Filters are rated on their ability to remove particles of a specific size from a fluid. The problem is that a variety of very different methods are applied to specify performance in this way. Pore size ratings refer to the size of a specific particle or organism retained by the filter media to a specific degree of efficiency. A filter that is marked ‘10 micron’ has some capability to capture particle as small as 10 micrometers. However, you do not know exactly what this means unless you also have a description of the test methods and standards used to determine the filter rating.
Nominal and absolute ratings are two different methods used to measure and classify the filtration efficiency of filters. Nominal ratings represent the average particle size that a filter is expected to capture efficiently. It indicates the filter’s ability to retain a significant portion of particles at a specified size but does not guarantee the capture of all particles above that size. For example, a filter with a nominal rating of 10 microns means that it can efficiently capture a substantial portion of particles that are 10 microns or larger, but it may allow smaller particles to pass through. Nominal filters are less expensive than absolute filters, but they are less effective in removing small particles. They are ideal for applications where the removal of larger particles is sufficient.
Absolute ratings, on the other hand, provide a more precise measure of a filter’s filtration efficiency. Absolute rating refers to the thinnest particle size guaranteed to be removed by the filter. Unlike nominal rating, absolute ratings do not allow for any particles above a certain size to pass through the filter. They represent the filter’s ability to retain a specified percentage of particles at a specific size. For instance, a filter with an absolute rating of 10 microns and an efficiency of 99% means that it can capture at least 99% of particles that are 10 microns in size, ensuring a higher level of filtration accuracy. Absolute filters are more expensive than nominal filters, but they provide a higher degree of filtration. They are ideal for applications where the removal of small particles is critical, such as in the pharmaceutical and semiconductor industries
There are no industry standard guidelines regarding how filter pore size, retention, or rating is determined. When assigning a filter its pore size, many factors are taken into consideration by the manufacturer including type of filter, removal efficiency, challenge testing, and ability to retain trapped particles.
The choice between absolute and nominal filters depends on the specific application. Absolute filters are more expensive than nominal filters, but they provide a higher degree of filtration. They are ideal for applications where the removal of small particles is critical, such as in the pharmaceutical and semiconductor industries. Nominal filters are less expensive than absolute filters, but they are less effective in removing small particles. They are ideal for applications where the removal of larger particles is sufficient.