Year of publication: 2008


Modern hydraulic systems, with their low gap tolerances and high operating pressures as well as finer filters, place ever greater demands on the filterability of hydraulic oils. In order to determine this precisely, the filterability test has been developed in accordance with ISO 13357 and included in the DIN 51524 standard for HLP hydraulic oils. Since spring 2008, the OELCHECK laboratory has been operating new test equipment that conforms to this standard.


Today’s hydraulic systems need filters. Whether you install the filter permanently in the system as a fullflow filter or as a, partly mobile, by-pass filter, you remove impurities and ageing products from the oil. By doing this, you safeguard the performance of the entire system and assure the longest possible
service life of components and oil. Modern hydraulic liquids have to be easy to filter. The filterability of an oil describes its characteristic differential behaviour under pressure when flowing through a filter. Even when developing hydraulic oils and filter materials, care is taken to ensure that the liquid
itself generates the lowest possible differential pressure increase.


In practice, this means that the filter or oil service life is not shortened, e.g. by the adhesion of gum-like, non-particulate components from the oil or from their reaction products on the filter, and it means that condensate does not affect the filterability.


Modern hydraulics work with increasingly lower gap tolerances and better surface quality. As a result, higher operating pressures and hydraulic components that work more precisely can be used for smaller tank contents. More effective filters guarantee the availability and reliability of the systems over a long period of time. Their average pore size has been reduced from what used to be 10 to 20 μm to 3 to 12 μm. The retention rate (ß value) has at the same time been increased for the depth filters. These “finer filters” can, however, block up in relatively shorter intervals and must then be replaced by new filter elements. The increase in the pressure differential, which is used to determine when to change the filters, depends on the filterability of the hydraulic fluids as well as the reduced ability of the finer filters to absorb dirt.


Using filtration tests, in which oil flows through a filter, often under intensified conditions, the interaction between the filter membrane and oil is examined in more detail. Most “static” filterability tests are carried out as internal tests, due to the lack of a standard, using different membrane pore sizes and
liquid volumes under a vacuum or pressure conditions. However, the results can vary enormously according to the procedure used and often depend on the individual experiences of the laboratory. Since the early eighties, scientists have been searching for a test capable of becoming a standard. This has
now been passed as the ISO 13357 filterability test part 1 and 2.


The OELCHECK laboratory is already using the corresponding test equipment. Using this equipment, it is possible to sufficiently differentiate the filterability of a new oil even as early as the development stage. In used oil analytics, the test produces valuable evidence regarding the change in filterability compared to the fresh product.

The test procedure

The standard test is intended for fresh hydraulic oils up to ISO VG 100. It is divided into the “wet” filterability test, in which the oil is replaced by water (part 1) and the test with “dry” oil (part 2).

300 ml of hydraulic oil is filtered through a dried 0.8 μm membrane filter under a precisely defined pressure. During this process, the filtration volume and time is recorded. The filterability is calculated by determining the volume - time relationships at the start, for a test duration of between 10 and 50 seconds (level I) and between 200 and 300 seconds (level II). The test ends when the required data has been recorded or the filtration time exceeds two hours. The filterability is indicated by the letter “F”. If the value is F > 50, the test is concluded as “passed” according to the standard. The closer “F”
is to 100, the better the filterability.

If F < 50, the test is deemed to have “failed”. If the filtration time exceeds two hours, the test liquid must be marked as “unfilterable”.

A typical test procedure

The figure below shows optimal filtration. At the start of filtration, an almost linear increase in the filtered volume per unit of time is recorded. This decreases due to the increasing pressure differential at the filter, as the filter becomes increasingly blocked, and it ends in a slightly declining curve progression.

Important decision guidance

Using the test developed for fresh oils, it is possible to differentiate newly formulated hydraulic oils as early as the development stage. The test is mainly only carried out for used oils (not least because of the relatively large quantities of oil) if, for example, the filter service life is noticeably too short following oil or filter changes. A change in filterability compared to fresh oil very quickly shows whether the composition of base oil and additives could be the cause of problems when all the other data sheet values are the same. If, for example, an HLP 46 fresh oil produces values of F=98 in the filterability test, this means that filterability is very good. If the F values sink below 50, reductions in the life service of the filters are to be expected. This means that the operating costs of the respective system increase and problems may occur due to sticky deposits or unsatisfactory oil purity. The cause for such a reduction in the filterability can be the forming of tribopolymers and the ageing products of oil itself. These form when the service life of oil increases or during extreme loads and are retained in the filter.

To assess used oils, the least expensive test after part 2, in other words, without adding water, is sufficient. The comparison with the fresh oil data indicates whether maintenance activities suffice or whether only an oil change can remedy the situation.

Experience shows that there is no unique correlation between filterability and other characteristic values of oil. WEARCHECK can use the filterability test to provide more precise evidence regarding the causes of filter changes. Filters and oils are becoming easier to compare. This applies in particular to
the selection of hydraulic oils for very large systems and to sensitive systems, such as the ones used in injection moulding machines or transfer presses.



Ensure that your lubricant supplier informs you of the key F value for the filter service life. Alternatively, let OELCHECK determine this value for you, e.g. for two comparable fresh oils. From now on, WEARCHECK offers the filterability test in accordance with ISO 13357 as a single analysis.