Cleanliness is a term used to describe the relative quantity of solid contaminant particles in any given system which is large enough to cause a seizure or fracture which is irreversible, e.g. piston pump. One can gauge a fluid’s cleanliness by referring to a number of internationally-agreed standards. Every machine has an optimum cleanliness level. This level will be a balance between the maintenance of the machine’s efficiency and the cost to maintain cleanliness.
Cleanliness doesn’t just refer to the particles that you can see – there are some that are invisible to the naked eye. Have a look at the comparison chart below
How is oil cleanliness measured?
If a gear system is filtered, particle count data may be useful. But if the gearbox is not filtered, particle count data doesn’t provide as much information as would other tests such as PQ/FW (Particle Quantifier / Ferrous Wear) or DR (Direct Read) Ferrography. Diesel engine oil is black and requires different techniques to the tried and trusted laser counting methods. FW/PQ, DR Ferrography and Wear Debris Analysis are probably wiser choices. (WDA is a vital tool in failure and warranty issues).
The most common standard used to rate cleanliness is the ISO 4406. This table shows the ISO Codes that are used to represent the number of contaminant particles present in 1ml of oil
| ISO 4406 Number | Particles per mm Greater than(>) | Particles per mm Less than(<) |
|---|---|---|
| 24 | 80,000 | 160,000 |
| 23 | 40,000 | 80,000 |
| 22 | 20,000 | 40,000 |
| 21 | 10,000 | 20,000 |
| 20 | 5,000 | 10,000 |
| 19 | 2,500 | 5,000 |
| 18 | 1,300 | 2,500 |
| 17 | 640 | 1,300 |
| 16 | 320 | 640 |
| 15 | 160 | 320 |
| 14 | 80 | 160 |
| 13 | 40 | 80 |
| 12 | 20 | 40 |
| 11 | 10 | 20 |
| 10 | 5 | 10 |
| 9 | 2.5 | 5 |
| 8 | 1.3 | 2.5 |
| 7 | 0.64 | 1.3 |
| 6 | 0.32 | 0.64 |
| 5 | 0.16 | 0.32 |
| 4 | 0.08 | 0.16 |
| 3 | 0.04 | 0.08 |
| 2 | 0.02 | 0.04 |
| 1 | 0.01 | 0.02 |
The three-number code relates to the number of particles that are >4µm, >6µm and >14µm in size. (1.0µm* = 1.0 x 10-6m = 0.001mm) * The unit ‘µm’ (micrometer) is commonly pronounced as simply “micron”. So, an oil that gave readings of…
- 72,064 particles > 4µm;
- 16,519 particles > 6µm;
- 541 particles > 14µm;
…would, after referring to the table, result in an ISO Code of 23/21/16.
Effects of Poor Oil Cleanliness Poor quality oil can:
- Damage interacting components via abrasion;.
- Reduce component life via erosion;
- Obstruct critical flow paths;
- Damage servo or proportional valves;
- Increase quality problems;
- Increase customer dissatisfaction;
- Reduce market perception;
- Damage business opportunities;
- Raise costs
What are the likely failures due to poor oil cleanliness?
Sudden or catastrophic – This is caused when a small number of particles invade a critical space and create a torque reaction large enough to cause a seizure or fracture which is irreversible.
Intermittent – Similar to that above but usually caused by smaller size structures. Intermittent events will eventually lead to sudden or catastrophic failures. Typical examples are temporarily blocked or unseated spool/poppet valves.
Degradation – Typically characterised by flow erosion, abrasion, polishing and general wear.
A Powerpoint presentation on Oil Contamination and Particle Counting 
When to use Particle Counting Analysis
Particle counting is probably one of the best tests there is for maintaining system cleanliness. The point of contention lies with whether or not particle count is an appropriate test for all fluid types and systems. Whilst it is an excellent method for determining the number and size of particles being generated, particle counting won’t tell you what the particles are. They could be metallic – both ferrous and non-ferrous – silica (dirt, dust), silt, filter fibres, bacteria colonies, varnish agglomerations, water, etc. Therefore, the decision to do particle count Oil testing should be based on the type of information you want to collect.
The unit types that benefit most from particle count testing are hydraulics, compressors, refrigeration compressors, turbines, automatic transmissions, natural gas engines, robotics, injection moulding machines and “filtered” bearing or gear systems.
Particle count is also a good test for diesel fuel, solvents, water-based hydraulic fluids and lubricants. It’s also useful on new oils for the above units as it tells you how clean the oil is when introduced to the system. Many users aren’t aware of the fact that new oil may actually have higher particle counts than the used oil from the machine. Oil can pick up dirt during the manufacturing process itself, where filtration is expensive and therefore an indication of the quality control applied during the process. Unwanted particles are also picked up during transport and distribution as new oils are transferred from container to container.
If a gear system is filtered, particle count data may be useful. But if the gearbox is not filtered, particle count data doesn’t provide as much information as would other tests such as PQ/FW (Particle Quantifier / Ferrous Wear) or DR (Direct Read) Ferrography. Diesel engine oil is black and requires different techniques to the tried and trusted laser counting methods. FW/PQ, DR Ferrography and Wear Debris Analysis are probably wiser choices. (WDA is a vital tool in failure and warranty issues).