An apparatus for controlling the metering of additives which are to be blended with a primary substance includes an element which determines how much additive is being discharged from an additive storage container. This element includes a weight detecting device which detects a first portion of the additive contained in a metering container and includes another weight detecting device for detecting the weight of a second portion of the additive contained in an inventory container which feeds the additive into the metering container. This element also includes a volumetric flow rate monitoring device for monitoring the flow rate of a liquid additive. Each of these detecting and monitoring devices generates respective electrical signals proportional to the respective monitored conditions. The apparatus also includes an element for sensing how much of the primary substance is being discharged from a primary substance container. This element includes both a main flowmeter which detects the actual rate of flow and a device for simulating a flow rate. Proportional electrical signals are generated by the flow meter and the flow rate simulating device. The apparatus also includes a control circuit which uses the electrical signals to control the amount of additive which is discharged and added to the primary substance. This determination is made based on data which is entered into the apparatus by means of manually actuated switches and keys. Control signals are generated by a microprocessor and associated program.