This paper describes a novel electrically passive fluid level gauge. It is based on a one-dimensional low-frequency acoustic gas resonator. The acoustical power is delivered to the resonator an the acoustic waveguide and a fibre-optic microphone is used as an acoustic pressure sensor. Distances of over 100 m between the electrically passive sensor and the electrically active controller were achieved in practice. The proposed method can be successfully implemented in systems where problems of residues, deposits, foams, etc, can be expected.
https://iopscience.iop.org/article/10.1088/0957-0233/9/9/017
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