Ratio control has many similarities to feed-forward control. Usually a “wild stream” flow rate is measured and another stream is manipulated to maintain a constant ratio to the wild stream. Selective and override strategies are often used to improve the safety of a process system. A prime example is a fired heater where it is important for combustion air to “lead” the fuel gas flow on increasing loads, but to “lag” fuel gas flow on decreasing loads. Split-range control allows the use of two or more valves as manipulated inputs, depending on the operating condition.