A bed of particulate solids is fluidized in a test chamber to yield multiple test samples for subsequent evaluation of segregation effects. A controlled stream of gas enters the chamber in a series of flow rate cycles each progressively increasing to a maximum rate of gas flow and then decreasing, the maximum rate increasing for successive cycles. An indicating function is formed from measurements of corresponding rates of gas flow and pressure across the bed. Upon termination of the fluidization, multiple samples are sequentially extracted from a single space at the bottom of the test chamber.