The purpose of the air change rate test is to determine the number of times the air is being exchanged within the cleanroom on an hourly basis. The Air Change Rate per Hour (ACH) is determined by measuring the total amount of airflow being supplied or returned from the cleanroom and by calculating the overall volume of the cleanroom.


  • A direct reading flowhood.


  • A manometer and a Pitot tube.


  • Determine if the cleanroom is under a positive or negative pressure.
  • If the pressure is positive, measure and record the supply airflow volume delivered through the filter using a flow hood. Use appropriate size capture enclosure for each filter application when using an airflow volume direct reading flowhood.
  • If the pressure is negative, measure and record the return / exhaust airflow volumes being captured by all return openings and process exhaust devices using the appropriate instrumentation for the application.
  • Measure the volume of the cleanroom space.
  • Calculate the Air Change Rate per Hour as per equation given below

Air Change Rate per Hour (ACH) 

ACH = Q / V

 Where: Q = Airflow m³ /hr

V = Volume m³

Air Change Rate per Hour (ACH)

 ACH = (Q • 60) / V

Where: Q = Airflow ft³ /min

V = Volume ft³

60 = minutes/hour


Reference links