Bubble Point Integrity Testing
Last Updated: 9/2021
Applies to: Beverage producers using single round membrane filters (e.g. cartridge or crossflow). This article explains integrity testing and offers a bubble point testing protocol.
WHAT IS INTEGRITY TESTING?
Integrity testing ensures the accuracy of absolute filtration media (ensures it is operating at its rated porosity/titer reduction rating). In other words, integrity testing validates that filtration media was not compromised before or during filtration. This is especially important if a filter is being used as a final sterile filtration step prior to bottling. Integrity testing is typically done immediately before bottling, and again after bottling to confirm that the membrane remained integral during the process.
There are three common manual tests used to assess the integrity of a membrane (cartridge) filter:
- Pressure Hold Test (best for single round membrane filters)
- Bubble Point Test (only for single round membrane filters)
- Pressure Diffusion Test (best and most accurate for both single and multiple round membrane filters)
WHAT IS A BUBBLE POINT TEST?
Bubble point tests may be used on single round cartridge or crossflow filters. A bubble point test is a non-destructive method of integrity testing that allows the user to correlate their results with manufacturer-determined values that indicate proper function. Bubble point testing is not a direct measurement of pore size.
A bubble point test returns the pressure at which the surface tension of water inside filter media is broken. When it is broken below the manufacturer-specified minimum value, this indicates something is awry. Each membrane filter has a minimum bubble point value included with it's technical documentation - it is important to know this is set by the manufacturer and will vary by filter and brand.
During a bubble point test:
- Filter media is wetted and water is held by capillary action within the pores of the filter media.
- Gas is applied to the upstream side of the filter while tubing downstream is submerged in a bucket of water.
- The bubble point is the pressure at which bubbles can be seen exiting the submerged tubing. This occurs when the pressure is high enough to overcome the capillary action of the largest pore(s) in the filter, creating a path for bulk flow of gas.
Bubble point testing was not applicable to the filter setup
Bubble point testing can be performed on any single round membrane filter setup. Bubble point tests cannot be performed on multiple round setups. Multiple round setups should only be integrity tested with a pressure diffusion test.
Readings were artificially low due to:
Adjuvants: As bubble point is a measure of surface tension, anything that affects the capillary action of water can produce inaccurate readings. This includes temperature, hard water (containing minerals), and adjuvants like cleaners or sanitizers.
Improper Filter Wetting: If the filter media was not properly wetted prior to the test, ensuring all gas is expelled from filter pores, the test will return artificially low values.
Bubble point test was conducted at the wrong time
Bubble point testing should be conducted only after sterilization, especially with steam. Improper cooling of cartridges after steam sterilization can cause severe warping that affects the integrity of the membrane. To avoid this, always conduct a bubble point test after steam sterilization and conduct a chemical sanitation after opening the housing.
BUBBLE POINT TESTING PROTOCOL
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.
- Filter and filter housing
- Hose barbs
- Compressed, filtered air, nitrogen or argon
- Pressure regulator
- The gauge connected to the pressure regulator should have subdivisions of at least 0.5 psi and the capacity to measure up to 60psi. A digital pressure gauge can also be used.
- Record the filter part number(s), lot number, and product information. Also include physical observations.
- Properly wet the filter to be tested expelling all air from the filter membrane. This is simple but critical:
- Run water through the cartridge housing.
- 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.
- Finally, stop the pump or switch off water source.
- Drain the water by opening the vents and valves. Close the inlet valve.
- Connect the gas source to the inlet port of the filter with a hose tail adapter.
- Connect a piece of flexible tubing to the outlet port of the filter with a hose tail adapter and submerge in a bucket of water
- Starting from zero pressure and 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 1-5 psi increments. Do not stop once the minimum bubble point pressure is reached.
- Observe the submerged end of the tubing to produce bubbles as the upstream pressure is slowly increased. Note the pressure at which bubbles appear at the end of the submerged tube as the bubble point.
- You will see sporadic bubbles at a pressure lower than the minimum bubble point value. Ensure that the bubble point pressure is noted only when a steady stream of bubbles is observed.
- Do not stop the test once the minimum bubble point pressure is reached. The bubble point is often much higher than the minimum value. Over time the bubble point pressure should decrease but never drop below the minimum value.
- Do not use CO2 for your test gas. It is a soluble gas that will give you inaccurate readings.
For a video explanation and demonstration of bubble point testing, please watch 2:10-4:33 in the video below:
How do you know when it’s time to bubble test your crossflow? As a general rule of thumb, a bubble test should be completed if/when: you experience failed filterability, NTU’s close to 1, the product is not clear, or the bottling line cartridges are plugging prematurely. Consult the manual of your specific crossflow for instructions on how to do this.