Abstract: A flow-through system for determining the particle size distribution of iron oxide particulate matter in a reactor water sample. The system includes a magnetic filter having a chamber through which a stream of reactor water. The chamber is surrounded by a winding which is coupled to an electrical circuit for generating a varying magnetic field which interacts with any magnetic particulates entrained in the water sample flowing through chamber. When a strong magnetic field is established, magnetic particles are filtered out of the water stream flowing through the chamber by magnetic attraction. Therefore, only particles which are non-magnetic or slightly magnetic pass through the magnetic filter. The particles which pass through the magnetic filter then enter a conventional particle distribution analyzer which counts and sizes the particles in the water stream flowing therethrough.