Bubble Point

When heating a liquid consisting of two or more components, the point where the first bubble of the vapor is formed is called bubble point. The boiling point and dew point are the same and are referred to as boiling point for single component mixtures.  For pore size determination, the bubble point is most widely used. The height of the water column in a capillary is indirectly proportional to the capillary diameter according to the theory of capillarity states. Normally the port size of the filter element can be established by wetting the element with the fluid and measuring the pressure at which the first stream of bubbles is produced from the upper surface of the element.

Bubble point pressure

The pressure at which the first gas bubble appears while decreasing pressure on a fluid sample is defined as bubble point pressure. With the hydrophobic membranes, bubble points are used to filter water and these are used for venting and filtration.

Bubble point test

The bubble point pressure is used in test called bubble point test. This is test used to determine the pressure at which the nonstop stream of bubbles is initially sees downstream of a wetted filter under gas pressure. In the appropriate housing the wetted filter is placed and the outlet fitting from the compressed air pressure regulator is connected to the upstream side of the test filter. In the upstream side of the test filter, the outlet fitting from the compressed air pressure regulators is connected and  from the downstream port of the test filter, a piece of flexible tubing is connected into a beaker filled with water. The pressure is gradually increased from the zero pressure to the test filter using the pressure regulator. As the upstream pressure is gradually increased in 0.5 psi (pounds per square inch), the submerged end of the tubing is examined for the production of bubbles. When the bubbles are produced from the tube at a steady rate, the bubble point of the test filter is reached.

Purpose

To estimate the pore size of the micro porous filters and to confirm the integrity of sterilizing membrane filters and filter systems this test is completed. The minimum pressure required to force liquid out of the pores is determined by the bubble point test, which is the measure of the pore diameter. Once the test has been performed, the filter passes the integrity test, if pressure is greater than or equal to the minimum expected bubble point. If the pressure is lower than minimum bubble point, filter has failed the integrity test.

Questions:

• What is bubble point?
• Explain bubble point test?