Patent ID: 6920787

Claim:
An apparatus for determining and/or monitoring the filling level of a filling material in a container and for determining a process variable of a filling material in the container, comprising: a membrane which can oscillate being fitted at the same level as the predetermined filling level; a unit which can oscillate and is attached to said membrane said unit which can oscillate being fitted such that it enters the filling material to a defined immersion depth; a transmitting/receiving unit which causes said membrane and said unit which can oscillate to oscillate at a pre-determined transmission frequency and which receives the oscillations from said unit which can oscillate; a control/evaluation unit which identifies that the predetermined filling level has been reached as soon as a predetermined frequency change occurs, or which determines the density of the filling material on the basis of the oscillation frequency of the unit which can oscillate; and a differential amplifier, wherein: said transmitting/receiving unit is a piezoelectric element in the form of a disk, on whose first face an electrode structure is provided, with the electrode structure having at least two transmitting electrodes and two receiving electrodes, and with the first transmitting electrode being opposite the second transmitting electrode, and with the first receiving electrode being opposite the second receiving electrode, essentially in each case symmetrically about a point, the inputs of said differential amplifier have the electrical signals which are tapped off from said two receiving electrodes applied, and said first transmitting electrode and said second transmitting electrode, as well as said first receiving electrode and said second receiving electrode are of opposite polarity.