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.