Abstract: A testing device is described for use in determining optimum operating conditions for a full size industrial continuous gravity settling unit and for the design of such settling unit. The testing device comprises an elongated, cylindrical settling column having concentric, inner and outer transparent cylindrical walls forming an annular space therebetween filled with transparent heat exchange liquid, with the inner wall defining a cylindrical settling cell. A cylindrical feedwell extends downwardly into the top end of the settling cell, an overflow outlet opening is provided in the inner wall at a location above the bottom end of said feedwell, a solids discharge opening is provided at the bottom of the settling cell and a rotating rake is provided at the bottom of the settling cell for compacting collected solids.