Reference

Principles of Liquid Filtration

Liquid Filtration

Liquid filtration involves the removal of contaminant particles in a fluid system. The grade of filter chosen for a specific application is usually determined by the size of the particle to be removed. Contaminant particles are measured using the "micron" unit of measurement.

Micron

A micron is a metric unit of measurement where one micron is equivalent to one one-thousandth of a millimetre 
[1 micron (1μ) = 1/1000 mm] or 1 micron (micrometer) = 1/1,000,000 of a metre.

Visualizing a micron

• a human red blood cell is 5 microns
• an average human hair has a diameter of 100 microns
• most humans cannot see anything smaller than 40 microns with the unaided eye.

The following chart relates to size of some common particles:

Lower Limit	Upper Limit	Contaminant
Micron	Micron
0.3	0.4	Smoke, Paint Pigments
0.4	0.55	Bacteria
0.55	0.7	Lung Damaging Paint
0.7	1.0	Atmospheric Dust
1.0	1.3	Molds
1.6	2.2	Flour Mill Dust
3	4	Cement Dust
4	5.5	Pulverized Coal
5.5	7	Commercial Dust
7	10	Pollen
10	75	Silt
75	1000	Sand

The micron unit of measurement is used not only to measure the size of a contaminate particle, it is also used to measure the size of the openings in filter media, hence, a media's micron rating. This system of measurement is more accurate when gauging woven filtration structures, such as monofilaments, than it is for gauging non-woven structures, such as felts.

Mesh vs. Micron

The old standard imperial system of gauging a woven filtration media's ability to remove contaminant particles was the mesh system. This system simply counted the number of strands or yarns per inch of woven media. Hence, a 100 mesh media has 100 yarns per inch of media. This system falls short because the actual window opening of a woven structure can vary as the diameter of the yarn varies. For example, a 50 mesh fabric with a yarn diameter of 100 micron would have a window opening of 410 micron, whereas a 50 mesh fabric with a yarn diameter of 200 micron would have a window opening of 310 micron. It's main value, now, is in the determination of a percentage of open area in a structure, which is calculated by using the yarn diameter and the mesh count in order to determine the potential flow rate of a liquid through a woven filtration media. The micron system, however, attempts to measure an exact window opening for a woven media and exact particle size retention for a non-woven media.