Many processes don't care about the absolute flow of any one stream — they care that two streams stay in a fixed proportion. Burner air-to-fuel, chlorine-to-water in disinfection, blending two reactants, diluting a concentrate. If the ratio drifts, you get incomplete combustion, undertreated water, off-spec product, or an explosion. Ratio control is the standard scheme that solves this.

The setup has two streams: a wild flow (uncontrolled, set by upstream demand) and a controlled flow (the one with a valve you can move). A flow transmitter measures the wild stream. That measurement is multiplied by a ratio setpoint R, and the result becomes the setpoint for the controlled stream's PID loop:

• SP_controlled = R × Flow_wild

When the wild flow rises, the controlled setpoint rises with it. The PID loop chases that moving target and the ratio holds. The ratio block itself is just a multiplier — no integral action, no tuning. All the work is done by the underlying flow loop.

Real example — combustion air on a natural gas burner. Steam demand swings the fuel valve (fuel is wild, driven by a master pressure controller). You want roughly 10 standard cubic feet of air per cubic foot of methane for clean burn, plus 10% excess air for safety, so R ≈ 11. Fuel flow doubles? The air setpoint doubles, the damper opens, combustion stays clean. Without ratio control, the air loop would lag the fuel change — you'd get a brief fuel-rich pocket, soot, and possibly a flameout.

Cross-limiting is the safety twist on this scheme. On a fuel increase, you raise air first, then let fuel follow the actual measured air. On a fuel decrease, you cut fuel first, then let air follow. The controlled stream is always slaved to whichever side keeps the mix lean. Every industrial burner control system (BMS) does this — it's why startup sequences light air before fuel.

Rule of thumb: ratio control only works if the controlled stream's PID loop is faster than the wild stream's disturbances. If the wild flow can swing in 2 seconds but your control valve takes 10 seconds to stroke, the ratio will lag and you've got nothing. Pair fast-acting wild streams with fast valves (small, electric, or high-Cv pneumatic with a positioner), and add feedforward if the lag is unavoidable.

Watch for measurement units: if one flow is mass and the other is volumetric, your ratio constant has to include density. Compressible gases at varying pressure need temperature/pressure compensation or your "constant ratio" silently drifts with the weather.