When grading oil filters, three main criteria must be considered: single-pass efficiency, multiple-pass efficiency, and the micron rating for particle pass-through.
Single-pass efficiency measures the ability of a filter to remove any contaminants or impurities on one single pass through the filter. Unfortunately, under actual driving conditions where dirty oil passes through the filter many times over several thousand miles, the single-pass test isn’t enough to measure a filter’s ability to protect your car’s engine. In cases like these, a multiple-pass efficiency is a better measurement. From tests and ratings, the best filters have a single pass efficiency of 98-99% while their multiple-pass efficiency is 96-98%.
Micron ratings, on the other hand, indicate what size of particles are permitted to pass through the filter. Ideally, the filter should have a micron rating of no more than 10-20 microns, with 10 microns being the optimal. Remember that the larger the particles that can get to your engine, the more damaging they are.
The most commonly used rating when it comes to filters is the Beta ratio. It is defined as the ratio of the number of particles going upstream of the filter to the number of particles going downstream, greater than a given size. For example, using the Beta ratio, a 5 micron filter with a Beta 80 rating will have an average of 80 particles that are larger than 5 microns upstream for every one 5 micron or greater particle. Using the beta ratio, a filter’s efficiency can be obtained by the formula (beta-1)/beta x 100.
Beta ratios, however, do not take into account field operating factors like changes in temperature or surges in pressure. In real life, these things affect filter and engine performance. Beta ratios also do not give any indication of the filter’s dirt capacity, the total number of contaminants it can trap throughout its life, or the change of the capture efficiency over time. In actual field conditions, regular monitoring of the cleanliness of the fluid using an ISO approved particle counting is used to determine the real efficiency of oil filters.
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