A fluid classifier for separating particles from a lading for operation with a lading tank including a filter unit for mounting in the tank and having capillary passages for fluid flow from the tank externally of the filter unit to the interior of the filter unit for providing a pressure differential across the capillary passages resulting from a difference in fluid level in the tank and within the filter unit, a drive for rotating the filter unit in the tank, and a pump for pumping fluid from filter unit interior to the outlet. Ultrasonic transducers positioned along the filter unit for agitating the lading adjacent the capillary passages. A method of controlling fluid flow through the capillary passages.