What is Diffusion Flow Testing?

In the realm of filtration systems, the efficiency of membrane filter cartridges is paramount for ensuring the purity of liquids and gases across diverse industries. To validate and optimize their performance, a crucial method comes into play–Diffusion Flow Testing.

But what exactly is diffusion flow testing, and why is it important? In this article, we will explore the intricacies of diffusion flow testing and its significance in maintaining the quality of membrane filter cartridges.

Hydrophilic VS Hydrophobic Filters h2

Distinguishing between hydrophilic and hydrophobic filters is crucial for comprehending diffusion tests. Hydrophilic filters possess membranes that attract water, whereas hydrophobic filters have membranes that repel water. Knowing the type of filter in use guides the testing approach.

While diffusion tests are commonly conducted on hydrophilic filters, there are instances where they are performed on hydrophobic filters. The procedures vary for each type.

Wetting fluid

While water is the predominant wetting fluid for conducting an integrity test, incorporating alcohol such as 70/30 v/v isopropanol (IPA)/water or 80/20 ethanol (EtOH)/water can offer advantages. This includes minimizing the need for additional tests caused by inadequate wetting, decreasing flushing time, and enhancing sensitivity.

The wetting fluid is the liquid used to thoroughly wet the filter before testing. For hydrophilic filters, which attract water, the wetting fluid is typically water. In contrast, for hydrophobic filters, which repel water, the wetting fluid is often a mixture of alcohol and water. The choice of wetting fluid is essential to ensure proper wetting of the filter membrane and accurate testing results.

Diffusion Test

The diffusion testing process involves measuring the flow of gas through the filter at differential pressures below the bubble point, where gas molecules move through water-filled pores in a wetted membrane following Fick’s Law of Diffusion. The diffusional flow rate is proportional to the differential pressure and the total surface area of the filter.

To assess the integrity of a filter, the gas that passes through the membrane is evaluated at a pressure of around 80% of the minimum bubble point. Gas flow is minimal in small area filters, while it becomes more significant in larger area filters. Maximum specifications for diffusional flow have been established for particular membranes and devices, serving as a basis for predicting results in bacterial retention tests.

Diffusion Test Procedure

The Diffusive Flow Integrity Test, conducted near the minimum retentive bubble point pressure, quantitatively measures diffused air volume through the membrane filter, offering correlation with bacteria retention.

1. Ensure the filter is completely saturated with the appropriate test fluid, such as water for hydrophilic membranes or an alcohol/water mixture for hydrophobic membranes.

2. Gradually increase the pressure on the upstream side of the filter to the recommended test pressure given by the manufacturer, generally at least 80% of the minimum bubble point specification.

3. Allow sufficient time for the system to reach equilibrium.

4. Measure the gas flow at the outlet using either an inverted graduated cylinder or a flow meter for a duration of one minute.

5. If the diffusional flow reading exceeds the specified limit, it indicates one of the following conditions:

• Incorrect pore size
• Non-ambient temperature
• Inadequate wetting of the membrane
• Defective membrane or seal
• Use of fluids that do not match the recommended specifications
• Insufficient time allowed for stabilization

Diffusion Test VS Bubble Point Test

The Diffusion Test and the Bubble Point Test are two distinct methods employed in assessing the integrity and efficiency of filtration systems, particularly membrane filters.

Diffusion Test

Diffusion Test, also called the Forward Flow test assesses diffusive gas flow across a wetted filter membrane under constant air or nitrogen pressure.

Principle: The Diffusion Test measures the flow rate of gas through the filter at differential pressures below the bubble point.

Process: Gas molecules migrate through the water-filled pores of a wetted membrane, following Fick’s Law of Diffusion.

Purpose: Evaluate the performance and integrity of the filter by assessing gas flow under specific conditions.

Application: Commonly used for hydrophilic filters, but occasionally performed on hydrophobic filters.

Bubble Point Test

The bubble point test is commonly employed for evaluating small filter assemblies. It involves gradually applying air or nitrogen pressure to the wetted membrane and measuring the gas flow across the membrane at each pressure stage.

Principle: The Bubble Point Test determines the pressure at which gas bubbles emerge from the wetted filter membrane.

Process: Gas pressure is gradually increased until bubbles appear, indicating the minimum pressure required for the filter to allow gas passage.

Purpose: Assess the maximum pore size and the structural integrity of the filter.

Application: Primarily used for hydrophobic filters but can be applied to hydrophilic filters.

When Does Filter Diffusion Flow Testing Become a Consideration?

Filter Diffusion Flow Testing is a sophisticated method employed to assess the efficiency and performance of filtration systems. Its application becomes particularly pertinent in several scenarios where precision and reliability are paramount. Here are key situations when Filter Diffusion Flow Testing becomes a crucial consideration:

Development and Quality Assurance

New Product Development: When designing and developing new filtration products or systems, incorporating Diffusion Flow Testing becomes an integral part of quality assurance. It ensures that the filtration system meets performance expectations and industry standards from the early stages of development.

Routine Maintenance and Monitoring

Regular Filtration System Maintenance: As part of routine maintenance procedures, especially in critical industries like pharmaceuticals and water treatment, Filter Diffusion Flow Testing is employed to monitor the ongoing performance of filtration systems. This proactive approach helps identify any potential issues before they escalate.

Compliance with Regulatory Standards

Pharmaceutical and Biotech Industries: In highly regulated industries such as pharmaceuticals and biotechnology, compliance with strict regulatory standards is non-negotiable. Diffusion Flow Testing is considered when ensuring that filtration processes adhere to these standards, providing the necessary data for compliance reporting.

Optimizing Filtration Efficiency

Water Treatment Facilities: Water treatment facilities utilize Diffusion Flow Testing to optimize the efficiency of filtration systems. Whether in municipal water treatment or industrial water processing, this testing method helps achieve the desired level of purity by fine-tuning filtration processes.

Conclusion

Filter Diffusion Flow Testing becomes a consideration whenever precision, reliability, compliance with standards, and the optimization of filtration systems are paramount. Whether during development, routine maintenance, compliance assessments, or troubleshooting, this testing method can ensure the effectiveness of filtration processes across various industries.

Brother Filtration has more than 10 years of experience as a filter manufacturer. We have experienced application and process support staff who will identify and assist with the correct integrity testing for your process. For more information on filter integrity testing, please feel free to contact us!