Technical perspective
A particle-count report may show an ISO code such as three numbers separated by slashes, while an older equipment manual may specify a single NAS class. Both expressions describe solid-particle contamination, but they are not two interchangeable labels for the same table. They group particle sizes differently and were developed around different counting and calibration conventions.
The practical question is not which code looks cleaner. It is whether the sample was representative, the particle counter and calibration were appropriate, and the reported code can be compared with the correct OEM or system target.
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What ISO 4406 reports
ISO 4406:2021 specifies a code for defining the quantity of solid particles in fluid used in a hydraulic fluid power system. In the commonly used automatic-particle-count format, the three scale numbers correspond to cumulative particle populations at 4 micrometres, 6 micrometres and 14 micrometres, expressed using the calibration convention indicated by the method and instrument.
Each scale number represents a concentration band rather than one exact particle count. Moving up by one ISO scale number broadly represents a doubling of particles within that channel. This makes the code compact enough for trending while preserving information about fine, medium and larger particles.
- Read all three ISO scale numbers; do not judge the sample from only the final number.
- Keep the particle-size notation and calibration basis with the result.
- Compare samples from the same point, under comparable operating and sampling conditions.
- Use the equipment or component manufacturer's current cleanliness target where one is available.
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How NAS 1638 differs
NAS 1638 is a legacy aerospace cleanliness classification that is still found in older OEM manuals, procurement specifications and maintenance practices. It evaluates particle populations across several size bands and is commonly reported as one class based on the most restrictive channel. SAE AS4059 is the current aerospace cleanliness standard that carries forward and explains relationships to the earlier NAS approach.
Because a single NAS class can be driven by whichever size range produces the highest class, it can hide the particle-size distribution that is visible in a three-part ISO 4406 code. Two samples with the same NAS class may therefore have different contamination patterns.
| Question | ISO 4406 | NAS 1638 |
|---|---|---|
| Typical report | Three scale numbers | A single class is commonly quoted |
| Particle grouping | Cumulative counts at three reporting sizes | Counts across multiple legacy size ranges |
| Common use | Hydraulic and lubrication systems | Legacy aerospace, defence and older industrial specifications |
| Current interpretation | Use the current ISO edition and stated calibration | Treat as a legacy requirement; review SAE AS4059 for current aerospace classification |
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Why a direct conversion can mislead
Conversion charts are convenient, but they are approximations. ISO and NAS codes do not use identical size channels or reporting logic. The result can also be affected by whether the instrument uses an older or current calibration convention. Reporting a converted value without the original measured counts can create false precision.
Keep the original result
Retain the raw or channel particle counts and the standard originally used by the laboratory.
State the calibration
Record the counter calibration and particle-size notation so historical trends are not mixed silently.
Use the target standard
Evaluate compliance against the standard actually named by the OEM, customer or maintenance procedure.
Explain any conversion
If a comparison is necessary, label it as an approximation and state the conversion basis.
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The sample can matter more than the code
Particle count is highly sensitive to sampling. A bottle exposed to dust, a sample taken from a stagnant drain, insufficient line flushing, aeration, water droplets or an unclean transfer device can change the result. A clean-looking fluid can still contain damaging fine particles, while a visibly dark fluid is not automatically assigned a poor cleanliness code.
- Choose a repeatable, live-zone sampling point that represents circulating fluid.
- Use a clean, compatible bottle and follow the agreed flushing procedure.
- Record whether the equipment was operating, recently filtered, topped up or disturbed.
- Repeat an unexpected result before making a high-consequence maintenance decision.
- Review water, viscosity, wear debris and filter evidence when the operating problem extends beyond cleanliness.
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Build a defensible cleanliness trend
Select one reporting standard for the programme, define equipment-specific targets, and keep the sampling point and method consistent. Where a legacy NAS limit must be retained, ask for the underlying particle-count channels as well as the NAS class. This preserves evidence for future comparison and makes a later transition to ISO 4406 or SAE AS4059 easier to manage.
Apply this guidance
