What is the difference between Absolute Rating and Nominal Rating?
Filters are rated based on their ability to remove particles of a specific size from a fluid, but this performance is specified in a variety of very different ways.
The size of a specific particle or organism retained by the filter media to a specific degree of efficiency is referred to as a pore size rating. A "10 micron" filter is capable of capturing particles as small as 10 micrometers.
However, you won't understand what this means unless you also have a description of the test methods and standards used to determine the filter rating.
Nominal and absolute micron ratings are the two most commonly used reported media ratings.
The absolute rating, or cut-off point, of a filter is the diameter of the largest spherical glass particle, normally expressed in micrometers (mm) that will pass through the filter under laboratory conditions. It denotes the size of the pore openings in the filter medium. At least theoretically, filter media with an exact and consistent pore size or opening have an exact absolute rating.
The absolute rating is not the same as the largest particle that will pass through a filter under operating conditions:
It simply determines the size of the largest glass bead that will pass through the filter under very low pressure differentials and non-pulsating conditions.
This is not usually the case in practice because pore size is affected by the shape of the filter element and is not always consistent with the actual open areas. Furthermore, the contaminants' actual shape is not spherical, and the particle's two linear dimensions can be much smaller than the nominal one, allowing it to pass through a much smaller hole (i.e. cylindrical particles with a thickness less than the slot opening of the filter).
The nominal rating refers to a filter's ability to remove a specified minimum percentage by weight of solid particles of a specific contaminant (usually glass beads) larger than a specified micron size, which is typically expressed in micrometers (mm). That is, 90 percent of 10 micron.It also denotes a nominal efficiency figure, or, more accurately, a degree of filtration.
Filter retention is greatly influenced by process conditions such as operating pressure, contaminant concentration, and so on.Many filter manufacturers use similar tests, but the use of nominal ratings has fallen out of favor due to the lack of uniformity and reproducibility of the basic method.
What’s the difference?
Pore sizes are given in nominal units. A filter with a nominal pore size rating can prevent the passage of a certain percentage (usually between 60% and 90%) of solid particles larger than the stated pore size, which is usually expressed in micrometers or microns.Filtration conditions such as operating pressure, particle shape, and particle concentration all have a significant impact on filter retention.
Filters with nominal pore sizes can be used in the following applications:
· Analyses of water quality
· Particulates in the air
Pore sizes in absolute terms under strictly defined test conditions, an absolute pore size rating specifies the pore size at which a challenge organism or particle of a specific size will be blocked by the filter with nearly complete efficiency (greater than 99.8 percent).
An absolute pore size rating test typically specifies the following conditions:
The test organism or particle size, the concentration of the challenge particle, the testing pressure used, and the detection method used to identify the particles that pass through the filter.
Another thing to keep in mind is that most pore size tests, such as the bubble point test, are performed in liquid.However, in air or gas, both the filters and the suspended particles operate differently. The filter's efficiency, or ability to capture particles, increases dramatically when used in air or gas filtration applications. This means that when used in gas applications, a filter with a nominal pore size rating can function as an absolute-rated filter.