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Know-how
Filter Parameters: Filtration Rating, Contamination Retention and Pressure Drop
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Filtration rating:
The filtration rating of a filter is defined differently according to the type and performance of the filter material built into the filter element.
With the aid of an optical microscope, the "fineness" of surface filters, such as wire mesh or fleece materials, can be determined simply by measuring their mesh or pore size. The fineness of these filter materials is, thus, characterized by their mesh size (for meshes) or their pore size (for fleece materials).
For filter materials that consist of many layers of fiber, e.g. the inorganic fiberglass papers frequently used in hydraulic filters, it is not usually possible to determine the pore size with an optical microscope.
The so-called filtration ratio is, therefore, used to characterize the fineness of these filter materials.
The filtration ratio βx (β-value) acc. to ISO 16889 (ISO 4572)
Definition:
The effectiveness of a filter used in fluid power technology is defined by the filtration ratio βx, also commonly referred to as the "beta value".
There is a relationship between the number of particles before and after the filter based on a given measurement volume in ml.
βx = Nbefore / Nafter
This dimensionless ratio is determined according to ISO standard 16889, the so-called multipass test. ISO 16889 thereby replaces test standard ISO 4572, which has been in use since 1981.
According to ISO 16889 an EPE H10XL filter element, for example, has the following filtration ratio:
ß1000 for particles > 10 µm (c )
If the filtration ratio of a filter is defined to be 1000, the filter in the previous example has a filtration rating of exactly 10µm.
This means: of 1000 particles > 10 µm, 999 particles are retained, 1 particle passes through the filter.
The particles that are not retained by the filter re-enter the circuit and can, thus, be retained by the filter material on the next pass through the filter.
This multi-pass principle represents most operating conditions of hydraulic and lubricating oil filters in practical operation.
To perform the multi-pass test, a special test dust is added to the test oil. This test dust consists primarily of quartz sand and is also referred to as ISOMTD test dust. Its composition and grain size distribution is defined in ISO standard 12193-1. All filter-relevant measured values, such as β-values, removal rates and contamination retention, are thus based on the characteristics of this test dust.
Comment:
In many manufacturers' instruction, one also encounters the term "nominal" filtration or the "nominal filtration rating".
This is an arbitrary designation defined by the given manufacturer. It is, thus, not comparable between the various manufacturers and not suitable for assessing different filters.
Filter materials made of fleece or filter paper are frequently labeled with a nominal filtration rating.
The filtration efficiency (removal rate):
The fineness of a filter can also be described in terms of the filtration efficiency. This is frequently used in general filter technology and defines the ratio of the number of removed particles compared to the number of particles preceding the filter.
Starting with the ß-value, the filtration efficiency is described as follows:
n = ( β-1 ) / β x 100 %
The following table shows the relationship between β-value and filtration efficiency:
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| 1 | 0% | | 2 | 50% | | 10 | 90% | | 75 | 98,75% | | 100 | 99,00% | | 200 | 99,50% | | 1.000 | 99,90% | | 10.000 | 99,99% |
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