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)||
|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|
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)
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.
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.