New approach for the reliable measurement of acoustic impedance of liquids in an acoustic waveguide

Continuous and reliable concentration measurement of liquids still is a great challenge as the sensor systems have to meet high industrial requirements. Apart from chemical sensors there are a couple of acoustic systems [1] that are well suited for a lot of industrial applications. Most of them determine the amplitude ratio of deflected ultrasound bursts at different boundaries as well as the sound velocity of the liquid in order to calculate its acoustic impedance and therewith its density. The advantages of acoustic sensors are their robustness and their fast response. Their disadvantages are sensitivity against variations of the reference material properties as well as abrasion or soiling of the boundaries. This contribution is about a new model-based method that uses the whole surface of an acoustic waveguide as reference boundary: It has turned out that the principal components of a signal at the end of the waveguide can be assigned to the different propagative acoustic modes (Fig. 1). Therefore it is necessary to use different simulation tools, e.g. FEM and modal analysis (Fig. 2). With this it is possible to determine the amplitudes of each mode by means of one measured signal at the end of the waveguide and Gauss Algorithm even if the transducer is of very simple kind [2]. Therewith it is possible to get redundant information -- one amplitude for each mode -- for the liquid impedance. In addition to that it is possible to generate an acoustic reference signal without information about the liquid impedance but dissipation if we use the fundamental mode. The different signal amplitudes and a model of acoustic wave propagation [2, 3] offer the possibility to distinguish between dissipation in the liquid and attenuation due to the impedance relations. Moreover, if there are enough analysable amplitudes available, variations of the reference material properties can be determined.