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Microporous Pleated Filter Integrity Testing: A Definitive Guide to Validation Methods

Views: 251 Author: LOONG FILTRATION Publish Time: 2025-11-19 Origin: Site

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WHY FILTER INTEGRITY IS NON-NEGOTIABLE ?

In highly regulated industries such as pharmaceuticals, biotechnology, and food & beverage, ensuring product safety and quality hinges on effective filtration. Microporous pleated filter cartridges are the last line of defense against microbial and particulate contamination.

To ensure these filters perform as specified, mandatory validation through Filter Integrity Testing is required. This post provides a detailed breakdown of the four primary integrity testing methods, their principles, and step-by-step procedures, helping professionals maintain regulatory compliance and operational excellence.

I. Understanding Filter Integrity: Selection Criteria

Filter integrity tests measure physical parameters (like air flow or pressure resistance) that correlate with the filter's microbiological retention capability (the bacterial challenge level). The choice of test method depends entirely on the filter's material property:

Filter Type	Primary Wetting Liquid	Recommended Test Methods
Hydrophilic (Aqueous Solutions)	Pure Water	Bubble Point, Diffusion Flow, Pressure Decay
Hydrophobic (Air, Gas, Solvents)	95% Ethanol or 60% Isopropanol	Bubble Point, Water Intrusion

II. The Four Standard Integrity Test Methods

Integrity Testing-LOONG FILTER

Bubble Point Test (BPT)

The Bubble Point Test is the oldest and most direct method. It measures the pressure required to force the wetting liquid out of the largest pores in the filter medium.

Principle: Gas pressure is slowly increased until the gas overcomes the surface tension of the liquid held within the largest pore, leading to a visible stream of bubbles. This pressure is the Bubble Point.

Step-by-Step Procedure (Referencing Figure 1):

Wetting: Fully wet the filter cartridge sample with the specified wetting liquid and drain excess fluid.
Setup: Connect the wetted filter into the test housing and ensure all connections are tightly sealed.
Pressurization: Slowly increase the gas pressure via the throttling valve (1).
Observation: Closely watch the gas outlet tube (5). The moment a sudden and sustained stream of large bubbles appears, stop increasing the pressure.
Reading: The reading on the pressure sensor/gauge (2) at this moment is the definitive Bubble Point Pressure.

Diffusion Flow Test (DFT)

The Diffusion Flow Test is highly sensitive and commonly used for quality control, as it measures bulk gas flow through all wetted pores, not just the largest one.

Principle: Gas is introduced at a constant pressure (below the Bubble Point). The gas diffuses through the liquid trapped in the pores. The volume of diffused gas is measured over time.

Step-by-Step Procedure:

Wetting & Setup: Fully wet the filter and connect the test apparatus.
Pressure Setpoint: Slowly raise the pressure to the Diffusion Flow Test Pressure (recommended 80% to 90% of the Bubble Point). Maintain stability.
Gas Collection: Invert a water-filled measuring cylinder (6) over the gas hose (5) to collect the diffused gas.
Measurement: After the filter begins to bubble uniformly, record the volume of gas collected per unit time. This volume is the Diffusion Flow value.

Pressure Decay Test (PDT)

The Pressure Decay Test is a faster, simpler version of the Diffusion Test, relying on pressure loss instead of flow measurement. It is often performed by automated integrity testers.

Principle: The system is pressurized to a constant volume. Any decrease in system pressure over a fixed period is due to gas diffusion through the wetted filter. Excessive pressure drop indicates a flaw (leak).

Step-by-Step Procedure:

Wetting & Setup: Fully wet the filter and connect the test apparatus.
Pressure Setpoint: Slowly raise the pressure to the Pressure Decay Test Pressure (recommended 80% to 90% of the Bubble Point).
Start Timing: Record the initial pressure reading. Close the pressure regulating valve and start the timer (recommended test time: 10 minutes).
Measurement: After the elapsed time, record the final pressure reading.
Result: The difference between the initial and final pressures is the Pressure Decay value.

Water Intrusion Test (WIT) for Hydrophobic Filters

Integrity Testing-LOONG FILTER

The Water Intrusion Test is the standard method for hydrophobic (water-repellent) filters, typically used for sterile gas or vent filtration.

Principle: Applying a low pressure forces water to enter the dry filter medium. The volume of water that intrudes into the pores over time is measured. A lower intrusion rate indicates higher integrity.

Step-by-Step Procedure (Referencing Figure 2):

Setup (Dry Filter): Install the dry filter sample (13) into the housing (12) and close all valves.
Water Fill: Open the diaphragm valve (3, 4, or 11) and fill the housing and measuring tube (10) with purified water until water overflows (1). Close the valve.
Pressurization: Open the throttling valve (7) and slowly raise the pressure (6) to the Water Intrusion Test Pressure (recommended 80% of the water breakthrough pressure). Maintain stability.
Timing & Measurement: Once the liquid level in the measuring tube stabilizes and begins a steady descent, record the initial liquid level and start the timer.
Result: After the detection time ΔV (recommended 6 minutes), record the final liquid level.

Water Intrusion (WI) Calculation:

The Water Intrusion value (WI) is calculated using the formula:

Summary Table: A Quick Reference

Method	Filter Type	Measured Parameter	Integrity Indication
Bubble Point	Hydrophilic/Hydrophobic	Pressure at First Bulk Flow	Correlates with Largest Pore Size
Diffusion Flow	Hydrophilic	Gas Flow Rate at Set Pressure	Total Filter Area & Pore Integrity
Pressure Decay	Hydrophilic	Pressure Drop Over Time	System Leak Rate & Integrity
Water Intrusion	Hydrophobic	Volume of Water Ingress Over Time	Resistance to Water Penetration

Conclusion and Further Reading

Filter integrity testing is a critical validation step for any controlled process. By correctly applying the Bubble Point, Diffusion Flow, Pressure Decay, or Water Intrusion methods, you ensure that your filtration systems consistently meet the required microbial retention standards.

Do you need to develop Standard Operating Procedures (SOPs) for these tests, or calculate the pass/fail limits for a specific filter grade? contact sales@loongfiltration.com