How to plan differential pressure monitoring in a cleanroom

Why does a cleanroom need differential pressure?

Differential pressure is the controlled pressure difference between two adjacent zones. When the cleaner room sits at overpressure, air always flows from clean to less clean and carries particles out of the critical zone. Without this staggering, contamination could enter every time a door opens.

GMP Annex 1 targets a pressure difference of 10 to 15 Pa between consecutive grades, with roughly 5 Pa as a minimum. ISO 14644‑4 gives similar figures for adjacent zones and requires a documented design of the airflow direction.

How do you build a pressure cascade?

A pressure cascade stages the rooms from the highest to the lowest cleanliness grade. Every airlock and transition gets a defined pressure step, so the pressure decreases continuously from the production zone out to the surroundings.

• Fix a reference point: usually the unregulated corridor or ambient at 0 Pa.
• Step the pressure down consistently from inside out, without skipping a level.
• Use airlocks as buffers so opening a door does not reverse the airflow.
• Balance supply and exhaust per room so the setpoint stays stable.

Which sensors and measuring points are right?

Differential pressure transmitters connect two rooms via short tubes. Cleanrooms suit sensors with a range of about 0 to 60 Pa and an accuracy of around ±1 Pa, so small setpoints are resolved reliably.

• Place the measuring point in a calm spot, not next to a supply diffuser or door.
• Route the reference port to the neighbouring zone, keep tubes short and kink-free.
• Calibrate sensors regularly and log every check.
• Add a local display so staff can read the pressure at any time.

How do you set limits and alarms?

GMP Annex 1 requires continuous monitoring of differential pressure with recording and alarming. A two-stage concept works well: a warning threshold before the tolerance band is left, and an alarm when the critical minimum is undershot.

• Warning: pressure approaches the tolerance limit, e.g. below 8 Pa at a 12 Pa setpoint.
• Alarm: minimum pressure undershot or flow reversal, e.g. below 5 Pa.
• Add a few seconds of delay to avoid false alarms when doors open.
• Send alarms to the control system and staff, log acknowledgement and cause.

Frequently asked questions