1) Gravimetric Measurement (ISO 4405)
2) Particle Size Distribution Analysis (ISO 4406 or NAS 1638)
3) Maximum Particle Size Analysis (ISO 4407).
This post will discuss the second method (above).
Particle Size Distribution Analysis (ISO 4406 or NAS1638)
Particle Size Distribution Analysis is a reporting method to gauge both the size and number of contaminant particles in a calculated quantity of hydraulic fluid. A fluid sample is either taken directly out of a hydraulic system or a known quantity of fluid is used to dislodge contaminants out of a hydraulic component. This fluid is run through a particle counting instrument to size and count contaminant particles.
These particle ‘counts’ can then be normalized by comparing the total component volume and to a corresponding ISO 4406 ‘code’ level of particle contamination. Levels of five and 15 microns of contamination are reported on a logarithmic scale corresponding to and ISO 4406 ‘code’ for the number of particles greater than or equal to these respective sizes per milliliter of fluid.