Abstract: The invention is a submersible laser scattering instrument that measures particle total volume, particle total area and Sauter mean diameter. It allows a single calibration for all particle sizes 1.2-250 &mgr;m. A beam of laser light is directed across a void where a sample of water containing particles is admitted. After passing through the water, the light which is forward scattered out of the direct beam falls on two detectors at the same time. The first detector has an active surface shape which is configured to produce an output signal proportional to total particle area at varying total particle areas. The second detector has an active surface shape which is configured to produce an output signal proportional total particle volume varying total particle volumes. Each detector falls within in a radius surrounding the unscattered beam of light.