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Designed structure of the liquid-level monitoring system. (PZT = Piezoelectric ceramic; D = Diameter; L = Length; Z2 = Aluminum alloy acoustic impedance; ZA = Air acoustic impedance; ZW = Water acoustic impedance RA = Aluminum alloy–air interface reflection coefficient; TA = Aluminum alloy–air interface transmission coefficient; RW = Aluminum alloy–water interface reflection coefficient; TW = Aluminum alloy–water interface transmission coefficient.).
