Ratio Control Loops

Ratio Control Loops

Based on hardware, ratio control is quite often confused with cascade control. The basic operation of ratio control, however, is quite different.

Ratio control is often associated with process operations in which two or more streams must be mixed together continuously to maintain a steady composition in the resulting mixture. A practical way to do this is to use a conventional flow controller on one stream and to control the other stream with a ratio controller that maintains flow in some preset ratio or fraction to the primary stream flow.

A preset ratio regulates the flow of the controlled variable. For example, if the ratio is 10 to 1, then for every gallon per minute of the uncontrolled variable that is flowing, ten gallons per minute of the controlled variable are allowed to flow.


Use the signal from the uncontrolled flow transmitter, or wild flow, as the ratio input of the ratio controller. Multiply the value by an adjustable factor or ratio setting to determine the set point of the flow controller. The process variable to the controller is the flow of the controlled stream. The output from the ratio controller adjusts the control valve.

You can use a ratio control loop with any combination of suitably related process variables, and the control action selected is normally proportional plus integral. The response to process upsets of a control loop with ratio control is the same as the response found in a single-feedback loop…