What is a Micron?
The unwanted particles found in today's fuels are measured in microns. A micron is a unit of linear measure in the metric system used to measure distance from one point to another. The particles and contaminants that we are trying to filter out are far smaller than can be seen with the naked eye, magnifying glass or even a low powered microscope. A micron rating for a fuel filter is a generalized way of quantifying the ability of the filter's media to remove contaminants of a particular size.
For example, a filter with a micron rating of 7 will filter out contaminants of 7 microns or larger and a filter with a micron rating of 3 will filter out contaminates of 3 microns or larger. Filter efficiency refers to a measurement of the filter’s retention efficiency. The measurement is typically the number of particles retained by a filter, compared to the number of particles present in the fuel that will pass through the filter. A 7 micron fuel filter with 98.7% efficiency rating removes 98.7% of the particles at or above the 7 micron size.
In accordance with the World Wide Fuel Charter, Cummins and other engine manufacturers specify a minimum fuel cleanliness level of 18/16/13, with a maximum water content of 200 parts per million. The means of expression is a three number coding system (i.e. 18, 16, 13) that corresponds to the numbers of particles of a size greater than 4, 6 and 14 microns per milliliter respectively. The number 18 expresses that there will be a maximum of 2,500 particles equal to or larger than 4 microns; the number 16, that there will be a maximum of 640 particles equal to or larger than 6 microns; and, the number 13 that there will be a maximum of 80 particles equal to or larger than 14 microns in a milliliter of fuel. The critical particle sizes for modern injection systems are 4 and 6 microns. These particles are smaller than what any human could detect by the naked eye.