Abstract: The invention relates to a portable on-line fluid contamination monitor. An inlet and an outlet are provided for connecting the monitor to a fluid control circuit and a measuring device is included for withdrawing fluid from the circuit. An optical sensor assembly is used to view fluid withdrawn from the circuit and to determine the level of contamination by observation of the particles therein, the optical sensor assembly viewing the fluid through a window by means of a light source disposed to project light through the window and the fluid. A light sensor disposed on the opposite side of the window from the light source is arranged to detect particles in the fluid passing across the window by sensing the degree of light obscuration caused by particles in the fluid.