³⁵The ISA Handbook of Control Valves cites this equation as being valid for conditions where the valve’s downstream pressure (P₂) is equal to or greater than one-half the upstream pressure (P₁), with both pressures expressed in absolute units. In other words, P₂ ≥ 0.5P₁ or P₁ ≤ 2P₂. An upstream:downstream pressure ratio in excess of 2:1 usually means flow through a valve will become choked.