A Liquid Density Sensor Based On AlN Piezoelectric Micromachined Ultrasonic Transmitter Insensitive to Liquid Viscosity

Abstract: To overcome the limitations of conventional liquid density sensors, MEMS-based approaches have been developed. However, the viscosity-density coupling effect often compromises accuracy in high-viscosity liquids. Although various decoupling strategies have been proposed, they often suffer from complexity and inefficiency. This study presents an AlN-based PMUT liquid density sensor insensitive to viscosity interference. The sensor employs two identical PMUTs, functioning as transmitter and receiver, respectively. An ultrasonic wave generated by the transmitter is reflected by the liquid surface and detected by the receiver. Theoretical calculations demonstrate that when the excitation frequency remains constant, the amplitude of the received electrical signal exhibits a specific relationship with liquid density, while the viscosity-induced signal amplitude variation becomes negligible. Therefore, after calibrating the PMUT liquid density sensor by fitting the relationship between received signal amplitude and density across 0-100% glycerol solutions, the device can measure densities within this range regardless of liquid type and viscosity. Experimental results show that the sensor can accurately measure the density of the propylene glycol solutions using the glycerol-calibrated fitting formula, with the calculated density error rate between measured propylene glycol solutions using the glycerol-calibrated fitting formula and actual densities remains below 0.125%, demonstrating measurement insensitivity to viscosity differences between the two liquids. The proposed method achieves maximum error rates of less than 2.5% in high-viscosity environments (80%-100% glycerol solutions), which is 20% that of other density measurement methods based on resonant frequency. The developed PMUT liquid density sensor exhibits a measurable density range from 0.789 to 1.261 g/cm3.