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Absolute rating vs. nominal rating for filters

Filters are rated on their ability to remove particles of a specific size from a fluid, but 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.

The two most used reported media ratings are nominal and absolute micron rating.

Absolute rating

The absolute rating, of cut-off point, of a filter refers to the diameter of the largest spherical glass particle, normally expressed in micrometers (mm), which will pass through the filter under laboratory conditions.
It represents the pore opening size of the filter medium. Filter media with an exact and consistent pore size or opening thus, theoretically at least, have an exact absolute rating.

The absolute rating shouldn't be confused with the largest particle passed by a filter under operating conditions: the absolute rating simply determines the size of the largest glass bead which will pass through the filter under very low pressure differentials and nonpulsating conditions.

This does not usually apply in practice: pore size is modified by the form of the filter element and it is not necessarily consistent with the actual open areas. Furthermore the actual form of the contaminants are not spherical and the two linear dimension of the particle can be very much smaller than its nominal one, permitting it to pass through a very much smaller hole (i.e. cylindrical particles with a thickness less than the slot opening of the filter).
The passage of oversize particles in this manner depends very largely on the size and shape of the opening and on the depth over which filtering is provided.

Most of filters generate a filter bed: contaminants collecting on the surface impart a blocking action decreasing the permeability of the element bad improving filter efficiency. When the blocking is so severe that the pressure drop is excessive, the flow rate through the system decrease seriously. This explains why the performance of a filter can often exceed its given rating based on the performance of a clean element and why test figures can differ widely with different test conditions on identical elements.

It may be argued that the term absolute rating is not a realistic description. Strictly speaking the term absolute indicates that no particle larger than that rating can pass through the filter, limiting the type of media to those of consistent pore size where they show 100% retention of particles.

Nominal rating

The nominal rating refers to a filter capable of cutting off a nominated minimum percentage by weight of solid particles of a specific contaminant (usually again glass beads) greater than a stated micron size, normally expressed in micrometers (mm). I.e. 90% of 10 micron.
It also represents a nominal efficiency figure, or more correctly, a degree of filtration.
Process conditions such as operating pressure, concentration of contaminant etc, have a significant effect on the retention of the filters. Many filter manufacturers use similar tests but, due to the lack of uniformity and reproducibility of the basic method, the use of nominal ratings has fallen into disfavor.

Mean filter rating

The mean filter rating refers to the measurement of the average pore size of a filter element. It establishes the particle size above which the filter starts to be effective. It is determined by the bubble point test and it is more meaningful than a nominal rating and, in casa of filter elements with varying pore size, more realistic than an absolute rating.

Source: 'Filters and Filtration Handbook', T Christopher Dickenson, Elsevier, January 1, 1997

Related topics:

Beta Ratio

Bubble point

Sediment filters

Oil filtration and Oil-Block filters

Zeta potential

For more information about filters provided by Lenntech check our replacement filters web page.
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