The changing trends in the filtration industry call for proper filtration selection as well as a knowledge of filter ratings and cyclic flow.

Let’s consider some of the changes affecting the filtration industry and end users. A standard multipass ISO 16889:1999 test procedure provides a basis for the comparison of the relative performance characteristics of various filter elements. The results from such a test, however, may not be directly applicable to most actual operating conditions. Thus the reason for NFPA/ISO to develop a standard for cyclic flow tests procedure. The cyclic flow multipass procedure ISO/WD 23369.2 has generated a great deal of discussion in the industry. The procedures for executing the test as well as the information that can be gathered are popular topics of conversation.

It is not likely that industry leaders in the fluid power field would go to this great of an effort unless they believed that fluid contamination was one of the main reasons for many hydraulic system and component failures. Recent changes in education and testing standards have directed more attention to filter elements that help the end user meet a predetermined target ISO Cleanliness Code per ISO 4406-1999.

Filter Selection
This can be the most difficult process of all, because of the complexity of filter ratings and nomenclature. Filters are rated in a number of ways by manufacturers; the three most common being nominal rating, absolute rating and beta rating.

Nominal Rating
A nominal rating is an arbitrary micrometer value indicated by the filter manufacturer.

Absolute Rating
An absolute rating is the diameter of the largest solid spherical particle that will pass through a filter under specified conditions. This is an indication of the largest opening in the filter element.

Beta Rating
While the nominal and absolute ratings are not supported by industry standards, the Beta rating comes from the Multipass Method for Evaluating Filtration Performance of a Fine Filter Element (ISO 16889:1999). While the filter is being tested, particle counters accurately measure the size and quantity of upstream particles per known volume of fluid (number of particles upstream) and the size and quantity of particles downstream of the filter (number of particles downstream). The number of particles of a given size upstream divided by the number of particles of the same size downstream gives a result called a Beta ratio. For example, a Beta ratio of particles above 3 µm in size with 50,000 upstream particles and 250 downstream particles would be represented as 3 = 50,000 / 250 = 200. In words, this filter would have a rating described as “Beta sub 3 equals 200.” This result, known as a Beta ratio, can also give the efficiency of a filter. It is simply (1- 1/) x 100 = percent capture efficiency (using the same example: (1-1/200) x 100 = 99.5 percent capture efficient. See Table 1.

Table 1. Beta vs. Efficiency

A standard multipass method for evaluating the performance of hydraulic fluid power filter elements under steady-state conditions had been developed as ISO 4572:1981. This test procedure provides a basis for the comparison of the relative performance characteristics of various filter elements. This standard had not been revised since its last approval as an ISO standard in 1981. The ISO group worked on the revision for many years and finally approved the new standard as the Method for Evaluating Filtration Performance of a Fine Filter Element (ISO 16889:1999). Some of the major revisions included a new test dust; this dust replaced the old AC Fine Test Dust that is no longer available, a new particle counter calibration procedure using a new calibration fluid which is National Institute of Standards and Technology (NIST)-traceable, a new specification for an on-line particle counter, and a new validation procedure. The ISO 4402 particle counter calibration standard became the ISO 11711:1999 standard.

Cyclic Flow
Tests have shown that in many instances the capabilities of a filter element are severely reduced when subjected to varying cyclic flow conditions. It is therefore important to evaluate the performance of a filter for applications under cyclic flow conditions. A cyclic flow multipass test procedure for hydraulic filters, ISO/WD 23369.2, is being developed (currently a working draft) to supplement the basic steady-state flow test for filter elements to be placed in-service with cyclic flow operating conditions.

The proposed multipass method of evaluating performance of a filter element under cyclic flow conditions (ISO/WD 23369.2) is basically the same as a standard multipass test, except an electrically controlled valve is installed to allow the test flow to be cycled from zero flow to full flow. The recommended flow cycle rate of 0.1 Hz, or one cycle per 10 seconds, is a result of an industry survey and a broad range of test results.

With this new procedure, the industry hopes to best simulate the filter element in the so-called real-world operating condition. Operating conditions in a laboratory environment differ from operating conditions in an actual service environment. The contamination level is much greater in the laboratory than in the field of actual use. When ISO/WD 23369.2 is completed there will be a standardized procedure for handling cyclic flow conditions.

The perfect filter would have no pressure drop, hold an unlimited amount of dirt, be small enough to fit anywhere in a system, give great ISO cleanliness codes per ISO 4406:1999, have high capture efficiency and cost nothing. Obviously this combination cannot exist, and the pursuit of the perfect compromise has always been the challenge for filter manufacturers.

For many years, filtration companies have been trying to educate end users on why filtration is important and how it helps the end user protect his equipment, save him money in the long run, or both. Many end users understand that there are often trade offs to lower micron rating and higher capture efficiencies of filters.

Education on filtration and contamination control fundamentals is just as important as the type of component or system to buy. This leaves us to ask the main question: what does the end user want for his filtration systems to accomplish at the end of the day?


  1. National Fluid Power Association
  2. Paul-Munroe Hydraulics, Inc. Lightning Reference Handbook, Seventh edition. January 1990.
  3. ANSI/ (NFPA/JIC) T2.24.1-1991.
  4. Society of Automotive Engineers. Particle Counting in Fuels, Lubricants and Power Fluids. One-day seminar of Dr. Holger T. Sommer.
  5. Downs, D. “Keeping Hydraulic Fluid Clean,” Plant Engineering, March 1, 1997.
  6. Eleftherakis, J. and Khalil, A. Multipass Beta Filtration Testing For the 21st Century. Fall 2000, INJ.