The ISO Technical Committees responsible for fluid power (ISO/TC131/SC6) have recently introduced new standards for the measurement and assessment of fluid cleanliness (Table 1). While primarily developed for fluid power, the new standards are equally applicable to the lubricating oil sector. These standards have been developed to increase the precision in the measurement of contamination and filtration performance, and to extend the amount of information from the tests. Additionally, the user can have greater assurance and confidence in the data, and designers or specialists can make a more informed judgment in the selection of the most appropriate filter for his system. This article will focus on the impact the new standards have on selecting candidate filters based on performance criteria.

The Role of the Filter
To appreciate the importance of the filter in the management of the system, consider the primary function of the filter: It has to protect the components from the damaging, critical clearance-sized particles (particles that can penetrate and interfere with the working clearances of components). The filter should control the fluid cleanliness to a level that is equal to the performance, life and reliability of the system required by the user. It should allow fluid to pass through at the given flow with the minimum pressure drop (DP) to minimize stress and energy losses.

A filter must control the levels of all contaminant particles at and above the size critical to its operating system. If the filter fails to provide the necessary control of damaging-sized particles, then their presence in the system will lead to a substantial increase in the number of particles generated within the system through a chain reaction of wear. Particles entering component working clearances will become work hardened and produce more wear particles. This makes the capture of these particles, by the filter, essential to sustain the good health of the system.

ISO 16889 Multi-Pass Test
The multi-pass test is a means of determining a filter’s performance; it measures the ability of the filter to remove particles of test dust over a wide particle-size range. This gives a series of Beta ratios for the filter as opposed to a percentage efficiency. The Beta ratio is defined as:

bx (number of particles upstream >xµm)
(number of particles downstream > xµm)

Where x is the determining size of particle for the Beta ratio.

The need to remove particles consistently across a wide size range was recognized by the ISO Working Group responsible for updating the multi-pass filter test. The group also stipulated that detailing comprehensive performance data was necessary so users can make informed selections. The data that can be made available is as follows:

  • Pressure drop/flow,
  • Variation in particle removal abilities (Beta ratio) with size,
  • Variation in performance with time and differential pressure,
  • Weight of test contaminant retained by the filter,
  • Filtration ratings over a wide size range, and
  • Filtration performance over a wide range of particle sizes.

Previously, the only performance data a manufacturer had to publish about the filter was based upon its ability to remove 10 µm particles. The current data requirement is a substantial improvement. Most manufacturers published ratings at other sizes but there was no consistency. Users were often confused when comparing performance data of seemingly identical filters.

One of the most important advances with the new multi-pass standard is the publishing of the micrometer(c), [or µm(c)] ratings at specified Beta ratios across a wide size range. Now, users can quickly see how the filter performs over a specified range of particle sizes and can compare the performance of candidate elements. The standardized ratings are the micrometer size (µm) where the Filtration Ratio (or Beta) is equal to or greater than 2, 10, 75, 100, 200 and 1,000 (Figure 1).

Figure 1. Filter Ratings

Note: To differentiate data obtained from APCs calibrated to the new standard, ISO 11171, micron size is reported as µm(c) rather than µm.

The slope of the curve effectively defines the pore size distribution (PSD) of the filtration medium; the wider the PSD the less able the filter is to control those critically sized damaging particles. When the contaminated hydraulic fluid passes through the filtration medium, it takes the line of least resistance. This will be through the larger pores in the medium and any holes created by poor manufacturing techniques. Therefore, the desired level of control will not be achieved. Figure 2 compares the performance of two candidate filters of very similar single point ßx>200 rating, in this case at 6 µm(c). However, substantially different filtration characteristics exist with Element A having a much more narrow pore size distribution as shown by the steeper Beta ratio/size curve. It is able to control the critically sized particles better than Element B.

Beta = 1000 Rating

Users of filters have historically liked the simplicity of a single point rating (the size where a specified Beta ratio is attained). Consequently, the number of different Beta ratings used has proliferated. In the early 1970s, ßx>2 (nominal) was used by some followed by the ßx>75 (absolute - all particles removed) in the late 1970s. ßx>75 then became the most widely used, and was introduced solely to meet the measurement limitations that were experienced at the time. This was followed by ßx>100 and ßx>200.

What value is most appropriate? Selection should be based on the filter’s ability to remove critically sized damaging particles. Therefore, the highest Beta ratio that can be reliably measured should be selected. Ideally, ß = (infinity - no particles in the downstream fluid) is the target. However because of particle count statistics in the multi-pass test, the absolute rating cannot be measured with any degree of confidence. In the 1970s and 1980s, a compromise was reached between what is desirable and what can be consistently achieved for the range of filters assessed by this test.

Figure 2. Filtration Profiles of Two
Nominally Identical Filters

The introduction of ßx(c)>1000 now enables users of hydraulic filters to select elements on their ability to exercise the necessary control over clearance-sized particles. This is essential to achieve the required system reliability and life. It also enables the user to discriminate between elements that achieve the desired control and those that do not.

Figure 2 shows the advantages of this approach. If the manufacturer’s published rating of ß>200 at 6 µm(c) is compared, then there appears little difference between Elements A and B at 5.8 and 6.4 µm(c) respectively. It is only when the ratings at higher Beta ratings are considered that the differences in performance are clearly seen. Element A gave a ß>1000 rating at 7.5 µm(c), but this rating could not be obtained for Element B at the sizes monitored.

The effectiveness of the hydraulic filter to control clearance-sized particles is essential if the desired level of reliability is to be realized in the hydraulic system concerned. This demands care in the selection of the most appropriate filter for the system.

The changes in the ISO multi-pass test method and associated standards provide greater precision in measurement. More detailed information from the test has to be published than previously required.

However, the multi-pass test is carried out in a laboratory under steady-state conditions and does not represent strenuous operational conditions. It is a preselection requirement that provides the user and specifier of filter standards with more consistent information.

A filter’s ability to remove contaminant must ultimately be judged on the level of fluid cleanliness it maintains in the operating system, and this requires regular monitoring. The more critical the system, the stronger the filter element and the more frequent the monitoring required.

Further Reading

DTI. (1984). “Contamination Control in Fluid Power Systems, Vol. 1 Field Studies.” Department of Trade and Industry, NEL, East Kilbride, Glasgow, United Kingdom.

Farris, J.A. (1971). “The Importance of Silt Removal Filtration in Mobile Vehicle Hydraulic Systems.” Society of Automotive Engineers, Inc.

Day, M.J. (1995). “Hydraulic Filters - why laboratory performance may not be duplicated in the field.” Hydraulikdagar ’95, Linkoping University.