The self-resonant frequency of a metal tuning fork is excited by a small piezo-ceramic plate. Level sensors are typically divided into two different measuring systems: Propagation time measurement of a reflected airborne ultrasonic signal by Piezo-ceramic transducers on the one hand, and immersed transducer measurement by piezo-ceramic sensors on the other. Level Sensors – Immersed Transducer Measurement This enables the measurement of both liquids and gases. The advantage of this measuring method is that the measurement is independent of the sound propagation speed and therefore also the medium. The flow speed is then proportional to the reciprocal value of the propagation time difference in and opposite to the direction of flow. In each case there is a superimposition of sound propagation speed and flow speed. They send or receive alternately pulse-shaped ultrasonic wave packages against the direction of flow and in the direction of flow. The traveling wave time principle always employs two piezo-ceramic transducers positioned offset to the direction of flow. Only one piezo transducer sending and receiving at different times is needed for this measuring method. The greater the flow speed of the liquid, the greater the frequency shift between the radiated and the reflected wave front. The Doppler evaluates the frequency shift of the ultrasonic waves, which are reflected by stray particles in the liquid. In both cases a piezo-ceramic transducer positioned on the tube wall generates ultrasonic waves, which are then transmitted into the liquid diagonally to the direction of flow. In ultrasonic flow rate measurement a distinction is made between two different measuring principles: the Doppler effect and the analysis of the traveling wave time. This application can be used for a variety of purposes in industrial automation, for example, for distance measurement, as a limit monitor or as a counter (an object is registered every time it passes through the measuring range). The distances that can reasonably be measured using these systems are between 0.25 m and 10 m. Directed radiation is desired in almost all cases. If the wavelength is shorter than the transducer, it is radiated straight (in a close angle). If the wavelength resulting from the frequency is large in comparison to the size of the transducer, the result is a spherical-shaped radiation of the wave front. The higher the frequency, the higher the resolution and the shorter the range, and vice versa. The frequency used by the transducer (resonance mode) here depends on the desired range and the resolution. The piezo-ceramic part normally has an adaption layer on the radiating side to match the acoustic impedance of the transducer to that of the air. The piezo-ceramic part operates here as sender and receiver (time multiplex mode). The sound wave is reflected by this object and received again by the same piezo-ceramic part. A piezo-ceramic disc sends a short ultrasonic air wave in the direction of the object whose distance is to be measured.

Piezo electric ceramics