A proportioning system mixes a fixed percentage of foam concentrate with the fire water. The resulting foam solution mixture is expanded with air in the foam generator. Lower proportions produce wet, heavy and flowable foam, whilst rigid foam with good adhesive and insulating properties is achieved at higher proportions. In any case, the volume of foam generated is many times that of the volume of the foam solution.
Induction ratespecifies the percentage ratio of foam concentrate added to water. At a 3% induction rate, for example, 3 parts foam concentrate are added to 97 parts water. The foam concentrate is added by means of a proportioner or pump premixer, either at the water pump, in the hose line, in the foam generator or in fixed installations in the central foam station. The induction rate is set at the proportioner or the pump premixer and usually varies (according to the type of foam concentrate, type of fire, flammable material and the application device used) between 0.5% and 6%. When using synthetic foam concentrates as wetting agents, up to 1 % is added.