The above came from ISO Code Info.quote:
An understanding of what these codes mean requires a discussion of the structure and nomenclature of the codes. The structure can be broken down as follows:
16 / 15 / 12
The first number, "16", is the range code number that corresponds to a range of the number of particles present in an oil that are greater than 4 microns (µm or micrometers). That count is typically based on the number of particles per milliliter (ml). The second number, "15", is the range code number corresponding to the number of particles/ml that are greater than 6 microns and the third number corresponds to the number of particles/ml greater than 14 microns in size. It should be noted that the first range code includes the particles from both the second and third range codes. Likewise, the second range code includes the particles counted in the third range code.
The actual numbers of particles counted per milliliter in each size category (4, 6, and 14 microns) are converted to the appropriate ISO Code via the chart in Table 2.
For example, if a particle counter determined that there were 501 particles/ml greater than 4 microns, the corresponding ISO code would be 16. If 199 particles/ml were greater than 6 microns, the corresponding ISO code would be 15. Finally, if the counter determined that 27 particles/ml were greater than 14 microns, then the corresponding ISO code would be 12. The overall cleanliness of the lubricant would be reported as 16/15/12. Most particle counters do this conversion automatically. The following characteristics should be noted:
* The number of particles/ml will always correspond to the same ISO code independent of the particle size being measured.
* The overall ISO code structure - XX/YY/ZZ - will always correspond to the particle counts at the sizes:
o XX - greater than 4 microns
o YY - greater than 6 microns
o ZZ - greater than 14 microns
* Each ISO code increase represents roughly a doubling of the particle count
* However, an increase of only 1 particle/ml at a particular size can also give the appearance by the ISO code that the oil is twice as dirty
It should be noted that the ISO Cleanliness Code was revised in 1999. The earlier code used only two range values corresponding to particles greater than 5 microns and 15 microns. This would be reported in the following format: YY/ZZ. Additionally, some companies also used a range code for particles greater than 2 microns, and it would be reported similarly to the current code structure with the particle sizes corresponding to 2, 5, and 15 microns as opposed to the new 4, 6, and 14 microns. This change was due to the phasing out of AC Fine Test Dust (ACFTD) as the test method and replacing it with Medium Test Dust (MTD) and to improvements in the test procedure. There is no net effect on filter performance and minimal effect on how particle counts are reported with this change. The new reporting standard may be referred to as ISO 4406:1999 or ISO 4406 (MTD). The previous standard may be referred to as ISO 4406 (ACFTD). Results reported as a two "digit" code could be assumed to be the older standard at 5 and 15 microns. If results are reported as a three "digit" code it is assumed to be the new standard at 4, 6, and 14 microns. Many labs will report the appropriate sizes to avoid confusion.
Users should be aware of this as particle counters calibrated to the old standard are still widely used and will be for some time. Particle counters calibrated to the new standard are available and will become more common with time. Particle counters that are calibrated and report to the new standard will meet the ISO 11171 calibration standard.
I found it enlightening as I had no clue what the three numbers were in the particle counts vs. the two numbers expained in the kleenoil link provided by Filterguy.