Particle counting – determine fluid cleanliness to ISO4406 / NAS / SAE4059. Achieve cleaner oil by monitoring it’s cleanliness with oil analysis.

This article will answer the following questions.

  • Is my lube oil contaminated?

  • Why do lubricating oils become contaminated?

  • Is my oil filter working and which oil filter should I use?

  • What is a ISO4406, ISO4407, NAS1638 or SAE4059 Particle count?

  • What sizes of particles are tested in ISO 4406 Analysis?

  • How small are the particles my filter removes?

Particle count – Particle counts measure the relative cleanliness of a fluid. This can be performed at different particle sizes from 4 microns all the way up to 70 microns. The most common cleanliness method is the ISO code system that measures the number of particles at 4, 6 and 14 microns. Traditional thinking suggests particles between 6 and 14 microns are the most damaging to equipment, but current thinking suggests smaller particle sizes such as 4 microns may also be damaging to systems with small machine tolerances, hence these are measured too. To easily express the cleanliness and allow comparison of large numbers. The code is expressed from smallest to largest from left to right, so a code of 20/19/15 means a code of 20 (500k to 1m particles in 100ml sample) greater than 4 microns, a code of 19 (250k to 500k) particles greater than 6 microns per 100ml sample and an iso code of 15 (16k to 32k) particles greater than 14 microns in a typical 100ml sample container.

Particle counts on reports are typically expressed per ml rather than 100ml since early 1990s, and the 100ml example above is just for simplicity to explain the number of particles in the sample bottle.

Common CausesPotential Result
Increased cleanliness code (dirtier lubricant)
  • Water contamination
  • Incorrect sampling and flushing before sampling
  • Inefficient filtration
  • Recent oil change
  • Dirt ingress through seals / header tank
  • Engine gelling and seizure
  • Poor fuel efficiency / mpg
  • Blogged filters
  • Reduced filter life
  • Increased wear
  • Carbon deposits.
No of particles /mlNumber of particles / 100ml bottleISO cleanliness code
5M – 10M500M  –  1000M30
2M – 5M200M  –  500M29
1.3M – 2M130M  –  200M28
640K – 1.3M64M  –  130M27
320K – 640K32M  –  64M26
160K – 320K16M  –  32M25
80k to 160k8M  –  16M24
40k to 80k4M  –  8M23
20k to 40k2M  –  4M22
10k to 20k1M  –  2M21
5k to 10k500K  –  1M20
2.5k to 5k250K  –  500K19
1.3k to 2.5k130K  –  250K18
640 to 130064K  –  130K17
320 to 64032K  –  64K16
160 to 32016K  –  32K15
80 to 1608K  –  16K14
40 to 804000  –  800013
20 to 402000  –  400012
10 to 201000  –  200011
5 to 10500  –  100010
2.5 to 5250  –  5009
1.3 to 2.5130  –  2508
64  –  1307
32  –  646
16  –  325
8  –  164
4–83
2–42
1–21

particle count significance Particle counting determine fluid cleanliness to ISO4406 / NAS / SAE4059. Achieve cleaner oil by monitoring its cleanliness with oil analysis.

Cleanliness of a lubricant is important as dirt and debris ingress can considerably decrease component life within machinery.

The three tables to the above show how to estimate the increased performance / component life gained by the improvement in cleanliness.

A value of 1 suggests the improvement in cleanliness has little effect on component life, whilst a value of 3 means theoretically, assuming the only failure type was contamination the component would last up to 3 times longer than if the cleanliness was not improved. In most cases the improvement will not be this large because of the difficulties in e.g. filtering the hydraulic lubricant to 4 codes less (16 times less particles) to achieve a 3 fold improvement.

Note these values above will vary between equipment manufacturers, load, operating temperature, lubricant type and maintenance practices in use. Some examples of typical cleanliness of the new oil and cautionary value are shown below. In many instances the new oil requires additional filtration to reach the target cleanliness codes and hence why oil changes should be avoided as the sole method of reducing high cleanliness codes. Some older manufacturers guidelines will quote only a 2 code e.g. 18/15. This is derived from when the particles used to be measured at 5 and 15 microns rather than 4, 6 and 14. Therefore please treat these target codes as expressing the last two values limits e.g. XX/18/15.

Application TypeTypical new oilUsual cautionary value
Construction equipment hydraulics20/18/1522/20/17(OEMs often quote 18/16/13 as ideal target)
Servo Valves20/18/1518/16/13
Gearbox21/20/1722/21/17
Steam turbine20/18/1519/17/14

word image 1 Particle counting determine fluid cleanliness to ISO4406 / NAS / SAE4059. Achieve cleaner oil by monitoring its cleanliness with oil analysis.NAS 1638 (particle count) / SAE4059. NAS 1638 is an older system that later became SAE 4059 and is predominantly used as an aircraft standard for cleanliness of fluids. The NAS code gives a maximum threshold per size at 100ml rather than a cumulative (i.e. greater than values) count as in ISO 4406. This was later dropped and became SAE 4059 which adopted cumulative codes with the same maximum values allowing for overlap with ISO 4406 measuring systems.

word image 2 Particle counting determine fluid cleanliness to ISO4406 / NAS / SAE4059. Achieve cleaner oil by monitoring its cleanliness with oil analysis. Since most particle counters tend to use smaller volumes than 100ml and then multiply up, and the fact the overall code uses a worst-case scenario coding system can lead to a couple of specs of dirt in the sample giving an instant code 12. Equally, the system fails when dealing with anything but the cleanest oils, hence its use in aviation where ultra clean oils are required. In industrial lubricants the use is limited as a new hydraulic of 20/18/15 would have an overall SAE4059 of ~10, whilst a barrelled gear oil may be 23/21/18 or ~12. Hence a new industrial gear oil and a dirty gear oil cannot be trended as the value is always ~12 as the max value.