Can an ISO cleanliness code be converted to ppm? Cold rolling mill coolant system debris (ash) is reported in ppm. These systems range from 10,000 to 25,000 gallons in size. We often check the system filtration by particle counts at 4,6,14 um. What is the correlation between the two and can it be converted?

Ppm (“parts per million”), refers to concentration of “something” in “something”, and is most often expressed in mg/kg or mg/l. In my opinion, you cannot convert ISO code to ppm unless you separate 4, 6, and 14  particles and determine their weight in a particular volume, and then calculate their concentration in a solution (oil in this case).

Anyway, why would you prefer cleanliness reported in ppm? How would it be possible to distinguish particle sizes, which is the primarily goal of ISO testing, from a single ppm account? I don't know how such reporting would be more practical and usable from the well-established ISO tables?