Bubble Point Integrity Testing

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Before purchasing the right filter cartridge, we need to test the integrity of the filter cartridge, and bubble point testing is one of the common methods used.

You may have a lot of questions about bubble point testing, so Sanitek will answer them and let you know the definition of bubble point testing, the process, and what can go wrong.

Bubble Point Integrity Testing

What is Bubble Point?

Bubble point, as a concept, serves as an indirect gauge of the largest pore within a filter matrix. Specifically, it hinges on pore dimensions, the filter medium’s wettability, surface tension, and contact angle.

At its core, bubble point relies on the principle that liquid is retained within filter pores due to surface tension and capillary action. The minimal pressure requisite to expel liquid from these pores serves as an indicator of their diameter.

It denotes the pressure threshold at which air traverses a dampened membrane filter. This pressure level correlates with the membrane’s pore size, making it a valuable tool to affirm the pore size and structural integrity of a membrane or filtration apparatus.

What is Bubble Point

What is Integrity Testing?

Integrity testing ensures the accuracy of absolute filtration media, ensuring that it operates at its rated porosity or titre reduction rating. In essence, integrity testing verifies that the filter media remains uncompromised prior to and during the filtration proces.

Typically, integrity testing occurs immediately before bottling and is repeated post-bottling to confirm the membrane’s integrity throughout the process.

Three common manual tests are employed to evaluate membrane cartridge filters’ integrity:

  • Pressure retention test, most suitable for single-round membrane filters
  • Bubble Point Test, applicable only to single-round membrane filters
  • Pressure Diffusion Test, offering the best and most accurate assessment for both single and multi-round membrane filters

What is The Bubble Point Test?

In order to determine the bubble point of a filter, we must perform a bubble point test, which is a common procedure. The bubble point test technique involves testing the open pores of the filter before measuring the pressure required to pass through the largest pores on the upstream side of the filter.

To perform the test, we first wet the filter membrane with isopropyl alcohol. Then, we gradually increase the gas pressure on the filter membrane.

Eventually, a stable gas bubble will appear, also known as the first bubble. This bubble will emerge from the largest hole in the filter membrane. At this point, the measured gas pressure is the bubble point of the filter.

The key to a successful bubble point test is to wet the filter membrane. If the membrane is dry, air will pass through the membrane prematurely and interfere with bubble formation.

The diffused airflow through the filter is generated at a pressure below the bubble point and is measured as the first bubbles form. In this way, the bubble point is measured as accurately as possible.

Throughout the bubble point test:

  • The filter media undergoes wetting, with water being held within its pores due to capillary action.
  • Gas is introduced to the upstream side of the filter, while the downstream pipe remains submerged in a water-filled bucket.
  • The bubble point signifies the pressure level where bubbles become visible upon exiting the submerged pipe. This moment arises when the pressure surpasses the capillary action of the largest pores within the filter, enabling a significant gas flow pathway.
Throughout the bubble point test

What is The Bubble Point Pressure?

For thermodynamics, the bubble point can be known as the temperature at a given pressure that first bubbles when the vapour is made when heating a two or more-component liquid. For a singular component, the dew and bubble point are the same and named the boiling point.

It acts as a mirror, reflecting the size of the pores and overall integrity. When pressure is applied to the bubble point pressure, it overcomes capillary forces in the filter pores, which causes the wet liquid to drain and creates a large airflow.

By determining the exact bubble point pressure, manufacturers in industries where product purity is maintained and contamination is prevented, such as pharmaceutical and electronics manufacturing, can evaluate a filter’s ability to retain particles of different sizes and contaminants.

Uses of Bubble Point Testing

Bubble point testing is critical for the pharmaceutical industry to verify the integrity of sterilizing grade filters. It is a non-destructive test method that ensures that membrane filters maintain a specified pore size and effectively remove contaminants.

By performing this test, pharmaceutical manufacturers can verify the reliability and effectiveness of their filtration process, thereby ensuring product quality and safety.

In addition, the Bubble Point Test is applicable to all types of filters used in pharmaceutical manufacturing, such as disc filters or cartridge filters, providing a versatile tool for quality control.

Problems That Can Occur With Bubble Point Testing

Bubble point testing can be performed on any single-round membrane filter setup. Bubble point testing cannot be performed on multi-wheel installations. Integrity testing can only be performed on multi-wheeled units by pressure diffusion testing.

The reasons for artificially low readings are as follows

  • Adjuvant: Since the bubble point is a measurement of surface tension, any factor that affects the capillary action of the water will result in an inaccurate reading. This includes temperature, hard water (containing minerals), and adjuvants such as detergents or sanitizers.
  • Improper filter wetting: Test results can be artificially low if the filter media is not properly wetted prior to testing to ensure that all gases are expelled from the filter pores.
  • Bubble point testing performed at the wrong time: Bubble point testing should only be performed after sterilization, especially if steam sterilization is used. Improper cooling of the cartridge after steam sterilization can result in severe deformation of the cartridge and compromise the integrity of the filter membrane. To avoid this, always perform a bubble point test after steam sterilization and chemically sterilize the cartridge after opening the housing.


  • You will see sporadic bubbling at pressures below the minimum bubble point value. Be sure to note the bubble point pressure only when a steady stream of bubbles is observed.
  • Do not stop the test after the minimum bubble point pressure is reached. The bubble point is often much higher than the minimum value. Over time, the bubble point pressure will gradually decrease, but will never fall below the minimum.
  • Do not use carbon dioxide for the test gas. It is a soluble gas and will cause inaccurate readings.


Sanitek is your trusted filtration partner. We offer cartridge integrity testing equipment and other related services. Contact us today and we will provide you with an excellent filtration solution within 24 hours.

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