Every hydraulic, turbine and gear system is bought and maintained against a cleanliness target, and that target is almost always written as three numbers separated by slashes: 18/16/13, 16/14/11, 22/21/18. The numbers are an ISO 4406 code. They exist so that a machine builder, a filter supplier and an oil supplier can describe the same oil the same way, and so that a particle count taken from a sample bottle becomes a specification somebody can act on.
What the three numbers count
The positions are fixed and always in the same order: particles 4 micrometres and larger, then 6 and larger, then 14 and larger. The bands overlap deliberately — a 20-micrometre particle is counted in all three — which is why the three numbers always step down from left to right.
Each position is a band code, not a raw count. Code 18 does not mean eighteen particles; it means the count landed inside one row of a table. Each row is double the row below it, so one code step is a factor of two. Moving a system from 18/16/13 to 17/15/12 halves its contamination. Moving it to 16/14/11 halves it again.
The code table
Counts are per 100 millilitres. The scale continues above and below the rows shown. Some instrument printouts give counts per millilitre instead, which is one hundredth of these figures for the same oil.
| Code | More than | Up to and including |
|---|---|---|
| 22 | 2,000,000 | 4,000,000 |
| 21 | 1,000,000 | 2,000,000 |
| 20 | 500,000 | 1,000,000 |
| 19 | 250,000 | 500,000 |
| 18 | 130,000 | 250,000 |
| 17 | 64,000 | 130,000 |
| 16 | 32,000 | 64,000 |
| 15 | 16,000 | 32,000 |
| 14 | 8,000 | 16,000 |
| 13 | 4,000 | 8,000 |
| 12 | 2,000 | 4,000 |
| 11 | 1,000 | 2,000 |
| 10 | 500 | 1,000 |
Reading a code
18/16/13 means: between 130,000 and 250,000 particles of 4 micrometres and larger per 100 millilitres; between 32,000 and 64,000 of 6 micrometres and larger; between 4,000 and 8,000 of 14 micrometres and larger.
The right-hand number is the one that matters most for bearings and servo valves, because 14 micrometres is the order of the clearances a particle has to bridge before it does damage.
A code is a range, not a measurement. Two oils both reported as 18/16/13 can differ by nearly a factor of two, which is why a code alone cannot settle an argument about whether oil is fit to return to a system. The underlying count can.
How NAS 1638 differs
NAS 1638 predates ISO 4406 and still appears on older drawings and in some aerospace work. It uses five size bands — 5 to 15, 15 to 25, 25 to 50, 50 to 100, and over 100 micrometres — and classes oil from 00 to 12, counting per 100 millilitres. Cleaner oil means a lower class, the opposite of ISO, where a higher code means dirtier oil.
There is no exact conversion. The two standards cut the size spectrum at different points, so a NAS class maps onto a band of ISO codes rather than a single one. NAS 7 is roughly ISO 18/16/13 and NAS 6 is roughly 16/14/11. Treat those as orientation, not arithmetic.
SAE AS4059 replaced NAS 1638. It keeps the class numbers and moves the size bands closer to ISO 4406, so a table quoting AS4059 sits between the two systems.
Targets in common use
These are widely quoted starting points. The machine builder's own requirement overrides any of them, and a system with servo valves or high-pressure vane pumps usually needs the cleaner end of the range.
| Application | Typical target |
|---|---|
| General industrial lube systems | 22/21/18 |
| Heavy gear and engine oil | 18/16/13 |
| Hydraulic and gearbox oil | 16/14/11 |
| Diesel fuel | 14/13/11 |
| Servo and proportional valve systems | 16/14/11 or cleaner |
What this means when specifying purification
A purification unit is specified against a starting code and a target code, not against a filter rating. The useful questions are how many passes at what flow are needed to move the 4- and 6-micrometre population down one or two code steps, and whether the machine can run online while the system stays in service — a unit that only treats drained oil cannot help a reservoir taking in water through a leaking seal.
Establish the starting count, the target code and the fluid volume before the model is chosen. Without those three figures a recommendation is a guess.
