Measured water flow that exceeds the test limit during Water Intrusion testing can be caused by a range of root causes, some of which are listed below. It can be seen that there are several factors which can lead to false test failures.

Possible root causes for false failures are:

• System leaks (filter housing, fittings, tubes, etc.)
• Insufficient test time
• Insufficient stabilization time
• Temperature influence
• Reversible partial wetting of the filter membrane (due to condensation of moisture within the pores or excessive pressure events)

More unlikely for unused filters (pre use test):

• Foreign substances / contaminations deposited on the filter

Root causes for true failures are:

• Filter defects
  
• Compromised O-ring seal with the housing

The WIT is performed on a non-wetted (“dry”) hydrophobic filter. The upstream side of the filter assembly is completely filled with water, covering the entire filter.  An air test pressure lower than the actual water intrusion (or water breakthrough) pressure of the largest membranes pores is applied to the system. The membrane pores remain “dry” during the test. The test pressure drives a transport of water vapors from the water phase across the filter membrane following the pressure differential. Transport of liquid water through all wetted pores and wetted flow pathways or defects will also occur. The WIT quantitatively measures the sum of water vapor (evaporation) and liquid water flow through the hydrophobic filter.

For a sterilizing grade filter, the maximum allowed flow (integrity test limit) is derived from the generic filter validation and correlated with the bacteria retention capability of the filter.

For more information, please see Pall Publication, USD 3033 Application Note: Best Practices for Successful Filter Integrity Testing Using the Water Intrusion Test (WIT) Method