Ratio control is used to ensure that two or more flows are kept at the same ratio even if the flows are changing.

Applications of ratio control:

• Blending two or more flows to produce a mixture with specified composition.
• Blending two or more flows to produce a mixture with specified physical properties.
• Maintaining correct air and fuel mixture to combustion.

Examples of ratio control:

The controlled flow(FIC-102) is increased and decreased to keep it at the correct ratio with the wild flow.

The "wild flow" (FI-101) is the flow not controlled by this loop. It may be controlled by some other control loop.

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.

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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