AIRFLOW VELOCITY – UNIDIRECTIONAL:- The purpose of the filter face airflow velocity test is to document the average airflow velocity and uniformity of the filter face in unidirectional airflow.

 INSTRUMENTATION AND EQUIPMENT:- An airflow velocity instrument.


  • Measurement readings shall be taken at a distance from the face of the filter system.
  • The measurement area shall be a cross-sectional area of the filter face normal to the airflow. This area shall consist of the net effective filter media area, which is exclusive of the filter frame.
  • When using a single point measurement instrument, such as a hot wire anemometer, divide the net filter face into grids of equal area of not greater than 0.09 square meters (1.0 square foot).
  • When using a multi-point instrument, such as tube array, divide the net filter face into grids of equal area of not greater than 0.37 square meters (4.0 square feet). The grid area shall be as specified in the contract documents.
  • Measure and record the airflow velocity at each grid point 150 mm (6 in.) from the filter face. Special care is necessary to keep the sampled area unobstructed during the airflow measurement. The use of a support stand is recommended with single point type measuring instruments. The use of standoff legs of the appropriate length is recommended for a tube array.
  • Take the measurement for a minimum of 5 seconds or the instrument manufacturer’s minimum specified time, using the average during that period as the measurement.
  • Calculate the Relative Standard Deviation (RSD).



  • The average airflow velocity for the filter should be within ±10% of that specified in the documents.
  • The Relative Standard Deviation should not exceed 15% unless otherwise specified in the documents.


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