Mass flow control systems and methods for controlling the mass flow rate of a gas through a primary conduit are disclosed. One mass flow control system includes a primary conduit for directing a flow of a gas and an adjustment system configured to divert a portion of the gas from the primary conduit to a secondary conduit and provide an adjustment signal that changes when a composition of the gas changes. A mass flow controller is operatively coupled to the primary conduit to control a primary flow rate of the gas. The mass flow controller includes a valve to control the primary flow rate of the gas and a control loop configured to receive the adjustment signal and control the valve to provide the primary flow rate of the gas at a set point.