Apparatus designed to determine and/or monitor at least a process variable of a medium in a vessel, wherein said apparatus comprises at least - a unit capable of vibrating with at least a membrane that can be made to produce mechanical vibrations, - at least three rods (10a-10d), which are attached in a force-locked manner to the membrane perpendicularly to a basic surface of the membrane, - a housing, wherein the membrane forms at least a part of a wall of the housing, and wherein the rods (10a-10d) are directed towards the interior of the housing, - at least a drive/reception unit, which is arranged in the end area of the rods (10a-10d) facing away from the membrane, wherein the drive/reception unit is designed to excite the unit capable of vibrating, by means of an electrical excitation signal and by means of the rods (10a-10d), to produce mechanical vibrations, and to receive the mechanical vibrations of the unit capable of vibrating and to convert them to an electrical reception signal, - wherein the drive/reception unit is designed to make the rods (10a-10d) produce mechanical vibrations by moving the rods (10a-10d) together or apart by means of the drive/reception unit at the correct frequency, and wherein the rods (10a-10d) are fixed to the membrane in such a way that the vibrations of the membrane result from the vibrations of the rods, and - an electronic unit, which is designed to generate an excitation signal from the reception signal, and to determine the at least one process variable at least from the reception signal, wherein at least one of the rods (10a-10d) is secured on the membrane at a place along the basic surface of the membrane, wherein at said place the second derivative of the deviation of the membrane from the rest position - as a function of the place along the basic surface - is essentially equal to zero. - a unit capable of vibrating with at least a membrane that can be made to produce mechanical vibrations, - at least three rods (10a-10d), which are attached in a force-locked manner to the membrane perpendicularly to a basic surface of the membrane, - a housing, wherein the membrane forms at least a part of a wall of the housing, and wherein the rods (10a-10d) are directed towards the interior of the housing, - at least a drive/reception unit, which is arranged in the end area of the rods (10a-10d) facing away from the membrane, wherein the drive/reception unit is designed to excite the unit capable of vibrating, by means of an electrical excitation signal and by means of the rods (10a-10d), to produce mechanical vibrations, and to receive the mechanical vibrations of the unit capable of vibrating and to convert them to an electrical reception signal, - wherein the drive/reception unit is designed to make the rods (10a-10d) produce mechanical vibrations by moving the rods (10a-10d) together or apart by means of the drive/reception unit at the correct frequency, and wherein the rods (10a-10d) are fixed to the membrane in such a way that the vibrations of the membrane result from the vibrations of the rods, and - an electronic unit, which is designed to generate an excitation signal from the reception signal, and to determine the at least one process variable at least from the reception signal,