Part 4: Understanding oil filter specifications and ratings.

Unscrupulous sellers of filters have been known to use the perceived complexity in the specifications of filters to misrepresent their products to ignorant buyers. However, a basic understanding of the terms and various rating systems can give the operator of hydraulic equipment the ability to purchase wisely.




Filter media is the actual material used to capture the contaminant in a filter element. Whatever media is used, it usually starts out in sheet form and is then folded or pleated to expose more surface area to oil flow. This is done to increase contaminant carrying capacity and to reduce pressure differential across the filter.pall-corp-filter-cutaway-web

The most common media include wire mesh, cellulose, fibreglass composites or other synthetic materials.

Wire mesh filters are constructed of either standard or stainless steel in a woven mesh. Because of the limitations in wire size and construction techniques, they are not available in very fine micron ratings. However, they have an advantage of being the only common type of filter element you can clean and then reuse.

Cellulose filters are made from wood pulp. The “paper” used for cellulose filters isn’t much more sophisticated than the paper used for writing. They are however cheap. The downside of this type of media is that it is not very efficient nor do they have a high dirt holding capacity. Cellulose also absorbs water, which may or may not be a benefit. They can be used to remove water from the system, however if this is not the desired effect and there is water in the system, the filter element will still absorb water. The pressure differential will increase and the filter will appear clogged.

Fibreglass is a type of synthetic filter media. In general, synthetic media performs considerably better than cellulose media because the fibres can be manufactured in a more uniform size distribution and thinner than cellulose media. Multiple layers of differing quality can be used to create a medium that is both extremely efficient and offers great dirt holding capacity.


Filter efficiency

Filter ratings are the most often misunderstood area of contamination control.

A nominal rating is an arbitrary size value assigned to a filter by the manufacturer. Tests have shown that particles as large as 200 microns will pass through a nominally rated 10-micron filter. Because the nominal rating is arbitrary, it has no value.

Another common rating for filters is the absolute rating. An absolute rating gives the size of the largest particle that will pass through the filter or screen. Essentially, this is the size of the largest opening in the filter although no standardized test method to determine its value exists. Still, absolute ratings are better that nominal ratings for representing the effectiveness of a filter.

The Beta Rating is the most commonly used efficiency rating in the hydraulics industry. This rating is based upon the Multipass Method for Evaluating Filtration Performance of a Fine Filter Element (ISO 16889:1999).


(Ref: Machinery Lubrication)

Particle counters accurately measure the size and quantity of upstream particles per known volume of fluid. Similarly, the size and quantity of particles downstream of the filter are also measured. The beta ratio is defined as the particle count upstream divided by the particle count downstream at the rated particle size. Using the beta ratio, a five-micron filter with a beta 10 rating, will have on average 10 particles larger than five microns upstream of the filter for every one particle five microns or greater downstream.

The efficiency of the filter can be calculated directly from the beta ratio because the percent capture efficiency is ((beta-1)/beta) x 100. A filter with a beta of 10 at five microns is thus said to be 90 percent efficient at removing particles five microns and larger.

Note that the beta rating is meaningless without quoting the size at which it is measured.

A filter’s beta ratio does not give any indication of its dirt-holding capacity, the total amount of contaminant that can be trapped by the filter throughout its life, nor does it account for its stability or performance over time. Operating conditions such as flow surges and changes in temperature are also not accounted for, nevertheless, beta ratios are an effective way of gauging the expected performance of a filter.

According to ISO 16889: 1999, absolute filters must have βx ≥ 75 (x = specified micron size), , which is 98.7% efficiency. Anything below βx ≥ 75 is considered a nominally rated filter. Such filters can drop as low as a beta ratio of 2, or 50% efficiency. As they get more and more clogged, nominal filters eventually start behaving like absolute filters. Nevertheless, inefficient filtration allows hydraulic oil to become lapping fluid which introduces further contamination to a hydraulic system and should be avoided.

Best Practice

Hydraulic oil cannot be “too clean”, and filter quality should be as good as space, cost and pressure drop will allow. If you have a reliable trustworthy hydraulic specialist, get their opinion on your filtration requirements. (Our details are on our website

Reference on our Website

A reminder that all parts of this series will be available on our website after they have been published for  quick and easy reference.

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