| 摘要 |
PCT No. PCT/DE90/00718 Sec. 371 Date May 20, 1991 Sec. 102(e) Date May 20, 1991 PCT Filed Sep. 19, 1990 PCT Pub. No. WO91/04467 PCT Pub. Date Apr. 4, 1991.The apparatus comprises at least two oscillating rods which are secured spaced from each other to a diaphragm, the edge of which is connected to a screw-in piece. On the side of the diaphragm remote from the oscillating rods a clamping screw is fixedly connected to the diaphragm so that the clamping screw and diaphragm assume a position with respect to each other in which the axes of symmetry coincide. The clamping screw is surrounded by a stack of annular piezoelectric elements, electrode rings and insulating rings. The stack forms the excitation and receiving transducer of the apparatus. The stack-shaped excitation and receiving transducer is clamped and secured between the diaphragm and a hexagonal nut. The stack bears on pressure studs which are arranged between a metal ring terminating the stack and the diaphragm. The clamping force is chosen so that the diaphragm is simultaneously concavely prestressed. The excitation transducer stimulates the oscillating rods to oppositely directed oscillations at their natural resonant frequency transversely of their longitudinal axes. The receiving transducer detects a change of the oscillation behavior of the oscillating rods and this is utilized to determine and/or monitor the predetermined level. Both the change of the oscillating frequency and that of the oscillating amplitude of the oscillating rods may be utilized.
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