What is ratio control and how is it implemented?

Ratio control maintains a fixed ratio between two flow streams. Implementation options: ratio station multiplies wild flow measurement by ratio setpoint to generate flow setpoint for controlled stream (most common); or divide measured flows to calculate actual ratio, control this to ratio setpoint. Applications: fuel-to-air ratio in combustion, reactant stoichiometry, blending operations, and steam-to-feed ratios. The wild stream (usually larger or uncontrolled) varies; controlled stream follows. Ratio may be adjusted by higher-level controller based on quality feedback.