Low-Frequency Piezoelectric Acoustic Sensing for In Situ Glycerol Monitoring

This study investigates the use of 5 kHz piezoelectric ceramics (PZT) for real-time, in situ monitoring of glycerol concentration during biodiesel production. A matched pair of PZT in a through-transmission setup enabled ultrasonic amplitude measurements across glycerol-water mixtures (0-100%). Simultaneous viscosity and density assessments captured viscous and inertial effects. Amplitude responses exhibited reproducible exponential decay with glycerol concentration, consistent with predictions from the inhomogeneous Helmholtz equation and Stokes-Kirchhoff model. A multidimensional exponential model incorporating concentration, viscosity, and density accurately represented the nonlinear attenuation profile. Results highlight the sensitivity of low-frequency ultrasound to viscoelastic properties and support its applicability to compact, robust, and calibration-ready sensing in industrial biorefinery environments.