How do you select between linear and equal percentage valve characteristics?

Characteristic selection depends on process and system characteristics. Equal percentage (most common): use when process gain decreases with flow (typical for pressure and level), when valve pressure drop is small portion of system, and for most liquid level and gas pressure applications. Linear: use when process gain is constant or increases with flow, when valve takes majority of system pressure drop, and for flow loops with minimal downstream pressure variation. The goal is consistent loop gain (product of process and valve gains) across operating range for stable control with consistent tuning.