Bubble point tests

An easier test which is also nondestructive is the bubble-point test. The maximum pore size may be determined by measuring the gas pressure required 

The bubble point equation is essentially the capillary rise equation  

0 = liquid-solid contact angle (0 = 0° for perfectly wetting fluids  

For water, with a surface tension of 72 dynes/cm, and a contact angle of 0, equation (1) reduces to  

For isopropanol, which wets almost all polymers, the equation becomes (with the same units)  

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Bubble point test A test to determine the maximum pore si2e opening of a filter. Buffer A solution or liquid whose chemical makeup neutralizes acids or bases without a great change in pH. 

Mean filtration rating Derived from Bubble Point test method. This data should be used as a guide only to compare overall retention capabilities between fabrics and should not be considered a guarantee of the particle size that the fabric will retain. 

A commonly used simple method for determining if there are any cracks or pinholes in microporous membranes is the so-caUed bubble point test. It has been used by many organic membrane manufacturers and users alike and is also being adopted by some inorganic membrane manufacturers. The method utilizes the Washburn equation... 

Figure XII. Schematic of a bubble point test apparatus.

Figure XIX Maximum and mean flow pore determinations by bubble point test.

Perform a pressure hold test with the sterilized filter. Perform the filtration of the product to be sterile filtered using normal production conditions. After the filtration step, the filter should be tested again with the bubble point test. 

Set up for media prehltration and final filtration bubble point test per SOP. 

Bubble-point test -for pore size [METALLURGY - POWDERMETALLURGY] (Vol 16)... 

Aseptic Lltration is necessary for parenteral formulations. Because both lipids and the structure of liposomes are unstable at high temperatures, conventional terminal steam sterilization is not suitable for liposome formulations. Thus, the membrane aseptic Lltration is the most reliable method for sterilizing liposome formulations. Since the possibility exists for the membrane being defective, it is advisable to test the integrity of the assembled unit by carrying out a bubble-point test. This test... 

The bubble point test is a popular single-point physical integrity test for disc filter membranes based on Eq. (21). A fdter medium is wetted with a liquid, and test gas pressure is slowly raised until a steady stream of bubbles appears from a tube or hose attached to the downstream side of the filter and immersed in water (Fig. 9). The pressure at which the bubbles first appear is recorded as the bubble point and is related to the largest pores in the fdter medium. A pore size can be calculated from Eq. (21) however, it must be realized that the bubble point test does not measure the actual pore size, but only allows correla-... 

Figure 9 Basic bubble point test setup.

The bubble point test, while popular, has some deficiencies that must be realized. First, there is variation in the operator detection of the test end point that is, the first appearance of gas bubbles rising in the liquid. Some operators are able to see smaller bubbles than others. In a recent study, a panel of seven observers recorded the initial detection of a steady stream of air bubbles rising from a capillary held under water as the air pressure was gradually increased. The observers, who had received different degrees of training, identified the simulated bubble point as occurring at air flows of 5 to 50 mL/min corresponding to air pressures of 34 and 38 psi, respectively, for a 90-mm disc filter membrane [56]. 

How will the bubble point test be run What will be the conditions of the pressure hold test ... 

The bubble point test is simple, quick and reliable and is by far the most widely used method of characterizing microfiltration membranes. The membrane is first wetted with a suitable liquid, usually water for hydrophilic membranes and methanol for hydrophobic membranes. The membrane is then placed in a holder with a layer of liquid on the top surface. Air is fed to the bottom of the membrane, and the pressure is slowly increased until the first continuous string of air bubbles at the membrane surface is observed. This pressure is called the bubble point pressure and is a characteristic measure of the diameter of the largest pore in the membrane. Obtaining reliable and consistent results with the bubble point test requires care. It is essential, for example, that the membrane be completely wetted with the test liquid this may be difficult to determine. Because this test is so widely used by microfiltration membrane manufacturers, a great deal of work has been devoted to developing a reliable test procedure to address this and other issues. The use of this test is reviewed in Meltzer s book [3],... 

