This is a kind of balanced proportioner, that is, the inlet pressure of water and foam concentrate is the same, so that the pressure difference of water flow through the water orifice plate (or venturi tube) and the foam flow through the foam orifice plate is the same. The accurate proportioning rate can be achieved as long as the flow areas of the two parts are matched.

The foam concentrate is stored in a pressure vessel with a built-in bladder. There is a branch of water flow is led from the upstream of the proportioner into the interior between the bladder and the pressure vessel. The foam concentrate in the bladder is extruded under the pressure of water, and the extruded foam concentrate pressure is consistent with the upstream water pressure of the proportioner, so that the water pressure and the foam pressure are the same. The proportioning rate usually changes with the brands and variety of the foam concentrates, because the viscosity of the foam concentrate will affect the flow through the orifice plate. Strictly, the certification of the proportioner is only for a certain type of foam concentrate used for testing. The bladder pressure proportioning tank is available in standard and wide flow types on request. The wide flow type is usually used in foam-water sprinkler systems.