Noninvasive locally and temporarily resolved measurement of sound velocity for process monitoring

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In times of increasing scarcity of resources, a detailed understanding and monitoring of technical processes is becoming increasingly important. Of particular interest are mixing processes in metallurgy, crystal growth and electrochemistry, which are - for example - needed for steel production or the production of silicon wafers. Information about the mixing process can be obtained by measuring the concentration of the different components as a function of time and location. Non-invasive measurement techniques allowing a combined determination of concentration with temporal and local resolution are currently unknown or solely work under strong confinements in transparent fluids. Therefore, the aim of the research project is to develop a completely new method for the monitoring of mixing processes by measuring the temporarily and locally resolved sound velocity using ultrasound. With the knowledge of the speed of sound at each measuring point, the corresponding concentration can be determined. The proposer achieved for the first time to measure the sound velocity in a fluid with moving scattering particles using a single ultrasonic transducer without the use of a reflector at a known position (back wall echo). The method uses the focus position, which depends on the sound velocity, as a second piece of information besides the time of flight. The focus position can be measured indirectly by determining the time of flight corresponding to the maximum of the echo signal maximum. The maximum position of the echo signal amplitude, in turn, indicates the time of flight to the focus, because the strongest echo signal is generated in the sound field maximum. By using simulated (or measured) calibration curves, the time of flight suffices to determine the sound velocity and the location of the ultrasonic focus in combination. This is the basis for the development of a novel measurement method. In the underlying project, ultrasonic arrays are used. The array elements are excited with slight time delays to move the focus position along the acoustic axis. This allows a locally and temporarily resolved measurement of sound velocity. The method is intended to be used for a vast variety of fluids including fluids with strong attenuation and scattering particles of different sizes and concentrations. Measurements during the mixing process of two liquids are intended for proof of concept. However, a much larger potential of applications can be expected. So, for example, chemical reactions can be monitored or temperature distributions can be determined. Thus, the new method can also be applied in the chemical and thermal process engineering, biochemical engineering or in food technology.