Although bubble point measurements can be used to determine the pore diameter of membranes using Equation (7.1), the results must be treated with caution. Based on Equation (7.1), a 0.22-pm pore diameter membrane should have a bubble point of about 200 psig. In fact, based on the bacterial challenge test, a 0.22-pm pore diameter membrane has a bubble point pressure of 40-60 psig, depending on the membrane. That is, the bubble point test indicates that the membranes has a pore diameter of about 1 pm. 

Figure 7.8 Correlation of P. diminuta microbial challenge and bubble point test data for a series of related membranes [6]. Reprinted from T.J. Leahy and M.J. Sullivan, Validation of Bacterial Retention Capabilities of Membrane Filters, Pharm. Technol. 2, 65 (1978) with permission from Pharmaceutical Technology, Eugene, OR...

Figure 7.9 An illustration of the model of Williams and Meltzer [7] to explain the discrepancy between membrane pore diameter measurements based on the microbial challenge test and the bubble point test. Reprinted from R.E. Williams and T.H. Meltzer, Membrane Structure, the Bubble Point and Particle Retention, Pharm. Technol. 7 (5), 36 (1983) with permission from Pharmaceutical Technology, Eugene, OR...

Bubble Point tests are usually carried out to characterize a membrane or porous material consistency or quality they are also a common procedure to determine the maximum pore size. 

Bubble point test. For separation applications, it is critical that the membranes are free of defects such as hairline cracks or pinholes. A simple technique widely used in the membrane industry to detect any defect or very large pores in organic or inorganic membranes is the bubble point test. 

The critically important pore size and its distribution can be determined by a host of measurement techniques. The bubble point test is used primarily for delecting any defects or hairline cracks and for estimating the average pore size. Traditionally mercury porosimetry and nitrogen adsorption/desorpiion have been the workhorse for determining the pore size distribution of a porous membrane. They arc, however, usually limited to a pore diameter range of >3 nm and 1.5-100 nm, respectively. Determination... 

When the wetting fluid is expelled from the largest pore, a bulk gas flow will be detected on the downstream side of the filter system (Fig. 7). The bubble point measurement determines the pore size of the filter membrane, i.e., the larger the pore the lower the bubble point pressure. Therefore, filter manufacturers specify the bubble point limits as the minimum allowable bubble point. During an integrity test, the bubble point test has to exceed the set minimum bubble point. 

Fig. 7 Manual bubble point test set-up. (Reprinted from Technical Report No. 26, Sterilizing Filtration of Liquids 1998 by PDA.)...

Bubble point It relates to the largest pore diameter of the membrane. The bubble point is the smallest pressure difference at which the first gas bubbles appear from a liquid-saturated membrane pressurized by an inert gas. The bubble point test is also used for checking the physical integrity of the membrane for the presence of defects such as cracks or pinholes. 

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. 

Since the effective pore size is estimated from the molecular diameter of globular proteins which are retained 90% by the membrane, it is obvious that larger pores do exist. The measurement of a membrane s bubble point (see the section on the bubble point test in Chapter 2) permits calculation of the maximum pore size in the skin of the membrane. 

Oily waste waters suitable for treatment by UF contain 0.1 to 10% oil in a stable emulsion. A limited amount of free oil can be processed but usually quantities above 1 to 5% are removed with a centrifuge prior to UF. The difficulty with free oil or unstable emulsions is that the oil accumulates at the membrane interface and may form a continuous layer which preferentially wets the membrane over water (the surface tension is lower). In this case, the membrane will pass oil and retain water. (See Chapter 2 on the bubble point test). The secret of successful UF is to maintain discrete and stable emulsoid particles of oil (generally over 0.1 m in size) which are larger than the membrane pore size (0.01 p or below). When this is the case, oil in the permeate will generally be less than 10 to 50 ppm. 

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