What particle size channels does NAS 1638 use, and how do they differ from ISO 4406?

NAS 1638 counts particles in five size ranges: 5–15 µm, 15–25 µm, 25–50 µm, 50–100 µm, and >100 µm, reporting counts per 100 mL of fluid. ISO 4406 uses three cumulative size thresholds — particles ≥4 µm(c), ≥6 µm(c), and ≥14 µm(c) — reported as range numbers per mL. The different size channels and volumetric basis make direct numerical conversion approximate; no exact equivalence exists between NAS classes and ISO 4406 codes.

How many cleanliness classes does NAS 1638 define, and what does Class 6 represent?

NAS 1638 defines 14 cleanliness classes: Class 00, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12. Class 12 is the dirtiest; Class 00 is the cleanest. Class 6 permits a maximum of 128 particles per 100 mL at the 5–15 µm range, 32 at 15–25 µm, 8 at 25–50 µm, 2 at 50–100 µm, and 1 at >100 µm. Class 6 is approximately equivalent to ISO 4406 code 17/15/12 — suitable for medium-pressure hydraulic circuits with gear pumps and directional control valves.

What NAS 1638 class is required for servo valve hydraulic circuits?

Servo valve hydraulic circuits require NAS Class 5 or cleaner (equivalent to approximately ISO 4406 16/14/11 or tighter). Servo valves with spool-to-bore clearances of 3–5 µm are sensitive to particles in the 5–15 µm range — the predominant size channel at the NAS Class 5 boundary. Class 6 is acceptable for proportional valves with looser clearances; Class 5 or better is required for servo valves operating at high cycle frequencies where particulate wear accelerates clearance opening.

Why was NAS 1638 superseded by SAE AS4059 for aerospace applications?

SAE AS4059 (Aerospace Fluid Power — Cleanliness Classification for Hydraulic Fluids) was issued in 2005 and updated to AS4059F in 2013 to address limitations in NAS 1638. AS4059 aligns particle size channels with ISO 11171-calibrated APC methodology, enabling traceable measurement. NAS 1638 was based on optical microscopy counting — a manual method with operator-dependent accuracy. AS4059 also adds 2 µm(c) and 5 µm(c) size channels relevant to modern aerospace servo actuator clearances tighter than those addressed by NAS 1638's 5–15 µm lower limit.

How are NAS 1638 particle counts reported — per 100 mL or per mL?

NAS 1638 classifies particles per 100 mL of fluid sample, in contrast to ISO 4406 which uses particles per mL. When converting counts for comparison, the NAS count must be divided by 100 to obtain the per-mL basis used in ISO 4406. This volumetric scaling difference is one reason approximate conversion charts between NAS classes and ISO codes carry ±1 range number uncertainty — the volumetric basis and size channel boundaries do not align exactly.

Can NAS 1638 Class numbers be directly converted to ISO 4406 codes?

No direct mathematical equivalence exists. Published conversion tables provide approximate correlations — for example, NAS Class 6 ≈ ISO 17/15/12, NAS Class 8 ≈ ISO 19/17/14, NAS Class 10 ≈ ISO 21/19/16. These conversions are indicative only. At the borderline between classes, the same fluid sample may report different cleanliness levels under each system because of the different size channels and counting bases. Authoritative determination requires simultaneous measurement under both standards using ISO 11171-calibrated equipment.

What is the practical significance of NAS 1638 Class 4 for industrial hydraulic systems?

NAS Class 4 represents high cleanliness — maximum 32 particles per 100 mL at 5–15 µm, 8 at 15–25 µm, 2 at 25–50 µm, and 0 at >100 µm. Achieving Class 4 requires kidney-loop offline filtration with β₃(c)≥200 elements and a sealed reservoir with breather desiccant. Class 4 is required for high-pressure axial piston pump systems (>280 bar), high-speed servo valves in precision motion control, and electrohydraulic actuators in aerospace ground support equipment. A non-zero count at >100 µm indicates a seal failure or media bypass event requiring immediate investigation.

How does NAS 1638 treat differential versus cumulative particle counts?

NAS 1638 uses differential (size range) counts — particles are counted only within each defined size bracket: a 5–15 µm count does NOT include particles from the 15–25 µm range. ISO 4406 uses cumulative counts — the ≥4 µm(c) count includes all particles 4 µm and larger. A NAS 1638 count at 5–15 µm is therefore not numerically comparable to an ISO 4406 count at ≥6 µm(c) without applying the differential-to-cumulative conversion, which requires access to the full particle size distribution data from the APC.

What sample volume and sampling conditions does NAS 1638 require?

