AbstractThe imperfect pulse response technique, employing two measurement points, has been applied to the characterization of fluid mixing in systems where 0.5 mm glass beads are fluidized by liquids of viscosity (0.56 to 7.74 cp.) for bed porosities from 0.45 to 0.95. The frequency content of the input pulses was kept relatively high by experimenetal techniques. Frequency responses were determined from the measured residence time distributions. Axial dispersion coefficients for the axial dispersed plug flow (ADPF) model were calculated by moments and by frequency domain analysis, with considerable discrepancies at high degrees of mixing. Parameters for a time‐delay timedelay time‐constant (TDTC) model have also been estimated. A correlation for the significant time‐delay parameter has been developed for the range of viscosities and porosities. While both models can reasonably predict the frequency response, the TDTC model shows somewhat lower deviations and has the major advantage of predictable parameters through the correlation pres