Change in particle count reporting

Bently Tribology Services is reporting particle count test results in a new format, consistent with the change in the ISO 4406 standard in 2000. This note serves to explain the differences in the reporting from previous format, and to address frequently asked questions about the change.

1. Reported Changes

There are significant changes occurring in the reporting of particle count results. They are:

Distribution change

BTS has always reported particle counts per two widely recognized particle distributions ie. ISO 4406 and NAS 1638.

The ISO particle distributions will change as follows:

We have always reported the ISO distributions as particles per 1 ml of fluid (p/ml). This will not change. The ISO coding format (e.g 18\16\12) is unaffected. The particle sizes have changed, and the new standard requires the size to be reported as follows: >4µm©

NAS Distribution

BTS has always reported the NAS 1638 distribution in addition to the ISO distribution because it afforded the end user a greater picture of the particle distribution, particularly above 15 µm. Reuslts were also, per the standard, reported in differential count(as opposed to cumulative per ISO)reported as particles per 100 ml (p/100ml). The new format will be an extension of the ISO standard, up to 70 µm (c) and the count will be reported as per 1 ml (p/ml) to be consistent with the new changes.

Frequently Asked Questions:

1. Why do we have to change?
   For many years, particle counters deigned for testing hydraulic fluid power applications have been calibrated with a standard dust (AC fine test dust) that did not have traceability with the NIST (National Institute for Standardards and Testing). The normal procedure for making a calibration fluid was to mix some of this fine Arizona road dust (chosen because it most closely resembled real world contamination) into super clean Mil-spec 5606 hydraulic fluid. Many labs would report variances in these calibration batches. It was decided that it was in everyones interest if the NIST classifies and certifies the standard material.
   
   
2. Whats the big difference?
   In the early 1950’s when the ACFTD was first employed, the particles were initially counted manually. This involves someone actually counting the particles and then classifying into size ranges. Optical microscopy techniques for particle measurement consist of the maximum chord diameter technique. This technique measure the maximum straight line diameter of an irregular shaped particle, and reports its size as such. This technique is very effective, and is widely used today for ferrography.
   Modern particle counters do not measure chord diameter, rather the light blocking sensor diodes produce a shadow for the detector, the size of which is based on the surface area of the particle. When the NIST evaluated the ACFTD, they used a SEM (Scanning Electron Microscope) which calculated surface area of particles, similar to particle counters.