DP

Differential pressure between adjacent classified rooms establishes which way air moves, and therefore which way contamination cannot. A cleaner room held positive to a dirtier one leaks outward; the cascade across a suite is designed so that every leak path runs from higher cleanliness to lower. For products or operations that must be contained rather than protected, the cascade is deliberately inverted, and the two intentions must never be confused in the same suite without an airlock resolving them.

The measurement is a continuous condition, not a periodic check. Pressure is lost by an open door, a failing fan, a loaded filter or a change in the return path, and each of those happens during operation rather than during qualification. Monitoring with alarms, and a defined response when an alarm sounds, is what converts a design intention into a control — an unalarmed gauge records the failure for whoever reads it next.

Recovery matters as much as the steady state. The time a room takes to re-establish its cascade after a door closes is a property of the design that only in-operation testing reveals, and it is the interval during which the barrier does not exist.

• —Sets the direction of airflow and therefore the direction contamination can travel.
• —Positive cascade protects product; negative containment protects people and environment — never confused without an airlock.
• —Lost by open doors, fan failure, loaded filters and changed return paths, all in operation.
• —Alarms plus a defined response convert a design intention into a control.
• —Recovery time after a door closes is the window in which the barrier is absent.

EU GMP Annex 1 (2022) for pressure differentials between grades; EU GMP Part I Chapter 3; ISO 14644-4 for design.