Abstract: An apparatus for automatic, quantitiative determination of an impurity contained in a product with an uneven upper surface travelling in bulk on a conveyor belt. The apparatus includes a leveling system including a half-cylinder formed of a plastic sheet, and having an arcuate surface downwardly directed toward the belt with the cylinder axis perpendicular to the direction of the belt and having a length which is less than the width of the belt. The half-cylinder is fixed to a frame attached to a lever arm which is in equilibrium about a horizontal shaft. A jack is provided for adjusting the lever arm and enabling the arcuate surface of the half-cylinder to be brought into contact with product travelling on the belt, the jack having a head connected to the lever arm so as to enable the lever arm to oscillate as the leveling device follows variations of the thickness of product travelling on the belt.