A method for measuring particle size distribution in a fluid material, involving inserting a laser beam instrument directly in the fluid flow line, wherein the laser beam instrument focuses a laser beam on a window directly coupled with the fluid flow line, wherein the fluid flow line comprises a fluid having a plurality of particles of different sizes, measuring a diameter of at least one particle in the fluid flow line by reflectance of the at least one particle as the at least one particle passes through the focused laser beam, and determining a duration of reflection of the at least one particle, and obtaining a count of particles in each of a pre-set range group of particle sizes, wherein the count of particles is used to determine particle size distribution in the fluid flow line.