Bubble point pressure determines the point at which a gas bubble forms, and there is a test used with the bubble point theory for filter systems.
Bubble point pressure is defined as the pressure at which the first gas bubble appears while decreasing pressure on a fluid sample. Bubble point is often used to filter water through hydrophobic membranes. These are used for venting and filtration.
Bubble Point Test
The theory of bubble point pressure is often used in a test called the "bubble point test." According to the Scott Laboratories website, this 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." It is performed by first wetting the filter to be tested with the appropriate solvent (water for hydrophilic filters, alcohol for hydrophobic filters). The wetted filter is placed in the appropriate housing and the outlet fitting from the compressed air pressure regulator is connected to the upstream side of the test filter. The outlet fitting from the compressed air pressure regulator is connected to the upstream side of the test filter and a piece of flexible tubing is connected from the downstream port of the test filter into a beaker filled with water. Starting from zero pressure, the pressure is gradually increased to the test filter using the pressure regulator. The submerged end of the tubing is examined for the production of bubbles as the upstream pressure is slowly increased in 0.5 psig (pounds per square inch guage) increments. The bubble point of the test filter is reached when bubbles are produced from the tube at a steady rate.
This test is completed to estimate the pore size of microporous filters and to confirm the integrity of sterilising membrane filters and filter systems. The bubble point test determines the minimum pressure required to force liquid out of the pores, which 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," according to the Scott Laboratories website. As for obtaining the results, a series of these tests are performed and the highest stable values of the tests performed on membrane discs are considered the bubble point. Pressure is greater than or equal to the minimum expected bubble point. Once the test has been performed, if pressure is greater than or equal to the minimum expected bubble point, the filter passes the integrity test. If the recorded pressure is lower than the minimum bubble point, the filter has failed the integrity test.