Abstract: A method is provided for controllably delivering an additive fluid into a host fluid in a target system at a desired volumetric flow rate. The method comprises: providing a binary supply injector connected in an additive fluid delivery system between a pressurized supply of the additive fluid and the target system. The binary supply injector is controllable between an ON state which permits flow of the additive fluid therethrough and an OFF state which prevents flow of the additive fluid therethrough. The method involves iteratively repeating: estimating a volumetric flow rate of the additive fluid between the pressurized supply and the target system; and if the estimated volumetric flow rate of the additive fluid is greater than the desired volumetric flow rate, controlling the supply injector to its OFF state; or if the estimated volumetric flow rate of the additive fluid is less than the desired volumetric flow rate, controlling the supply injector to its ON state.