Continuous process monitoring in gaseous, liquid or molten media is a fundamental requirement for process control. Besides temperature and pressure other process parameters such as level, flow, concentration and conversion are of special interest. More qualified information obtained from new or better sensors can significantly enhance the process quality and thereby product properties. Ultrasonic sensors or sensor systems can contribute to this development.The state of the art of ultrasonic sensors and their advantages and disadvantages will be discussed. Commercial examples will be presented. Among others, applications in the food, chemical and pharmaceutical industries are described. Possibilities and limitations of ultrasonic process sensors are discussed.

A transmission line equivalent circuit for piezoelectric transducers has been modified to provide modeling of lossy piezoceramic transducers. A lossy transmission line is used to model the mechanical losses. The equivalent circuit parameters are derived from analogies between electrical transmission lines and acoustic wave propagation. Implementation of the equivalent circuit model in SPICE is shown. Simulations and measurements in the time and frequency domain of a low-Q material and a multilayered ultrasonic sensor using a low-Q piezoceramic transducer are presented.

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This paper presents a new ultrasonic density bility and is suitable for a wide range of tube diameters. The sensor comprises a transducer consisting of a piezoceramic disk mounted between two reference rods of quartz glass. Additionally, a second transducer is used as a sound receiver. The density is obtained from the reflection coefficient of ultrasound at the interface between the quartz glass rod and the liquid, and the transit time of sound between this interface and the second transducer. Parameters like a high long term stability and accuracy of f0.2% were achieved by an internal acoustic reference measurement and a high signalto-noise ratio. The latter was gained by using the sound radiated from the rear side of the piezoceramic disk as the reference. Advantages of the new sensor are discussed and experimental resnlts are given in this paper. Applications of the sensor comprise concentration measurement and ultrasonic mass flow measurement.