DETERMINATION OF ISO CLEANLINESS
Appendix A can be used to determine the ISO 4406 number from the particle count (I). The first digit is taken from the particle count of 5 micron particles and the second digit is taken from the particle count of 15 micron particles. In 1999 the ISO 4406 standard was changed per ISO
4406:99 to report three ·numbers which were the contamination by particles >4 micron, >6 micron and>14 micron, respectively.
The key, however, is the focus on particles less than 15 micron, which will typically pass through a standard 10 micron in line filter. For example, an oil with a contamination of 30,000 of 5 micron particles per ml and 1,000 of 15 micron particles per ml would have an ISO rating of 22117.