Abstract: An automatic liquid flow control device for use in the application of building insulation foam is disclosed comprising flow meters to measure the flow of resin and foaming agent through conduits to the application gun, a needle valve connected to the resin conduit and operated by a servo motor, a controller to control the servo motor and having a selector to allow selection of the desired ratio of resin to foaming agent, a comparator to compare the actual ratio of resin flow/foaming agent flow to the selected ratio with the controller automatically controlling the servo motor and needle valve to "slave" the resin flow to the foaming agent flow in order to adjust the resin flow/foaming agent flow to the selected ratio. The comparator also compares the actual flow through the resin and foaming agent conduits to predetermine flow values in order to determine abnormal operating conditions.