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Abstract

In a process for the detection of the limit level of liquids and bulk materials in containers and pipelines, the change in the oscillation behavior of a sensor which is excited to oscillation is detected electronically and used to trigger a switch command. As sensor there is used a parallel bimorph piezoelectric element (22) having a common central electrode (44) and two outer electrodes (46, 48). A first alternating voltage (U1) with an oscillator frequency (f1) is applied between the central electrode (44) and a first outer electrode (46), a second alternating voltage is measured as amplitude of oscillation (U2) with corresponding oscillation frequency (f2) between the central electrode (44) and the second outer electrode (48), and the measured amplitude of oscillation (U2) or oscillation frequency (f2) is converted into an output switch signal. In a device suitable for the carrying out of the process, the piezoelectric element (22) is inserted in liquid-tight manner into a plastic cover (24), the plastic cover (24) in its turn being fastened in liquid-tight manner to a plastic housing which passes through the wall (16) of a container or a pipeline, and the plastic cover (24) together with the piezoelectric element (22) extends into a protective tube (18) developed on the plastic housing (12).

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