Diffusion testing bubble point pressure

A completely wetted filter membrane provides a liquid layer across which, when a differential pressure is applied, the diffusive airflow occurs in accordance with Fick s law of diffusion (Fig. 8). This pressure is called test pressure and commonly specified at 80% of the bubble point pressure. In an experimental elucidation of the factors involved in the process, Reti simplified the integrated form of Fick s law to read as follows ... 

The pressure hold test can be set up as a variant of the bubble point test by using a pressure slightly below the estimated bubble point. It can be set up as a variant of the diffusion test with the pressure around 80% of the bubble point pressure, or it can be set up at lower pressures around 5-10% of the bubble point pressure. The test can be done over a very short period of time or over a protracted hold time. 

When a test gas (for example ambient air) is applied over a water moistened filter, just below the pressure level of the bubble point, test gas diffusion will occur through the water in the wetted membrane filter. This diffusion happens in all water filled pores, not only in the largest. This principle is the basis for two tests, which use different approaches to measure gas diffusion the pressure hold test and the diffusive-flow (forward flow) test. Other names for the same principle tests exist. These tests are performed at a pressure of about 80 % of the theoretical bubble point pressure of the filter. It is important that the largest pores are still filled with liquid. In this phase, diffusion occurs more or less linearly with the pressure drop over the... 

There is one complication in the bubble-point test referred to as "diffusional-flow." A small amount of gas-flow can result even through a pore is filled with liquid. The gas dissolves in the liquid in the pores at high pressure, diffuses across the liquid-filled pore in solution, and comes out of solution on the low-pressure side of the membrane. In practice, "diffusional-flow" is not even detected when small membrane areas are involved. Even for large areas, it is easily distinguished from the much larger gas-flow at the bubble point. 

The integrity of sterilizing fillers is most often validated and routinely monitored by nondestructive methods. The U.S., European, and U.K. guidelines on sterile filtration refer to four methods of integrity measurement filtration flow rate, bubble point tests, diffusion (forward flow) tests, and pressure hold tests. Each of these has its uses in determining that routinely used filters are per-forming to the same standards as those validated for the particular products and processes. 

The ideal relationship of two straight lines crossing one another can never arise because of certain unavoidable phenomena that occur during the test. First, there must always be gas flow at pressures below the bubble point this is a result of diffusion of gas through the liquid-filled membrane (see below). Diffusional flow is proportional to the applied upstream gas pressure. The second factor is associated with the distribution of pore sizes in commercial membranes. As with... 

Like bubble pointing, the diffusion test allows evaluation of the integrity of filter membranes. The membrane is first wetted and gas pressure on the upstream side of the filter is increased to that recommended by the... 

Airflow Technique An alternative method, the airflow technique, measures the flow of air passing through the membrane surface. Several variations of this test method are available from the various membrane suppliers, and the test is usually automated. This test may involve applying a constant feed-side gas pressure below the bubble point of the selected size hole. The operator then measures the diffused gas filtrate flow or the displaced water from the membrane. 

---