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If the physical characteristic of the mixed flow is measured, a PID controller can be used to manipulate the ratio value.

For example, a measurement of the density, gasoline octane rating, color, or other characteristic could be used to control that characteristic by manipulating the ratio.

The analyzer signal is conected to controller AIC-103. The output of that controller is the ratio. To manually adjust the ratio, the operator may place AIC-103 into manual mode and adjust the output.

The "controlled flow" is controlled by this loop with a setpoint equal to the measured wild flow multiplied by some value (FF-102).

The measured wild flow is multiplied by a value that may be fixed or may be adjustable by the operator. The result of the multiplication becomes the setpoint of the controlled flow controller.

This is a three level cascade. The controlled  flow, FIC-102, is the secondary. The ratio, FF-102, is the primary to FIC-102 but the secondary to AIC-103. AIC-103 is the top level primary controller.

The options, such as tracking, that apply to cascade control also apply to ratio control. The controlled flow controller is a "secondary loop" in a cascade pair with the wild flow measurement and ratio multiplication. (see cascade control information)

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