This is a negative pressure proportioner, usually using a venturi tube. When water flow through the tube will generate negative pressure, and then the foam concentrate is sucked into the proportioner. Due to the ratio of the flow area, so as to achieve accurate proportioning rate.

This kind of proportioner is sometimes affected by back pressure, and the flow range is relatively narrow.

The proportioning rate of the proportioner usually changes with the brands and variety of the foam agents, because the viscosity of the foam will affect the flow through the orifice plate. Strictly, the certification of the proportioner is only for a certain type of foam agents used for testing.

This kind of proportioner is usually used in semi-fixed system, and is commonly used in offshore platform foam extinguishing system as well.