Ratio control
Two flows held in a fixed proportion, where one flow's setpoint is a ratio times the other flow's measurement.
What it means
Ratio control keeps two flows in a fixed proportion. The wild flow is measured, and the ratioed flow's setpoint is that measurement times a ratio, so the ratioed loop tracks the wild flow automatically as the load changes.
It is the standard structure for blending, for air-to-fuel on a burner, and for reagent dosing — anywhere the relationship between two streams matters more than either absolute value. The ratio is the operator's handle: raise it and the proportion changes, whatever the total load is doing.
Ratio control often ends up as the slave of a cascade whose master is a third variable — a composition, a temperature. Then it stops being a fixed proportion and becomes the mechanism the outer loop works through, which is where it meets feedforward thinking.
Related
Where this comes up
Last reviewed 2026-09-18.