a self calibrating system for measuring concentration of a material carried in a fluid medium . the system has a flow cell means for measuring the transmissivity of a fluid medium passing therethrough , a first means for delivering a first fluid containing material carried in fluid medium , and a second means for delivering a clear dispersant fluid . the system also has a switching means for selecting among the delivering means , the switching means connecting one of the delivering means to the flow cell means , a first memory means for storing a table of c1 concentration and associated voltage values corresponding to the transmissivity measurement of the first fluid , a second memory means for storing a table of c2 concentration and associated voltage values corresponding to the transmissivity measurement of the clear dispersant , and calculating means for adding a c1 concentration value from the c1 table to a c2 concentration value from the c2 table thereby determining the concentration of the material in the fluid medium being measured by the flow cell . the system can have additional delivering means as is required for color printing . also provided is a method for measuring the concentration of toner particles carried in a fluid medium during a calibration period and during run - time use .