Abstract: A signal generator is electrically coupled with a first sensor for sensing the conductivity of water and electrically coupled with a second sensor for sensing the conductivity of a foam solution that included a concentration of surfactant. One or more switches repeatedly switch between the sensed conductivity of the water and the sensed conductivity of the foam solution. A conversion circuit converts the conductivity of the water into a first voltage and the conductivity of the foam solution into a second voltage, such that a single circuit path may be used to convert both conductivity values. A control circuit compares the difference of the first voltage and second voltage to determine a differential voltage that is utilized to control a surfactant control valve.