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Diffusion Test Processing Steps

Gas molecules migrate through the water-filled pores of a wetted membrane following Fick's Law of Diffusion at differential gas pressures below the bubble point. The gas diffusion flow rate for a filter is proportional to the differential pressure and the total surface area of the filter.

To determine the integrity of the filter, gas pressure is measured when it reaches approximately 80% of the minimum bubble point.

How to Define the Diffusion Criteria?

Diffusion specifications are provided by the manufacturer in the Certificate of Quality delivered with the filters.


DT is the total diffusion of the housing
DSis the diffusion specification of one filter cartridge (listed in the Certificate)
NCis the number of filter cartridges contained in the housing

Your filters and housing are integral if diffusion measured is lower than or equal to DT.

The Process

  1. For hydrophilic membranes, wet the filter with water. For hydrophobic membranes, wet the filter with an alcohol water mixture.
  2. Slowly increase pressure on the upstream side of the filter to the recommended test pressure provided by the manufacturer.
  3. Allow the system to equilibrate.
  4. Measure the gas flow at the outlet for one minute with an inverted graduated cylinder or a flow meter.
  5. If you get a diffusion flow reading higher than the specification, this indicates one of the following:
    • Wrong pore size
    • Temperature other than ambient
    • Non-integral membrane or seal
    • Inadequate stabilization time
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