ISO 4406:1999 is commonly referred to as the cleanliness code and consists of a three number rating system referring to the contaminant levels of correlating particles, 18/16/13. The particles are measured when oil flows past a fine laser beam, which has been programmed to look for particles that are 4, 6 and 14 microns in size. The volume of these are then reported.
How does this relate to the 18/16/13 principle?
18/16/13 refers to the range in which these particles must fall within in order to be considered 'clean'. The particle volumes are then cross-referenced with the ISO Standard Table to find out whether the oil can be considered clean, with lower numbers in the code indicating fewer particles in the oil;
- 18 = between 1300 - 2500 particles larger than 4 microns
- 16 = between 320 - 640 particles larger than 6 microns
- 13 = between 40 - 80 particles greater than 14 microns
In order to ensure your oil meets these standards you will need to have it regularly tested and polished where necessary.