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.

Abstract: Fire fighting apparatus for generating air compressed foam having both a water and a surfactant metering device for dispensing controlled and discrete quantities of both into a mixing conduit where they combine into a foam solution. The foam solution is combined with air prior to being injected within a compression chamber of an air compressor device. Foam is generated by compression of the air-foam solution and then is discharged through a discharge device. The air compression device is also controlled to dispense a discrete quantity of foam therefrom in correlation with the discrete quantities dispensed from the other two metering devices. The quantitative dispensing coordination of the air compression device with the two metering devices makes all three devices both relational and proportional in the cooperative generation of compressed air foam, and thus ensures prompt production of constant quality foam.