A computerized chemical application system for a water treatment system. The water treatment system includes a main header conduit through which water flows. Connected in shunt with the main header is a chemical introduction header of smaller diameter and a lower flow rate than the main header. A computer control is provided to sense the conductivity of a reference water flowing through the chemical introduction header. Injection means are provided to introduce a water treatment liquid into the chemical introduction header and to mix it thoroughly with the water flowing therein. The conductivity of the mixture is sensed, and based upon the differential conductivity and values relating conductivity to concentration for the water treatment liquid (which values are stored within the computer control) the amount of the water treatment liquid introduced is precisely controlled.