A micron is a unit of measure in the metric system equal to 1 millionth of a meter in length, the average size of the openings in a filter screen / media is represented in microns.
The average cross-section of a human hair is 50 microns and the human eye cannot see anything smaller than 40 microns in size.
50 microns – Diameter of a human hair
40 microns – Lower limit of visibility of a human eye
25 microns – White blood cells
8 microns – Red blood cells
2 microns – Bacteria
A micron rating is used to indicate the ability of a liquid filter to remove contaminants by the size of the particles.
For a micron rating to be truly useful, you must know the filters' removal efficiency of the specific particle size in question.
There are several variations of micron ratings, but the two most common terms used are Nominal and Absolute.
Nominal Micron Rating.
This rating expresses the ability of the filter to capture particles of a specified size at an efficiency between 50% and 90%.
For instance, a nominal rating of 50% at 10 micron simply means that filter captures 50% of contaminants 10 micron in size. Keep in mind that nominal micron ratings may range anywhere from 50 – 98.6% efficient at removing a specific particle size.
Absolute Micron Rating
This rating means that the filter is capable of removing at least 98.7% of a specific size particle.
This rating is determined through a single-pass or multi-pass test in which fluid containing measureable particles is passed through a flat sheet of filter material. Particles that pass through are measured and counted. This rating is more informative than the nominal micron rating.
As a general rule, the smaller micron rating for a filter is better, especially if you are on well water, but as with most everything, there is a trade-off. Flow capability usually drops off as the micron rating gets smaller, especially if the water has a lot of sediment.
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