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(Updated Feb, 2017) If you were to casually browse through Sterlitech's website, you may find that we have an incredible range of filter options. The casual browser may certainly find themselves overwhelmed looking for the right filter for their application. One of the most important aspects of filter selection is pore size, which determines the size of largest particles that can pass through the filter. Pore sizes are usually placed in one of two categories: nominal or absolute. So what's the difference? Nominal pore sizes A nominal pore size rating refers to a filter capable of preventing passage of a minimum percentage (usually between 60% and 90%)of solid particles of greater than the stated pore size, which is normally expressed in micrometers or microns. Conditions during filtration, such as operating pressure, shape of the particles and the concentration of particles, have a significant effect on the retention of the filters.  Sterlitech's glass and quartz fiber filters have nominal

Defining a Pore Size and Sterile Filtering; 0.2 Micron vs. 0.22 Micron.  What’s the difference?

If you were to spend a little time perusing Sterlitech’s selection of membrane disc filters, one thing we’re very proud of might just jump out at you: we have a lot of pore sizes. So many that you might wonder if it’s a little excessive that we carry both 0.2 and 0.22 micron pore sizes. After all, both are used to sterilize fluid passed through them. Can the tiny difference of 0.02 microns really change a filter’s performance characteristics that much?

To answer that question, we must first take a look at one of the methods used to test a filter’s performance: the bubble point test1. Standard tests to verify a filter’s stated pore size usually entail a bubble point test. This test pushes air under pressure through a submerged membrane (either in water or alcohol) to the point where air bubbles first begin to come through the filter membrane2. The largest pore, or pores, in the membrane will

Place a polycarbonate (PCT) or polyester (PET) membrane under an electron microscope and you'll see something similar to the picture here: a smooth surface perforated with neat, cylindrical holes. In this aspect, PCT and PET membranes stand out from other membrane types such as PTFE, nylon, or silver which provide irregular, tortuous paths for permeates to follow. So how are the regular little pores created? Are they drilled, punched, molded or torn into the membrane? If you guessed etched in with the help of a nuclear reactor, then you are absolutely right.

Every PCT or PET membrane filter starts as a roll of plastic film stock. The film is exposed in a controlled manner to charged particles in a nuclear reactor. The charged particles pass through the film, leaving behind sensitized tracks. The density of these tracks in the film depends on the amount of time that the film is exposed to the reactor. More time in the reactor with result in more tracks and greater pore density in the finished

One of the important characteristics in membrane selection is whether you want a membrane that is Hydrophobic or Hydrophilic. Here we'll define these terms, as well as provide some examples of membrane materials and applications for both types. Hydrophilic literally means “water loving.” Hydrophilic membranes will attract water, and in the process push away other molecules in order to allow water access to the membrane. This keeps contaminants away from the membrane allowing it stay clean and functioning for a longer period of time. Because of this trait hydrophilic membranes are especially well suited for medical applications and biological assays. Hydrophobic on the other hand, literally means “afraid of water.” These membranes will block the passage of water and are commonly used for applications involving separation of water from other materials, such as venting gases.