Bubble Point

Bubble Point is a practical, nondestructive test used for estimating the pore size of microporous filters and confirming the integrity of sterilizing membrane filters and filter systems. It is the most widely used non-destructive integrity test.

Bubble point is based on the fact that liquid is held in the pores of the filter by surface tension and capillary forces. The minimum pressure required to force liquid out of the pores is a measure of the pore diameter. The pressure required to force liquid out of a liquid-filled capillary must be sufficient to overcome surface tension and is a direct measure of effective tube diameter.

Formula for Bubble Point Test

Where:
P = bubble point pressure
d = pore diameter
k = shape correction factor
cos θ = liquid-solid contact angle
ó = surface tension

The bubble point test is a sensitive visual technique and is performed routinely as part of the Millipore Quality Assurance Program. The bubble point test detects minor filter defects and out-of-size pores and correlates with the bacteria passage

Bubble Point Procedure for Filters

1. Wet the filter with the appropriate fluid, typically water for hydrophilic membranes or an alcohol for hydrophobic membranes.
2. Place the filter into the holder and cover the membrane withte wetting fluid.
3. Pressurize the system to about 80% of the expected bubble point pressure which is stated in the manufacturer’s literature.
4. Slowly increase the pressure until a single, continuous stream of bubbles comes through the membrane.
5. Read the bubble point pressure from the pressure gauge.

1. Wet the filter with the appropriate fluid, typically water for hydrophilic membranes or an alcohol/water mixture for hydrophobic membranes.
2. Pressurize the system to about 80% of the expected bubble point pressure which is stated in the manufacturer’s literature.
3. Slowly increase the pressure until rapid continuous bubbling is observed at the outlet.
4. A bubble point value lower than the specification is an indication of one of the following:
1. fluid with different surface tension than the recommended test fluid
2. integral filter, but wrong pore size
3. high temperature
4. incompletely wetted membrane
5. non-integral membrane or seal