This kind of proportioner is similar to the in-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 into the proportioner, and then the venturi tube of the proportioner generates negative 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 used in foam fire engines, and proportioning rate is adjustable. However, with the development of remote water supply systems, this kind of negative pressure suction proportioner has been unable to meet the requirements. Because it's economical and practical, a lot of foam fire engines still use it.