What is a Forward Flow (FF) integrity test?

The Forward Flow Test is designed to test the integrity of sterilizing and virus grade hydrophilic and hydrophobic membrane filters.  This qualitative test is based on measuring the gas flow across a completely wetted membrane at a defined constant test pressure on the upstream side.  When the downstream side of the membrane is at atmospheric pressure a diffusion flow of gas is established due to the pressure differential.  The Forward Flow test is Pall’s recommended integrity test for capsule and cartridge filters.

Do I need a holder to use the Single-Use TFF (SUTFF) modules? Can it be used without a holder?

Yes, the SUTFF module requires a SUTFF module holder.  The SUTFF module must be placed in the SUTFF module holder and compressed to activate the seals.  The SUTFF module will leak if it is used without a module holder.  Refer to FAQ: What is the part number of the SUTFF module holder?

What are the possible root causes of a false failure of the Water Intrusion Test (WIT)?

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 

What is the best way to lubricate the O-rings of a filter before an integrity test is performed?

Pall recommends the use of water to lubricate the O-rings of a filter in order to ease installation into the housing.

Pall does not recommend the use of alcohol (or an alcohol/water mixture) as it could come into contact with the filter membrane, causing a hydrophilic spot that will allow water passage, resulting in a false failure test result, especially in a water intrusion test.

Additionally, if any alcohol gets trapped between the two O-rings grooves, there will be a localized area where the alcohol will expand when the filter is sterilized (autoclave or steam-in-place).  This can potentially damage the filter adapter, or impact the filter to housing seal.

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 

What should I know if I wish to perform an integrity test with water at high temperatures?

In general, Pall does not recommend performing an integrity test at temperatures above 50°C when using water as the wetting fluid.

Forward Flow measurements at temperatures above 50 °C are considerably higher than at ambient temperature, and are more difficult to keep constant to the ±1 °C test specification.  This may introduce inaccuracies in the measurement and in calculating appropriate test limits.  When integrity testing is performed at elevated temperatures, the use of jacketed housing or a heating jacket is recommended to keep the temperature stable during the test.

While Pall can provide calculated test limits for elevated temperatures when water is used as the wetting fluid, additional testing should be performed to confirm these limits.  In addition, monitoring actual test results by the end user can also show that these limits are appropriate.

Please contact a Pall representative if you would like more information on developing Forward Flow and Bubble Point limits for all wetting fluids at elevated temperatures.

What is a Bubble Point (BP) integrity test?

The Bubble Point Test is designed to detect the largest pores of hydrophilic and hydrophobic membrane filters.  The Bubble Point test is based on measuring the gas flow across a completely wetted membrane at increasing gas test pressure, until the point at which the wetting fluid is expelled from the pores, and bulk flow is measured. 

The Bubble Point test is considered a subjective test, and the results can vary depending on the algorithm of the test instrument.  The Bubble Point method is the preferred integrity test method for testing filter discs, as the Forward Flow is often too low to be accurately measured. 

Why does my filter need to be dry before a Water Intrusion Test (WIT)?

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

When should the Water Intrusion Test be selected for integrity testing hydrophobic filters?

The Water Intrusion Test is a practical and validated test which can be used for in-situ integrity testing of hydrophobic gas filters. This test is conducted with deionized or higher quality water, without the need for low-surface-tension flammable solvents (such as Isopropyl Alcohol or Ethanol). Due to occupational risks, environmental regulations, safety guidelines and cost associated with handling these low surface tension solvents, water intrusion is becoming the method of choice for integrity testing hydrophobic microbial rated filters for air or gas applications.

Water Intrusion testing is the preferred test where the hydrophobic microbial rated gas filter:

  • Is integrity tested in-situ
  • A pre-use test is performed (especially after sterilization)
  • Alcohol use is restricted or not allowed in the production area

The Forward Flow or Bubble Point test, is the preferred method to integrity test filters in applications where:

  • A small area filter is used
  • An off-line filter integrity test is performed
  • To confirm filter integrity following a water intrusion test failure evaluation

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


How does the water level in the filter housing assembly affect the Water Intrusion Test (WIT)?

To successfully perform a Water Intrusion Test (WIT), the entire length of the filter has to be covered by (submerged in) water for the duration of the test.

During pressurization of the filter housing assembly to the water intrusion test pressure, the water level drops upstream of the filter. The reason is due to compression of the membrane pleats and elimination of gas bubbles during initial pressurization.

This can leave a portion of the filter exposed to pressurized air, which will freely flow through the exposed area and result in a false test failure.

To troubleshoot the cause of a test failure, Pall recommends:

·       To refill the filter housing with water and repeat the test, or

·       Before the test, increase the upstream volume, to have more water above the filter, to compensate for the reduced water levels resulting from the compression.

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