Hydraulic Filter ratings are an often misunderstood area of contamination control. On several recent occasions, I have witnessed someone describing a Hydraulic Filter Element by its nominal rating. A nominal rating is an arbitrary micrometer value given to the Hydraulic Filter by the manufacturer.
Actually these values have little effect. Tests have shown that particles as large as 200 microns will pass through a nominally rated 10-micron Hydraulic Filter. If someone tries to sell you a Hydraulic Filter based on an "excellent" nominal rating of five microns, run away. Be sure that your fitler supplier provides clear information about filtration products that includes a beta rating, but more on that in a moment.
FAbsolute Rating of Oil Filter
Another common rating for Hydraulic Filter Element is the absolute rating. An absolute rating gives the size of the largest particle that will pass through the Hydraulic Filter or screen. Essentially, this is the size of the largest opening in the Hydraulic Filter although no standardized test method to determine its value exists. Still, absolute ratings are better for representing the effectiveness of a Hydraulic Filter over nominal ratings.
Beta Rating of Hydraulic Filter
The best and most commonly used rating in industry is the beta rating. The beta rating comes from the Multipass Method for Evaluating Filtration Performance of a Fine Hydraulic Filter Element (ISO 16889:1999).
To test a Hydraulic Filter, particle counters accurately measure the size and quantity of upstream particles per known volume of fluid, as well as the size and quantity of particles downstream of the Hydraulic Filter.
The 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 Hydraulic Filter with a beta 10 rating, will have on average 10 particles larger than five microns upstream of the Hydraulic Filter for every one particle five microns or greater downstream.
The efficiency of the Hydraulic Filter Element can be calculated directly from the beta ratio because the percent capture efficiency is ((beta-1)/beta) x 100. A Hydraulic Filter with a beta of 10 at five microns is thus said to be 90 percent efficient at removing particles five microns and larger.
Caution must be exercised when using beta ratios to compare Hydraulic Filters because they do not take into account actual operating conditions such as flow surges and changes in temperature.
A Hydraulic Filter's beta ratio also does not give any indication of its dirt-holding capacity, the total amount of contaminant that can be trapped by the Hydraulic Filter throughout its life, nor does it account for its stability or performance over time.
Nevertheless, beta ratios are an effective way of gauging the expected performance of a Hydraulic Filter Element.