The Water Intrusion Test measures water flow through a submerged filter when pressure is applied to the upstream side of the filter housing. Because this test can only be performed on a hydrophobic filter, the WIT measurement for an integral filter is primarily evaporative flow of water through the pores of the membrane.

When a hydrophobic filter becomes partially wetted with a low surface tension liquid such as an alcohol water mix, or condensate (from autoclave or Steam-In-Place (SIP)), then the WIT may result in a false failure.  This is due to a water channel forming through the membrane in areas where it has become wet, resulting in the free flow of water.

If the filter has become partially wet, it must be restored to a fully dry state before a WIT can be performed successfully.  Flowing compressed air through the filter for several hours is often required.  Alternatively, oven drying can be performed. Please contact your local Pall representative for the appropriate filter drying conditions.

Pall recommends the following to prevent a hydrophobic filter from becoming wet:

  • Keep the filter away from potential sources of low surface tension liquids such as alcohol mixtures.
  • If the filter is autoclaved, use a slow exhaust cycle and a vacuum drying cycle.
  • If the filter is subjected to SIP, use a cooling gas such as air or nitrogen following SIP.

In certain applications, post-use testing using WIT is impractical due to product contamination on the filter, i.e. bioreactor exhaust filter.  In these cases, Pall recommends performing a post-use Forward Flow integrity test.

For more information, please see Pall Publications:

USD 3033 Application Note: Best Practices for Successful Filter Integrity Testing Using the Water Intrusion Test (WIT) Method

USTR2821 Application Note: Scientific and Technical Report – In Situ Drying of Pall Hydrophobic Air Filters Prior to Moist Heat Sterilization