Particle counting – determine fluid cleanliness ISO4406 / NAS / SAE4059

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 Causes Potential 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 /ml Number of particles / 100ml bottle ISO cleanliness code
5M – 10M 500M  –  1000M 30
2M – 5M 200M  –  500M 29
1.3M – 2M 130M  –  200M 28
640K – 1.3M 64M  –  130M 27
320K – 640K 32M  –  64M 26
160K – 320K 16M  –  32M 25
80k to 160k 8M  –  16M 24
40k to 80k 4M  –  8M 23
20k to 40k 2M  –  4M 22
10k to 20k 1M  –  2M 21
5k to 10k 500K  –  1M 20
2.5k to 5k 250K  –  500K 19
1.3k to 2.5k 130K  –  250K 18
640 to 1300 64K  –  130K 17
320 to 640 32K  –  64K 16
160 to 320 16K  –  32K 15
80 to 160 8K  –  16K 14
40 to 80 4000  –  8000 13
20 to 40 2000  –  4000 12
10 to 20 1000  –  2000 11
5 to 10 500  –  1000 10
2.5 to 5 250  –  500 9
1.3 to 2.5 130  –  250 8
64  –  130 7
32  –  64 6
16  –  32 5
8  –  16 4
4–8 3
2–4 2
1–2 1

particle count significance Particle counting   determine fluid cleanliness ISO4406 / NAS / SAE4059

Cleanliness of a lubricant is important as dirt and debris ingress can considerably decrea

se component life within machinery.

The three tables to the right 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 Type Typical new oil Usual cautionary value
Construction equipment hydraulics 20/18/15


(OEMs often quote 18/16/13 as ideal target)

Servo Valves 20/18/15 18/16/13
Gearbox 21/20/17 22/21/17
Steam turbine 20/18/15 19/17/14

word image 1 Particle counting   determine fluid cleanliness ISO4406 / NAS / SAE4059NAS 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 ISO4406 / NAS / SAE4059 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.