Frequently Asked Filters Questions

What is the difference of ABSOLUTE and NOMINAL filtration?
What is a BETA value?
How can I determine the retention rate in % when I have the BETA value?
How long is the service life of my filter element?
What does a differential stability of 3000 psi mean?
Why do I need Dirt holding capacity?

Q: What is the difference of ABSOLUTE and NOMINAL filtration?
A: ABSOLUTE filtration is defined by ISO 4572 and starts at a separation degree of 98.7% or higher. This reflects in a BETA value of 75 or bigger.

NOMINAL filtration is not defined and can be everything smaller BETA 75. That means ABSOLUTE filtration guarantees a defined retention rate, NOMINAL not.
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Q: What is a BETA value?
A: The Beta value is a separation degree of a filter element determined in the so-called Multipass test to ISO 4572. It is a non-dimensional number which can be calculated by dividing the number of particles (of a certain size) before the filter element by the number of particles after the filter element. E.g. you count 1000 particles bigger or equal 10 micron berfore the filter and only 5 particles bigger or equal 10 micron after the filter that brings to 1000/5 = 200. So we have a BETA value of 200 for 10 micron particles.
Which is stated ß(10) = 200
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Q: How can I determine the retention rate in % when I have the BETA value?
A: The retention rate (R) can be calculated as follows : R = (1-1/BETA) x 100
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Q: How long is the service life of my filter element?
A: This depends on many variables. Only by knowing the dirt content (cleanliness level) and dirt ingression rate of your hydraulic system you can calculate the service life of a filter element. For an exact calculation please contact the filter experts at HYDAC.
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Q: What does a differential stability of 3000 psi mean?
A: By differential pressure stability we mean the max. differential pressure (pressure difference before and after the element) which a filter element or its weakest part can withstand without damage. It can be the filter media or the inner support tube. This value is determined according to a test procedure defined in ISO 2941.
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Q: Why do I need Dirt holding capacity?
A: Besides the Beta value dirt holding capacity (DHC) is an important criteria in comparing the performance of filter elements. The DHC of a filter element can only be determined through a Multipass test. Usually this is referred to as the actual contamination retention. Some manufacturers give the specific DHC, expressed in [mg / cm_], where the filtration area must be known in order to be able to make a comparison. The higher the DHC the longer the service life.
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