We show that, for small added masses, the sensitivities of bulk-mode, surface-mode, and flexural plate-mode gravimetric ultrasonic sensors are respectively 2/ρλ, ≊1/ρλ, and 1/2ρd, where ρ is the density of the sensor substrate material, λ is the wavelength, and d is the plate thickness of the flexural-mode sensor.

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We describe the fluid loading of Lamb waves propagating in a 3 mm×9 mm composite membrane consisting of low-stress silicon nitride deposited by low-pressure chemical vapor deposition (2.0 μm thick) supporting an aluminum ground plane (0.3 μm), a piezoelectric zinc oxide layer (0.7 μm), and a pair of 100-μm-period interdigital Al transducers (0.5 μm). When this device is operated in air, the phase velocity of the lowest antisymmetric mode is 474 m/s; it changes to 304 and 322 m/s, respectively, when the membrane is loaded on one side with de-ionized water and with methanol. The large fluid-loading effect, in excellent agreement with theory, and the stability of an oscillator made with this membrane (stability better than 1 part in 106) suggest the utility of the device as a sensitive densitometer and pressure sensor.

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