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The pores of microporous membrane filters act as small capillaries. When hydrophilic membranes come into contact with water, capillary action associated with surface tension forces causes the water to spontaneously enter and fill the pores. In this manner, the membranes are easily wetted and allow the bulk flow of water through the pores. Once wetted, hydrophilic membranes will not allow the bulk flow of air or other gasses, unless they are applied at pressures greater than the membrane’s bubble point.

Hydrophilic membrane filters are typically used with water and aqueous solutions. They can also be used with compatible non-aqueous fluids. Hydrophilic membrane filters are typically not used for air, gas or vent filtration since the filters would block flow if inadvertently wetted, by condensation for example.

When hydrophobic membranes come into contact with water, surface tension forces act to repel the water from the pores. Water will not enter the pores and the membranes will act as a barrier to water flow, unless the water is applied at pressures greater than the membrane’s water entry pressure. Low surface tension fluids, such as alcohols, can spontaneously enter and fill the pores of hydrophobic membranes. Once all the air in the pores is displaced, there are no longer any surface tension forces and water can easily enter the pores, displace the low surface tension fluid, and pass through the membrane. The membrane will then allow bulk flow of water for as long as the pore remain water filled. If the membrane is allowed to dry (i.e. air enters the pores), then it must be pre-wet with a low surface tension fluid again prior to use with water.

Hydrophobic membrane filters are typically used with compatible non-aqueous fluids. They are also commonly used as air, gas, or vent filters. Hydrophobic membrane filters are sometimes used with water or aqueous solutions; and, in these applications, they must first be prewet with a low surface tension, water miscible fluid prior to use.

Nominal pore size ratings are provided as a general indication of filter retention. It is understood that some quantity of particles greater than, and equal to, the nominal pore size ratings will pass through the filters into the filtrate. Some manufactures may associate nominal pore size ratings with percentage filtration efficiencies. Nominal pore size ratings vary from manufacturer to manufacturer and, consequently, are not necessarily equivalent. Filters from different manufactures with similar nominal pore size ratings may not actually exhibit similar retention characteristics.

Absolute Tamaño de poro ratings are typically based on retention studies performed using challenge suspensions of standard microorganism cultures or particles of known size. Absolute Tamaño de poro ratings represent the size of the smallest microorganisms or particles completely retained during these studies. Absolute Tamaño de poro ratings are almost always correlated to bubble point Especificaciones that are used for quality control during membrane manufacturing. For the most part, absolute Tamaño de poro ratings, especially those based on microbial retention, are comparable from manufacturer to manufacturer. There is more uncertainty for absolute Tamaño de poro ratings based on particle retention studies, especially for Tamaño de poro ratings <0.2µm, since there are no standard methods for these studies.

Regardless of pore size ratings, it is important to understand that application conditions do influence particle retention. Even filters with absolute pore size ratings can be operated in conditions that will allow unexpectedly sized particles to pass.