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Technical Profile

Cartridge filters are rated according to micron number referring to the particle size the filter will trap. A filter may be rated as nominal, absolute, or mean. Nominal rating refers to the filter's ability to trap a nominated minimum weight percent of solid particles greater than a specified size (for instance: 85% of 10 micron). It gives some indication as to the amount of filtration a specific filter may provide. Absolute rating refers to the largest particle that will pass through the filter under laboratory conditions (for instance: 99.99% of 10 micron). Absolute does not represent performance in the field, which may produce differing results due to physical characteristics and particle shape. Mean filter rating refers to the average pore size of the filter. Particles above this size will be trapped by the filter. It is often more meaningful than nominal rating and more realistic than absolute.

Cartridge filters are made of a variety of materials depending on the service. Multiple cartridges are typically grouped in filter housing. As particles collect on the outside of a filter they create a filter bed that lowers the filter's permeability and helps trap more particles. In many cases the overall life performance of a filter will exceed the rating as a result of the improvement of efficiency. Pressure increases as the filter bed builds up and volume decreases. Filter cartridges are replaced when volume decreases below a minimum or before pressure drop increases to a point where cartridge failure might occur.

Normally, nominal filtration is fine, but when very high quality water is needed, absolute filters may be needed. Low micron rated filters are more expensive and must be replaced more often. Therefore, the selection of particle size removal is an economic one balancing the cost against the benefit to downstream equipment or subsurface formation.