PI App notes


The challenge

Early detection of wear particles in lubricating and hydraulic fluids is critical to having a proper predictive maintenance program. It is this early detection and identification of wear particles that permits the extension of engine life and can minimize down-time of equipment. The challenge is to count the number of contaminant particles between specified size ranges, and to classify them into several types.


Applicable measures


Equivalent circular area diameter
Bounding rectangle width
Bounding rectangle length
Bounding rectangle aspect ratio
Range of acceptance

3 - 500 microns
0 - 1.00
0 - 1.00
2.0 - 100 microns
5.0 - 100 microns
1.0 - 50.0

Typical image

oil containing metallic and water droplet contaminants Image showing ECAD micron sizes. Calibration is 0.62 microns/pixel. The large circles are water droplets.


ISO 4406 Standard for Oil Cleanliness

The ISO 4406 standard was established in 1987 as a method of coding the level of particle contamination of hydraulic fluids. The most recent revision of the standard, 1999, has been changed to account for the use of a different calibration standard for optical automatic particle counters. The sizes to report now are specified as ≥ 4µm, ≥ 6µm, and ≥ 14µm. After the counts for these three sizes are determined, they are summarized in a kind of shorthand known as Scale Numbers:

ISO Code   17   / 15   / 12

The first of the 3 numbers relates to particles ≥ 4µm, and is a code indicating the approximate count: Scale Number 17 means that the count (expressed as a concentration) was between 640 particles / ml of fluid and 1300 particles / ml of fluid.

The second scale number relates to particles ≥ 6µm. In this case, the value 15 is indicative of a count between 160 particles / ml and 320 particles / ml.

The third scale number codes the amount of particles ≥ 14µm in size. A value of 12 indicates the count was between 20 and 40 particles / ml.

The ISO 4406 standard lists scale numbers that code a wide range of possible particle concentrations.


ISO 4406 results table The three 4406 concentrations are shown in the Scale Number shorthand as 15 / 15 / 14.

Particle Insight presentation at Reliable Plant conference

Reliable Plant description

Reliable Plant 2014, the annual conference and exhibition for machinery lubrication, oil analysis and reliability professionals, was held in San Antonio Texas on April 22-24, 2014. This industry event offered extensive pre-conference workshops and learning sessions covering today’s trends, technologies and issues. One of the session workshops covered the use of image analysis for this industry. Here are links to view the presentation given.

Reliable Plant article

YouTube    Particle Insight presentation

YouTube    Particle Insight presentation (Spanish)

Particle Classification results