One of the most commonly asked questions about oil filters for passenger cars and light-duty trucks is “What is the micron rating of the filter?”. The inquirer is typically seeking information upon which to make a determination as to the quality of the filter and whether it is appropriate for the application under consideration. Often, the concept of “micron rating” and it’s relationship to filter efficiency is not fully understood which can result in an answer that is not meaningful.
What is a Micron Rating?
A micron is a unit of measure in the metric system equal to 1 millionth of a meter in length. For perspective – The average cross-section of a human hair is approximately 50 microns.
A micron rating is used to indicate the ability of an oil filter to remove contaminants by the size of the particles that the filter will collect and retain. For example, a filter with a micron rating of 25 will filter out contaminants of 25 microns or larger in size and a filter with a micron rating of 5 will filter out contaminants of 5 microns or larger in size.
However, in order for a micron rating to be meaningful, two pieces of information must be present – the contaminant particle size (in microns) and the filter’s efficiency percentage.
Filter efficiency refers to a measure of a filter’s ability to collect and retain particles of a specific size. For example, if a filter can collect and retain 95% of particles that are 25 micron-sized or larger, the filter can be said to be 95% efficient at 25 microns. In other words, a micron rating of 25 at 95% efficiency. The same filter may also be able to collect and retain 20% of particles that are 5 microns or larger. In a such case, the filter can be said to be 20% efficient at 5 microns or have a micron rating of 5 at 20% efficiency.
Both pieces of information must be included when stating the micron rating of a filter.
Stating a micron rating without an efficiency is misleading. As noted earlier, a micron rating is used to indicate the ability of an oil filter to remove contaminants by the size of the particles. It is important to remember that a micron rating without an associated efficiency does not fully describe the performance of a filter. For example, the same filter described above could be called a 5-micron filter if its efficiency at collecting 5-micron sized particles was, for example, only 20%. If the recipient of the information only hears “5-micron filter” without the efficiency percentage, the micron rating is misleading and therefore meaningless.
The two most common terms used to describe micron ratings are Nominal and Absolute.
Nominal Micron Rating
Nominal Micron Rating expresses the ability of the filter to capture particles of a specified size in microns at an efficiency of 50%. A nominal rating of 50% at 10 microns simply means that a filter captures 50% of contaminants 10 microns in size. The statement of this efficiency is important to understanding the performance capability of the filter.
Absolute Micron Rating
Absolute Micron Rating describes filter performance that is capable of removing at least 98.7% of a specific size particle. This rating is determined through a standard test in which oil containing measurable particles is passed through the filter. Particles that pass through are measured and counted. This rating is more informative than the nominal micron rating. Again, the particle size and efficiency must be stated.
Passenger car and light-duty truck oil filters typically have an absolute micron rating in the range of 25-30 microns.
Misunderstanding Efficiency and Micron Rating can result in selecting a filter that is not appropriate for the application under consideration. The Micron Rating must be stated in terms of the efficiency of the filter to collect the specific sized particles.
It can be said that the backstop at a baseball field is 100% efficient at collecting baseballs that hit it. The backstop, which is the filter in this case, is therefore 100% efficient at collecting baseball-sized particles. However, if one were to throw marbles at the backstop, there’s a good chance that many of them are going to get through. The backstop may be much less efficient at collecting marble-sized particles. Understanding that efficiency must be stated in terms of particle size is the key to the meaningful interpretation of a micron rating. Without both pieces of information, the actual performance claims of a filter may be misleading. That’s the truth about micron ratings.