Abstract: The present invention relates in general to a method and apparatus for blending several chemicals or fluids together. This invention also relates in general to a method and apparatus for safely calibrating a flow meter used for addition of liquid chemicals into other liquid chemicals in a closed system to minimize the environmental and occupational risks that may be associated with the added chemicals. In a preferred embodiment, a fluid injection system is provided with an in-line calibration device to permit calibration of the flow meter without introducing the fluid to the environment. In another embodiment, a preferred in-line calibration device is described that can be used to calibrate existing fluid additive injection systems. A method is disclosed for accurately and safely metering in additives to a desired location by employing a closed loop injection/calibration system.