Abstract: A component is washed with a fluid to release particulate contaminant. The particulate-containing fluid is passed through a screen (22) having a known number of apertures of identical known size. The change in pressure caused by the accumulation of particles in the screen is measured and compared with the pressure when the screen is clean. From this, a control system (54) determines the number of particles in the fluid having a size greater than the aperture size. This is related to a unit volume by measuring the volume of fluid passing through the screen (22). If the screen blocks before all the fluid is passed through the screen (22), the fluid is pulsed in forward and reverse flow to re-distribute the particles on the screen. The fluid is then back washed from the screen to a second screen (28) having a known number of apertures of identical size. The number of particles per unit volume having a size greater than the size of the apertures of the second screen is then determined.