Ultrasonic pressure transducers

Ultrasonic Pressure Transducers. Advantage is taken of the fact that pressure influences sound propagation in solids, liquids, and gases, but in different ways. In solids, applied pressure leads to so-called stress-induced anisotropy, In liquids, the effects of pressure are usually small (relative to effects in gases), but the frequency of relaxation peaks can be shifted significantly,... 

This equation is the basis of both the electrovibration sind vibration potential listed in the electrokinetic phenomena of Table 9.10. In these cases, the root mean square (rms) voltage i=E L) is either measured as in the case of the colloid vibration potential or induced by an electrode and the rms pressure fluctuations (= AP) at the same fi quency are either induced by an ultrasonic actuator or measured with a pressure transducer, as in the case of electrovibration. 

Level. Level sensors that depend on a pressure difference between two points, one submerged and one in the vapor, are subject to the same installation considerations described for pressure measurement. The inferred level measurement is more accurate when determined by a differential pressure transducer using remote diaphragms than when calculated from the difference between two absolute pressure transducers due to errors from calibration and transducer drift. In either case, the measurement is affected by changes in the weight percent solids in the solution an increase in the liquid density would be interpreted as an increase in the level. Ultrasonic level sensors are not affected by the slurry density but are sensitive to fouling. 

Lekkala and Paayanen (1999) proposed metalized porous PP as ElectroMe-chanical Film (EMFi). By utilizing the EMFi material, Reinhard et al. (2007) reported that the transmitter emitted a sound pressure level up to 90 dB at a distance of 1 m, and the ultrasonic receiver had a sensitivity of 500 pV/Pa. The sensitivity of an ultrasonic receiving transducer without an FET reached -226 dB re 1 V/pPa, which exceeds that of a PVDF-based MHz-range hydrophone (Horino etal. 2012). 

The technique presented above has been extensively evaluated experimentally using ultrasonic data acquired from a test block made of cast stainless steel with cotirse material structure. Here we briefly present selected results obtained using two pressure wave transducers, with refraction angles of 45° and 0°. The -lOdB frequency ranges of the transducers were 1.4-2.8 MHz and 0.7-1.4 MHz, respectively. The ultrasonic response signals were sampled at a rate of 40 MHz, with a resolution of 8 bits, prior to computer processing. 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

Flow meters have traditionally been classified as either electrical or mechanical depending on the nature of the output signal, power requirements, or both. However, improvement in electrical transducer technology has blurred the distinction between these categories. Many flow meters previously classified as mechanical are now used with electrical transducers. Some common examples are the electrical shaft encoders on positive displacement meters, the electrical (strain) sensing of differential pressure, and the ultrasonic sensing of weir or flume levels. 

Let us now turn our attention to the application of the sound wave to a liquid since this is the medium of importance to the practising chemist. The sound wave is usually introduced to the medium by either an ultrasonic bath or an ultrasonic horn (see Chapter 7). In either case, an alternating electrical field (generally in the range 20-50 kHz) produces a mechanical vibration in a transducer, which in turn causes vibration of the probe (or bottom of the bath) at the applied electric field frequency. The horn (or bath bottom) then acts in a similar manner to one prong of a tuning fork. As in the case of air, the molecules of the liquid, under the action of the applied acoustic field, will vibrate about their mean position and an acoustic pressure (P = P sin 2k ft) will be superimposed upon the already ambient pressure (usually hydrostatic, Pjj) present in the liquid. The total pressure, P, in the liquid at any time, t, is given by Eq. 2.4. 

The compression of a powder is a complex process that is usually affected by different kinds of problems. These problems have been widely investigated and mainly concern the volume reduction and the development of a strength between the particles of the powder sufficient to ensure tablet integrity [82], The application of ultrasonic energy shows a great ability to reduce and even avoid these problems [83], Ultrasound refers to mechanical waves with a frequency above 18 kHz (the approximate limit of the human ear). In an ultrasound compression machine, this vibration is obtained by means of a piezoelectric material (typically ceramics) that acts as a transducer of alternate electric energy of different frequencies in mechanical energy. An acoustic coupler, or booster, in contact with the transducer increases the amplitude of the vibration before it is transmitted (usually in combination with mechanical pressure) to the material to be compressed. 

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