Abstract: An apparatus and method are described for determining precisly when the resin column in a deionized water generator needs to be regenerated. This is achieved by causing the untreated water to pass through a flowmeter and by measuring its electrical conductivity and temperature. After subtracting the value for pure water from the conductivity value and correcting for temperature, the two numbers are converted to an impurity concentration which is multiplied by the flow rate to give a value for weight of impurity per unit time. The latter value is integrated over time by accumulating sample values read at regular intervals, thereby generating a dosage value. This dosage may be displayed for viewing and/or compared with the known capacity of the system's resin column so that an alarm may be sounded when regeneration is needed.