Filter Leakage Test


This Filter Leakage Test is performed to confirm that the filter system is properly installed and that leaks have not developed during use.


The tests are performed by introducing an aerosol challenge upstream of the filters and scanning downstream of the filters and support frame or sampling in a downstream duct.


  • Aerosol Photometer


  1. Ensure that the filter system under test has unobstructed access for observation and testing.
  2. Check that the filter has no visible damage.
  3. Check that the filter is provided with a sampling point and the accessibility to the upstream of the filter is available for testing.
  4. Switch ON the AHU at least 30 minutes prior to testing.
  5. An artificially generated polydisperse or atmospheric aerosol should be introduced into the upstream airflow to achieve the natural upstream aerosol to reach the required homogeneous challenge concentration.
  6. Connect a tube from upstream port, of the filter under test, to the “upstream sample port” of the photometer in order to measure the upstream aerosol concentration. 
  7. Heat the DOP / PAO in the aerosol generator upto 410 – 420 0C till the aerosol starts generating. It will start getting sucked into the duct and form an aerosol/air mixture upstream of filter. Continue generation of aerosol till the upstream concentration reaches desired level.   
  8. The concentration of the aerosol challenge upstream of the filter should be between 10 mg/m3 and 100 mg/m3.
  9. Set the photometer to read 100% concentration of challenge aerosol at the upstream sampling point.


Iso-kinetic Sampling:

Iso-kinetic Sampling:
  1. Isokinetic Sampling is the sampling condition in which the mean velocity of the air entering the sample probe inlet is the same as the mean velocity of the unidirectional airflow at that location.
  2. The sample probe inlet size should be calculated from consideration of the sample flow rate of the measuring instrument and the filter exit airflow velocity so that the probe inlet air velocity approximates to the filter exit airflow velocity (i.e. Iso-kinetic Sampling)
  1. The sampling probe should be of square or rectangular configuration.
  2. Dp as shown in above figure is calculated as below

Dp = qva  / (U x Wp)

Dp is the probe dimension parallel to the scan direction (cm)

qva is the actual sample flow rate of the measuring instrument  (photometer) in cm3/s

U is the average filter exit airflow velocity (cm/s)

Wp is the probe dimension perpendicular to the scan direction (cm)

  1. Prior to the generation of the aerosol, place the photometer probe downstream of the HEPA filter until the instrument gets stabilized. The photometer has a sample flow rate of 1 cfm. The scanning of the downstream side will be read as a percentage penetration.
  2. Hold the probe at a distance of 3 to 5 cm from the filter face.
  3. Scan the filter and the filter perimeter by passing the probe in slightly overlapping strokes over the filter so that the entire area of the filter and sealing gasket is sampled.
  4. Scan Rate: The probe traverse scan rate should be approximately15/Wp cm/s.
  5. Scanning should be performed over the entire downstream face of each filter, the perimeter of each filter, the seal between the filter frame and the grid structure, including its joints.
  6. Verify the upstream concentration of challenge aerosol during the test and ensure that the current concentration is comparable to that of starting level. 


Designated leaks are deemed to have occurred where a reading greater than 10-4 (0.01 %) of the upstream challenge aerosol concentration. Alternative acceptance criteria may be agreed upon between the customer and the supplier.


  1. Factory repair should not block or restrict more than 1 % of the filter face area and no single repair should exceed 13 cm2 (2 in2)
  2. Field repair should not block or restrict more than an additional 3 % of the filter face area and no single repair should have a lesser dimension exceeding 3.8 cm (1.5 in.)


By agreement between the customer and supplier, the following information and data should be recorded:

a) Test method

b) Type designations of each measurement instrument and apparatus used and its calibration status;

c) Any special condition or departures or both from this test method and any special procedures agreed on between the customer and the supplier;

d) Measured upstream aerosol concentrations with their sample point locations and the corresponding time of measurement;

e) Sample flow rate

f) Calculated average upstream aerosol concentration and its distribution;

g) Calculated acceptance criteria applied for the downstream measurements;

h) Result of the downstream measurement for each clearly identified filter, area section, or measuring location;

i) Final result of the test for each defined location;

j) If there is no leakage then the test passed, otherwise if there is a leakage then report leak location, repair action, and the result of re-testing the location.

For More Article on SOP

  1. SOP For Airflow Velocity and Uniformity Test
  2. SOP For Temperature and Humidity Test
  3. SOP For Room Pressure Difference Test