Acoustic deposit thickness measurement

Figure 71.16 Deposit thickness measured with the acoustic method and flux versus the quantity of deposited matter...

As we do not know the acoustic velocity in particle deposit, the general principle of acoustic method for thickness measurement has to be modified. It was proposed to first measure the signal on the clean membrane, at t = 0, then the deposit echo on a fouled membrane (Figure 11.15). 

The goal of the study was to obtain several kinds of information (thickness, porosity) about a deposit. Therefore, two characterization methods were developed. First, the acoustic method gives time variations from a complex signal, which are qualitative variations. Second, the optical method gives thickness measurements, which are quantitative variations. Adaptation of the two methods on the same apparatus, a confined channel, enables a complete deposit characterization to be obtained. 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Because TSM oscillators have been around for over 50 years, quite a number of circuits to measure their response have been proposed, fabricated, and tested. The frequency of operation of TSM resonators (typically < 20 MHz) allows circuits to be constructed using ordinary components and printed circuit boards. Instruments and fixtures are commerciaUy available from a number of vendors (see Appendix D) that utilize fairly simple oscillator circuits incorporating the TSM resonator as the principal fiequency-control element. These systems are sold primarily for monitoring the deposition of metal films via evaporation or sputtering in a vacuum environment. The operator must typically input the density and acoustic impedance of the metal to be deposited, and the instrument then displays film thickness as deposition proceeds. These systems can also be utilized for gas-phase sensing applications, provided the TSM device is not coated with any particularly lossy materials these can cause so much damping that oscillation ceases. The systems provide information derived only from the resonant frequency there is no indication of damping except in the instance that oscillation ceases entirely. 

First experiments were conducted with day suspensions at 1 g in dead-end mode. The TMP was constant and equal to 80kPa. Figure 11.16 gives an example of results obtained with the acoustic method applied on the filtration module. The measured thickness and the flux are plotted versus time. The flux curve is a typical result encountered in dead-end filtration a rapid decrease at the beginning of the experiment and then more slowly until a stabilized value (which was not reached at the end of this experiment). The final thickness is 180 pm for a total deposited mass of 120 gm and a relative flux decrease of 66%. Reproducibility of the measurement was tested and gave good results. 

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