This kind of proportioner is similar to thein-line eductor, which is installed in the bypass between the outlet of thepump and the suction inlet. Part of the water at the outlet of the pump flows intothe proportioner, and then the venturi tube of the proportioner generatesnegative pressure. It can inhale the foam concentrate and infuse it into the suction inlet of the pump, and mix foam concentrate withwater.

This kind of proportioner is usually usedin foam fire engines, and proportioning rate is adjustable. However, with thedevelopment of remote water supply systems, this kind of negative pressuresuction proportioner has been unable to meet therequirements. Because it's economical and practical, a lot of foam fire enginesstill use it.