Filtration efficiency is normally expressed as the ratio of dirt entering a filter compared to the dirt exiting the filter of a specified size. The testing of filters is done under precisely controlled laboratory conditions so the results can be compared and statistically verified. Filtration tests attempt to accurately control the quality of the test dust, the flow rates, temperatures, measuring equipment and many other variables in order to ensure the repeatability of the test.

It is understandable that filters do not always perform in the field just as laboratory tests would suggest. Issues contributing to poor filter performance can include pulsating or surging flows, start-up contamination, and overall design flaws within the hydraulic system.

Pulsating flow — In the laboratory, filters are tested under steady state flow conditions. With the pulsating flow of real-world applications, significant reductions in performance can be experienced, Figure 4. Furthermore, if the magnitude and frequency of pulsations are great enough, they can actually cause premature failure of the filter. Therefore, precautions should be made to install filters to avoid subjecting them to pulsating flow.

Not only can efficiency vary with pulsating flow, but also different configurations of media can cause different efficiencies at increasing differential pressures. For example, Figure 5 depicts two elements that have the same micron rating (average efficiency) but differing maximum and minimum efficiencies and, consequently, varying dirt-holding capacity. The final point on the graph depicts element collapse. 

Contaminant ingression — Contamination concentration in a system is a function of many factors, but primarily the amount of contaminant in the fluid at startup and the rate of ingression. For most hydraulic systems, preventing contamination from entering the system is much more cost effective than removing it. No unfiltered air should be allowed to come into a hydraulic system. Unfortunately, many hydraulic system breathers consist of an open cover, tube or, a filler cap with no proper filter element inside. For details, refer to the recommendations in the accompanying sidebar.