The invention concerns a grader operating on the air separation principle and comprising: a housing (1); a rotor (2) mounted in the housing so as to rotate and comprising two rotor discs (5, 6) and blades (4) between the discs; and an outer channel (7) formed by the circumference of the rotor (2) and the housing (1) for feeding in solids and the fluid. In order to minimise turbulence detrimental to grading, it is proposed that the diameter of the rotor discs (5, 6) should be chosen to ensure that the outer edges of the rotor discs should protrude into the solids and fluid feed channel (7) and form the lateral boundaries thereof.