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They are the cornerstone of anything performance. Without filters, our fuel systems and engines alike would quickly deteriorate. As a home-based fuel producer, I want to be sure that what I am putting in my vehicle is as clean and pure as I can make it. What’s the big fuss? A filter is as good as it says right on the box, right? Prepare to learn everything you didn’t know about filters!


Filter ratings are often misunderstood. There are a few factors for deciding which filter to choose, some of which can be controlled and others that can’t. I have tried to find as much pertinent information on filters as I could and include it here, in one place.


  • Beta ratio and efficiency
  • Filter media
  • Build quality
  • Dirt holding capacity


  • Operational conditions
  • Environmental conditions


Filter Ratings:

To begin with, there are a few different ways that filter manufacturers rate their filters for efficiency.

  • Nominal ratings:

The nominal rating of a filter refers to its ability to remove a nominated minimum percentage by weight of solid particles greater than a given size (in micrometers or microns, µm). An example of this would be 98% of 20 microns. However, I believe a more common definition of nominally rated filters would be 50% of the nominal rating for particles of the given rating or larger. I have read that some tests show particles up to 200 microns pass through nominally rated 10 micron filters. Maybe a more exaggerated example would be better. A screen door will collect 10 micron particles, so it can be rated nominally as such. However, the screen door only captures 5% of particles 10 microns or smaller.

  • Absolute Ratings:

These are probably the better rating of choice when looking at buying filters. An absolute rating means that the filter is 98.7% or better at removing particles of a given size under laboratory conditions. It gives an accurate description of the largest size of particle that will pass through the filter media. These filters are usually much more expensive. Considered a better way to represent the effectiveness of a filter.

  • Mean ratings:

This is another rating that a few filters use in the industry, although not particularly popular. It uses a rating to show the filter is effective above a certain particle size. It seems to be a better representation than nominal ratings.

Beta Testing and Efficiency:

Beta ratings are the most commonly used ratings in the industry. They are derived from the multi-pass test methods for filter elements in ISO 16889:1999.The purpose of the multi-pass test is to provide a lab based procedure which will give reproducible data. The data can be used to interpret dirt-holding capability and contaminate particle efficiency of filter elements (beta ratio).

The multi-pass test begins by mixing a known amount (mass) of particles of known size into a hydraulic reservoir. The contaminated test fluid is pumped through the filter at a continuous and constant flow rate. During this test the differential pressure (difference between before and after filter pressure), upstream and downstream particle counts and amount of contaminate in the system are continually monitored. The test concludes when the differential pressures reach a specified limit or beta ratios (particle counts) no longer become acceptable. The test specifies that it is applicable to filters with an average beta ratio of 75 or greater. Each test is divided into ten smaller time frames. Particle counts are conducted during each time frame.


Using the particle counts from the time frames, one can calculate an average beta-ratio and a single beta ratio for the entire test. A beta value is calculated by counting the number particles upstream of the filter (size times microns and larger) and dividing that number by the amount of particles downstream (size x and larger). Calculating average beta ratios for each of the ten time period (sum particle counts and averaged for each period) and graphing them against differential pressure for each time frame allows for the beta ratio to be determined for different differential pressures. A single beta value for the entire length of the test can be determined by summing the average particle counts from each timeframe of upstream particle and dividing that by the sum of the average of the downstream particles. This is the value used to determine which particle size to rate the filter at. Each beta ratio particle count is done for one specific particle size.

Using the single beta ratios filter element percent efficiency for a given particle size can be determined. The math is:


There are standard whole number beta ratios which correspond to a specific percent efficiency.


While beta testing may look great on paper and may be reproducible in a lab, it does not take into account certain realities. Beta ratios do not account for operational conditions such as surges in flow or extreme temperatures nor does it account for stability over time. Depending on how the filter media is tested, it may not take into account build characteristics or flaws either.

Filter Media:

Dirt holding capacity is another crucial factor determined in the ISO 16889 multi-pass test. The dirt-holding capacity is how well a filter can retain particles once they have entered the filter media. This may be the most crucial factor in a filter. There are two main styles or filters: surface (membrane) and depth (bypass) filters.

Surface filters trap dirt on the surface and depth filters allow the oil to flow throughout the body of the filter and trap particles within the media.


In addition, both depth and surface filters have the following filtration properties:

  • Entrapment: Particles collect on the media due to pore size being smaller than the particle.
  • Adsorption: Natural electrostatic attraction between particles and media.
  • Impact: Particles impact media and become stuck due to adsorption.
  • Brownian Movement: This is less common, but it pertains to small particles suspended in a fluid having random movement in regards to flow.
  • Gravity: The natural force which helps larger particles settle out of oil.

Each filter has media with a specific pore size. As the pore size decreases, differential pressure increases. To compensate, the number of pores per unit of area must increase (porosity or pore density). Without enough porosity and too much differential pressure, a filter may bypass or fail.

Filter media is most commonly made of two different types of materials:

  • Cellulose: Generally a wood pulp material with inconsistent pore size and larger fibers. This material tends to have less holding capability than a synthetic material and tends to fail quicker in high acid and temperature environments.
  • Synthetic: Generally made of a fiber-glass style material. Has a very consistent pore size. Due to the fibers being much smaller, synthetic media has a much larger pore density and therefore a larger dirt-holding capacity. These filters are very durable and will withstand harsh environments.


 Filter Build Quality:

Last but not least, and certainly not to be forgotten, is the build quality. In the end, it really all boils down to this. You can make the best filter media, have the best advertising, and have the prettiest box. However, if you cut corners in the manufacturing process it’s all for naught. Russ Kinze performed an outstanding qualitative analysis on engine oil filters. It provides some pertinent information that would be difficult to replicate and some of the brand specific information carries over into hydraulic and fuel filters as well.

Engine Oil Filter Study

burst filter

Sweet! You know everything I know about filters. Now it all makes sense and you can go buy your buy your filters as an educated consumer…