombin with RNA and DNA aptamers [24]. The authors compared the binding of thrombin to RNA aptamer also by using filter binding method utilizing radiolabeling of RNA aptamer by 3 -P32 and also by... 

Urban warfare. Sensor networks are deployed in buildings and open air urban areas. Snipers can be localized by comparing the samples from multiple acoustic sensors. 

Precise measurement tools are necessary parts of most successful scientific and engineering enterprises. The sensing devices that we consider in this volume are such tools, capable of measuring physical, chemical, and biological quantities. What they have in common is that they all employ acoustic waves in their cq>-eration. The purpose of this introductory chapter is to provide an overview of these devices, and to answer the question why use acoustic sensors ... 

These devices are conveniently small, relatively inexpensive, quite sensitive, and inherently capable of measuring a wide variety of different input quantities. It is because of these far-reaching characteristics that we have written this book in order to bring a diverse audience of readers an understanding of acoustic sensor principles. 

Figure 1.2 Schematic sketches of the four types of acoustic sensors discussed in detail in this book (a) Resonant quartz crystal like that used in electronic communications systems (after Lu [6]) (b) Suiface-acoustic-wave delay line with selective absorptive coating (after Wohltjen and Dessy [3]) (c) Acoustic-plate-mode delay line made from quartz crystal (after Ricco and Martin [7]) (d) Thin-membrane flexural-plate-wave delay line made by microfabrication techniques from a silicon wafer.

The alternative sqiproach for getting information from these acoustic sensors is to measure the sensor characteristics passively that is, to supply an external... 

Figure 1.4 Measurement schemes used with the acoustic sensors illustrated in Figure 1.2.1.L. = insertion loss, fres = resonant frequency, Q = quality factor, and Zin = input impedance.

The surface-acoustic-wave sensor is also commercially available, either as a single sensor or as a part of an entire sensing system. The authors hope that informing potential users about acoustic sensors may stimulate the wider use of all the sensors that we discuss. 

The important point to note from this example is that the attenuaticHi is proportional to the square of the frequency. This prediction has been borne out experimentally with both bulk and surface waves for a number of materials. Since the loss increases rapidly with frequency, it is important to use high-quality materials for acoustic sensors operating at high frequency. 

Figure 3.1 Schematic sketches of the four types of acoustic sensors, (a) Thickness-Shear Mode (TSM) resonator (b) Surface-Acoustic-Wave (SAW) sensor, (c) Shear-Horizontal Acoustic-Plate-Mode (SH APM) sensor, and (d) Flexural-Plate-Wave (FPW) sensor.

In a flexural plate wave (FPW) device, an acoustic wave is excited in a thinned membrane. Figure 3.38 (page 112). As with the other acoustic sensors discussed — the TSM, SAW and APM devices — the flexural-plate-wave (FPW) device can sense quantities that cause its phase velocity, Vp, to change. A unique... 

One finds experimentally that the amplitudes of the displacements associated with a flexural wave of given total power are large relative to those of the other acoustic sensors discussed here. Peak-to-peak normal displacements up to 100 nm have been measured, using laser diffraction, when only a few milliwatts of wave energy were propagating in a plate a few micrometers thick. As a result of... 

It may be helpful to close this chapter by discussing what the TSM, SAW, APM, and FPW sensors have in common and in what ways they differ. We will also describe several additional types of acoustic sensors. 

There is a growing body of literature on the use of acoustic sensors for in-situ experiments, particularly in conjunction with electrochemical measurements... 

The relative importance of the mass-loading and viscoelastic contributions to the observed acoustic sensor response is an issue that has yet to be resolved. Capitalizing on these effects to improve chemical selectivity and detection sensitivity requires further characterization of sensor response, in terms of both velocity and attenuation changes, in addition to more accurate models describing how coating-analyte interactions affect relevant film properties. 

The use of polymer-coated acoustic sensors as chromatographic detectors (GIX, HPLC) has also been demonstrated [1,43,218]. In such applications, a lack of selectivity fcH a given analyte is actually beneficial, since the function of the coated sensor is to detect each and every species passing the detector after preseparation by the chromatographic column (see Chapter 6). 

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