FrozenMerc
Full Access Member
For the record. A 12 micron rating does not mean that is the largest particle the filter will pass. It is a statistical number, essentially the median value.
Nominal Micron Rating expresses the ability of the filter to capture particles of a specified size in microns at an efficiency of 50%. A nominal rating of 50% at 12 microns simply means that a filter captures 50% of contaminants 12 microns in size.
It can and will pass particles larger than 12 microns, but as the size of the particle increases, the more efficiently they are caught. For Example, a 12 micron filter will capture 50% of all 12 micron sized particles. It may capture 75% of all 16 micron particles, and it may capture 95% of all 20 micron sized particles.
Be careful with nominal micron rated filters. From "Fluid Power" -
"Filter quality varies as much as the construction and application of the units themselves. Filters are qualified by both the average size of particles they trap and the efficiency in which they’re trapped. Filter quality is expressed in the micron, which is a thousandth of a millimeter, and micron rating describes the theoretical maximum sized particle the medium will allow to pass. For example, a 10-μm filter should trap any 10-μm particle passing through it. In reality, this is not the case. Filter ratings are either nominal or absolute, and each describes the general capacity for the filter to remove particles of the specified size at a specified efficiency. A filter with a nominal rating is low efficiency, and a filter with an absolute rating is high efficiency. The problem is that any manufacturer can describe their filter with any micron rating as long as they publish the efficiency of the media, so you must be careful when selecting them.
Filter efficiency is often expressed by beta ratio, which is a ratio derived from a test measuring the number of particles upstream of the filter to downstream of the filter. Test dust is added to the oil of a sophisticated test rig that counts the number of particles before the subject and then counts the particles again after the fluid has passed through. Particle micron size must be specified when describing beta ratio, and conversely, no filter micron rating is worthwhile without knowing the beta ratio at which it was tested. Once filter beta ratio is calculated, we can further deduce the filter efficiency to decide if a filter is absolute or nominal.
If 100,000 5-μm particles are measured upstream of the filter and 100 particles are measured downstream of the filter, the filter
has a Beta Ratio of 1000, and is expressed as β5 1000.
• To now calculate efficiency, we subtract one from the beta ratio and then divide by the beta ratio.
For example: (1000-1)/1000 = 0.999 or 99.9%.
• Our theoretical test filter is 99.9% efficient at removing 5-micron particles.
Absolute filters must have βx ≥ 75 (x = specified micron size), according to ISO 16889: 1999, which is 98.7% efficiency. Anything below these numbers is considered a nominally rated filter, and such filters can drop as low as a beta ratio of 2, or 50% efficiency. Avoid nominally rated filters, if possible. When filter competence is low, the excess of particles turns the fluid into a liquid lapping compound, of which its abrasiveness further wears components, subsequently increasing the abrasiveness of the fluid itself. To be safe, use only absolute filters with >75 beta ratios."
Nominal Micron Rating expresses the ability of the filter to capture particles of a specified size in microns at an efficiency of 50%. A nominal rating of 50% at 12 microns simply means that a filter captures 50% of contaminants 12 microns in size.
It can and will pass particles larger than 12 microns, but as the size of the particle increases, the more efficiently they are caught. For Example, a 12 micron filter will capture 50% of all 12 micron sized particles. It may capture 75% of all 16 micron particles, and it may capture 95% of all 20 micron sized particles.
Be careful with nominal micron rated filters. From "Fluid Power" -
"Filter quality varies as much as the construction and application of the units themselves. Filters are qualified by both the average size of particles they trap and the efficiency in which they’re trapped. Filter quality is expressed in the micron, which is a thousandth of a millimeter, and micron rating describes the theoretical maximum sized particle the medium will allow to pass. For example, a 10-μm filter should trap any 10-μm particle passing through it. In reality, this is not the case. Filter ratings are either nominal or absolute, and each describes the general capacity for the filter to remove particles of the specified size at a specified efficiency. A filter with a nominal rating is low efficiency, and a filter with an absolute rating is high efficiency. The problem is that any manufacturer can describe their filter with any micron rating as long as they publish the efficiency of the media, so you must be careful when selecting them.
Filter efficiency is often expressed by beta ratio, which is a ratio derived from a test measuring the number of particles upstream of the filter to downstream of the filter. Test dust is added to the oil of a sophisticated test rig that counts the number of particles before the subject and then counts the particles again after the fluid has passed through. Particle micron size must be specified when describing beta ratio, and conversely, no filter micron rating is worthwhile without knowing the beta ratio at which it was tested. Once filter beta ratio is calculated, we can further deduce the filter efficiency to decide if a filter is absolute or nominal.
If 100,000 5-μm particles are measured upstream of the filter and 100 particles are measured downstream of the filter, the filter
has a Beta Ratio of 1000, and is expressed as β5 1000.
• To now calculate efficiency, we subtract one from the beta ratio and then divide by the beta ratio.
For example: (1000-1)/1000 = 0.999 or 99.9%.
• Our theoretical test filter is 99.9% efficient at removing 5-micron particles.
Absolute filters must have βx ≥ 75 (x = specified micron size), according to ISO 16889: 1999, which is 98.7% efficiency. Anything below these numbers is considered a nominally rated filter, and such filters can drop as low as a beta ratio of 2, or 50% efficiency. Avoid nominally rated filters, if possible. When filter competence is low, the excess of particles turns the fluid into a liquid lapping compound, of which its abrasiveness further wears components, subsequently increasing the abrasiveness of the fluid itself. To be safe, use only absolute filters with >75 beta ratios."
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