NAS 1638 specifies a minimum sample volume of 100 mL taken from the hydraulic system under normal operating conditions — with system pressure, temperature, and flow within operating parameters. Samples taken from stagnant reservoirs or cold systems underrepresent dynamic ingression particles from rod seals and actuator cycling. ISO 3722 sampling bottle conditioning procedures apply — bottles must be cleaned to at least two cleanliness classes below the target system cleanliness to avoid bottle-sourced contamination falsely elevating the NAS class result.

Does NAS 1638 specify a test dust for contamination level measurements?

NAS 1638 is a classification standard that defines acceptable particle count limits, not a test method standard, and does not specify a test dust for calibration. ISO 11171 (calibration of automatic particle counters using NIST-traceable PSL particles) provides the calibration methodology required for APC instruments used in NAS 1638 reporting. Older NAS 1638 measurements made with optical microscopy using AC fine test dust are not directly comparable to APC-based results because of fundamental differences between manual microscopy and electronic light-extinction particle counting.

Knowledge Center›Standards›NAS 1638

INDUSTRIAL STANDARD · 1964 (with subsequent amendments)

NAS 1638

Cleanliness Requirements for Parts Used in Hydraulic Systems

Hydraulic system components and fluids in aerospace, defense, and industrial applications.

KEY PARAMETERS

Class 00 to Class 12

Class range

5–15, 15–25, 25–50, 50–100, >100 µm

Size ranges

Particles per 100 mL

Count unit

ISO 17/15/12

NAS 6 ≈ ISO equivalent

01 /

NAS vs ISO 4406

NAS 1638 was the predecessor standard for hydraulic cleanliness specification, developed by the National Aerospace Standards committee. It uses a single-number class system (Class 00 to Class 12) based on particle counts at five size ranges: 5–15, 15–25, 25–50, 50–100, and >100 µm. NAS 1638 Class 6 is approximately equivalent to ISO 4406 17/15/12. ISO 4406 has largely superseded NAS 1638 in industrial applications but NAS 1638 remains in use in aerospace and defense specifications.

02 /

Class Definitions

NAS 1638 classes define the maximum particle count per 100 mL at each size range. Class 6: 5–15 µm = 32,000; 15–25 µm = 5,700; 25–50 µm = 1,012; 50–100 µm = 180; >100 µm = 32. Class 4: 5–15 µm = 8,000; 15–25 µm = 1,425; 25–50 µm = 253; 50–100 µm = 45; >100 µm = 8. Lower class numbers indicate higher cleanliness.

COMMON ENGINEERING MISTAKES

Treating NAS 1638 Class numbers and ISO 4406 Range Numbers as directly interchangeable. Different measurement bases and particle size channels make conversion approximate, not equivalent.

Applying NAS 1638 targets to modern servo and proportional valve systems specified to ISO 4406 without formal conversion verification. Clearance tolerance differences make direct substitution unsafe.

Using NAS 1638 for aerospace hydraulic system maintenance monitoring where the applicable standard is now SAE AS4059 in most OEM documentation.

ENGINEERING REFERENCES

STANDARD

NAS 1638, Cleanliness Requirements for Parts Used in Hydraulic Systems, Aerospace Industries Association

Primary cleanliness classification standard defining 14 classes (00–12) for hydraulic fluid particulate contamination based on differential particle counts per 100 mL at five size ranges: 5–15, 15–25, 25–50, 50–100, and >100 µm.

STANDARD

SAE AS4059F, Aerospace Fluid Power — Cleanliness Classification for Hydraulic Fluids, SAE International, 2013

Successor to NAS 1638 for aerospace applications; aligns particle size channels with ISO 11171 APC calibration, adds 2 µm(c) and 5 µm(c) channels for modern aerospace servo actuator specifications.

STANDARD

ISO 4406:2021, Hydraulic Fluid Power — Fluids — Method for Coding the Level of Contamination by Solid Particles, ISO Geneva

Parallel cleanliness classification system using cumulative particle counts at ≥4 µm(c), ≥6 µm(c), and ≥14 µm(c); commonly correlated with NAS 1638 classes in industrial hydraulic applications.

STANDARD

ISO 11171:2016, Hydraulic Fluid Power — Calibration of Automatic Particle Counters for Liquids, ISO Geneva

Calibration methodology for APCs used in NAS 1638 measurements; NIST-traceable PSL particle calibration ensures inter-laboratory reproducibility of particle count data supporting both NAS and ISO cleanliness reporting.

FREQUENTLY ASKED QUESTIONS — NAS 1638

ENGINEERING ARTICLES REFERENCING NAS 1638

NFPA T2.14 Hydraulic System Contamination Control: Component Sensitivity and Cleanliness Level Specification

11 min read

NAS 1638

NAS vs ISO 4406

Class Definitions

Related Engineering Content