Particle Counting (ISO)
Particle counting is actually a test for particle contaminant levels and not specifically wear debris. It does not distinguish between
wear and dirt particles, but if it can be determined that nonferrous contamination has remained stable, then an increase in the particle count must be
attributable to wear. A magnet can be used to modify the particle count to count ferrous debris only. There are various ways of doing this, but essentially
a magnet holds back the ferrous debris while the nonferrous debris is flushed from the sample, after which a ferrous debris particle count is performed.
Particle counts are invariably reported according to ISO 4406:99. Other standards do exist, but they are not as commonly used. ISO 4406:99 returns a three-digit
solid contamination code. The method of particle counting is not as important as performing the test properly. It is important to note that only results from the
same method should be compared.
ISO 4406 Standard
The ISO 4406/2000 classification of particle contents was introduced to facilitate comparisons in particle counting. Sudden breakdown in an oil
system is often caused by large particles (>14 micron) in the oil while slower, progressive faults, e.g. wear and tear, are caused by the smaller particles
(4-6 micron).This is one of the explanations why the particle reference sizes were set to 4 micron, 6 micron and 14 micron in ISO 4406/2000.
A typical sample of dirty oil contains in every 1 ml of oil:
- 45,000 particles >4 micron
- 12,000 particles >6 micron
- 1,000 particles >14 micron
The oil sample described above would have a contamination class of 23/21/17 - see Particle Contamination ISO classification table.