An ISO-4406 cleanliness rating for fluids is expressed as three numbers, such as 17/15/10. The numbers, left to right, are codes for the approximate number of particles— contained in a very small (1-milliliter) sample—that are 4 microns and larger, 6 microns and larger, and 14 microns and larger. A 17 code in the example indicates that the 1-milliliter sample likely contains between 640 and 1,300 particles 4 microns and larger.
The ISO system actually is based on analyzing 100-milliter (about 4-ounce) samples with a particle-counting instrument to determine the number of particles present in each of the three size ranges. The code numbers and their associated particle counts are a function of the number 2 being multiplied by itself. A 10 code in the example means that 210 or 1,024 particles 14 microns and larger were in the sample.
Each code actually represents a range of particle counts, so a 10 means that the 100-milliliter sample contains between 29 (512) and 210 (1,024) particles. Since the ISO system is based on particles/milliliter, however, those counts are divided by 100, meaning that a 10 code represents a 1-milliliter sample with between 5 and 10 particles in a given size range. The ISO-4406 chart rounds particle counts in some instances for convenience.