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.
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