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Bubble Point Integrity Test

What It Is, Why It's Important, and How to Run One

There are three major tests used to determine the integrity of a membrane filter: the Bubble Point Test, the Forward Flow or Diffusion Test, and the Pressure Hold Test. All three tests are based on the flow of a gas through a liquid-wetted membrane under applied gas pressures. Where they differ is in which part of the flow/pressure spectrum they examine.

The Bubble Point and Pressure Hold test is used to determine the integrity of a single membrane. While we will be focusing on the Bubble Point Test in this article, please note that a Pressure Diffusion Test is recommended for accurate integrity validation of multiple-round cartridge setups. A Multiple round cartridge housing may hold anywhere from 2–48 membrane cartridges at a time.

What is a Bubble Point Test?

A bubble point test is designed to determine the pressure at which a continuous stream of bubbles is initially seen downstream of a wetted filter under gas pressure. To perform a Bubble Point Test, gas is applied to one side of a wetted filter, with the tubing downstream of the filter submerged in a bucket of water. The filter must be wetted uniformly such that water fills all the voids within the filter media. When gas pressure is applied to one side of the membrane, the test gas will dissolve into the water, to an extent determined by the solubility of the gas in water.

Downstream of the filter, the pressure is lower. Therefore, the gas in the water on the downstream side is driven out of the solution. As the applied upstream gas pressure is increased, the diffusive flow downstream increases proportionally. At some point, the pressure becomes great enough to expel the water from one or more passageways establishing a path for the bulk flow of air. As a result, a steady stream of bubbles should be seen exiting the submerged tubing. The pressure at which this steady stream is noticed is referred to as the bubble point.

Why is an Integrity Test Important?

Validation of integrity on a membrane cartridge is typically done before bottling (after heat sterilization), and again after bottling to confirm that the membrane remained integral during bottling. If you experience microbial hits in your packaged product, this is one of the checkpoints that can then be marked off the list of potential microbial sources.

Proper Procedure: Setting the Stage

A bubble point integrity test is a procedure which measures the pressure needed to be applied to the upstream side of a filter causing bulk or open pore flow through the largest pores of a wetted filter.

The Bubble point measurement is taken by reading the pressure on the gas canister or inlet pressure gauge (they should be the same) when a steady stream of bubbles is observed from the outlet tube into the bucket of water. If this pressure value is the same or above the bubble point value provided for the brand of cartridge being tested, then the cartridge is integral.

Required Materials to Perform a Bubble Point Test

  • Compressed, filtered air, nitrogen or argon
  • Pressure regulator
  • Filter and filter housing
  • Hose barbs
  • Beaker
  • Tubing
  • Filter adapters
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Test Method: A Step-By-Step Guide

  1. Record the filter part number(s), lot number, and product information. Also include physical observations.
  2. Wet the filter to be tested with water by running water through your cartridge in the cartridge housing. Properly vent the housing to make sure it is full of water.
  3. To ensure all pores are filled with water and the test doesn’t fail due to improper wetting, bring the back pressure up to 5psi by slowly closing the outlet valve until your outlet pressure gauge reads 3-5psi while your pump is running, and water is flowing. Now vent the top to release any trapped air bubbles. Close the vent, open the outlet valve all the way. Repeat 2-3 times.
  4. After wetting the filter, drain the water by opening the vents and valves. Then close your inlet valve.
  5. Connect the outlet fitting from the compressed air pressure regulator to the upstream side of the test filter. This can be done at the top of the housing or at the bottom inlet valve. Check that the gauge which is connected to the pressure regulator has subdivisions of at least 0.5 psi, and has the capacity to measure up to 60psi. A digital pressure gauge can also be used.
  6. Connect the outlet fitting from the compressed air pressure regulator to the upstream side of the test filter.
  7. Connect a piece of flexible tubing from the downstream port of the test filter into a beaker filled with water.
  8. Starting from zero pressure, gradually increase the pressure to the test filter using the pressure regulator to 5psi. A stream of bubbles will escape. This is not a failure. Wait for bubbles to dissipate and continue increasing the pressure incrementally in
  9. WARNING: Pressurized containers present a safety risk to operators. Ensure proper safety procedure and tools are used and in all cases regulator and approved vessels are utilized per local regulation and common safe practice.

Observe the submerged end of the tubing to produce bubbles as the upstream pressure is slowly increased in 1-5 psi increments. Note the rate that the bubbles appear at the end of the submerged tube.

Starting from zero pressure, gradually increase the pressure to the test filter using the pressure regulator to 5psi. A stream of bubbles will escape that were collected at the outlet side of the filter. This is not a failure of integrity. Wait for bubbles to dissipate after a few seconds and continue increasing the pressure incrementally.

PRO Tip #1: The inlet pressure can be measured on the bottom inlet side or top of the housing. If you want to find out whether your gauges are working properly, these two gauges should have the same pressure during a filtration.

PRO Tip #2: Bubble Point is usually much higher than the minimum Bubble Point value of each membrane cartridge. We recommend reaching Bubble Point each time you test the filter rather than stopping once you reach the minimum Bubble Point value. With each recording of the Bubble Point Value, you should see this value decrease over time, but never drop below the minimum Bubble Point value.

PRO Tip #3: Do not use CO2 for your test gas. It is a soluble gas that will give you inaccurate readings.

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Please refer to the technical data sheet of the membrane you are using to confirm the minimum bubble point value. Here are examples of some of our Membrane cartridge Bubble Point Values:

FILTER GRADE

WETTING FLUID

MINIMUM BUBBLE POINT

ScottCart Membrane PES 0.45 micron

Water

27.5 psi

ScottCart Flow Membrane 0.45 micron

Water

23 psi

ScottCart Flow Membrane 0.65 micron

Water

13 psi

ScottCart Water Membrane 0.2 micron

Water

46 psi

Things to keep in mind:

  1. Ensure that the filter is thoroughly and uniformly wet such that all the pores are filled with water. Failure to wet the filter may result in premature air flow resulting in false failure.
  2. Diffusive flow of air through the filter will occur at pressures lower than the bubble point. Ensure that the pressure recorded is in fact the pressure at which a steady stream of bubbles is noted.
  3. If failure occurs, make sure that the filters were properly installed and that the O-rings are not cracked or out of place. Repeat the steps to re-wet the filter cartridge, replace the housing cover and retest.

And that’s it! You’re officially ready to start testing the Bubble Point value of your cartridge.

Need help choosing the right filtration and filter media products for your use? Check out our how-to guide on choosing the right filter media to start your initial filtration.