What Happened to NAS 1638?

Mike Day, Pall Corporation

National Aerospace Standard (NAS) 16381 is a particulate contami-nation coding system used in the fluid power industry to simplify the commu- nication of data from particle counters. It converts the particle counts at various size ranges into convenient broad-base classes. The particle numbers can range from single particles to many millions; therefore, a power series is used to cover the number range with a convenient number of classes.

NAS 1638 was conceived in the 1960s to control the amount of contamination delivered in aircraft hydraulic components, and became an American National Aerospace Standard in 1964. No coding system existed at the time for completed systems, so it was logical that it would be applied in this area.

NAS 1638 saw widespread acceptance in the 1970s and 1980s by industries where reliability was a prerequisite, in areas such as offshore oil production, iron and steel industries, etc. It led to the development of other coding systems, the most notable being the International Organization for Standardization (ISO) 44062.

If you obtain the latest issue of NAS 1638 you will find the following statements: “Inactive for new designs after May 30, 2001, see AS4059C” and “6.1.3 This standard should not be used with Automatic Particle Counting.”

This is a result of recent changes to the ISO contamination standards for automatic particle counter (APC) calibration3, which necessitated the review of NAS 1638 and resulted in its withdrawal for newly designed systems.

NAS 1638 Contamination Classes

NAS 1638 was the forerunner of other contamination coding. The concept of the code can be seen in Table 1 and it is based upon a fixed particle size distribution of the contaminant over a size range of >5 to >100 µm. This distribution was based on particle contamination inside delivered aircraft hydraulic components in the 1960s.

From this basic distribution, a series of 14 classes was created covering very clean to very dirty levels, where the interval between each class is double the contamination level. This principle is a feature of many of the classes that have developed since.

The method of counting the particles referenced the optical microscope method defined in ARP 5984, simply because this was the only method in existence at the time. Another method of measuring the concentration of particles is included in the standard of gravimetric analysis, which is the weight per volume (mg/100 mL) with nine codes for this rarely applied technique.

Since its inception in the 1960s, the use of finer rated filters in hydraulic systems meant that the distribution of particles did not follow NAS 1638 as the proportion of larger particles (>15 µm) was reduced through filtration. These relative low numbers generated significant errors in data at the larger sizes (>25 µm) due to the use of bottle samples5.

Trending these sizes was fraught with problems and inaccuracies. This, and the fact that all five of the size ranges were not needed, led to the development of ISO 4406 in the early 1970s. This system was originally based upon the numbers of particles at >5 µm and >15 µm, but has since changed with the updates to the ISO standards.

Another criticism of NAS 1638 was that there was little guidance on how to use or apply it. For this reason, and to generally improve the standard, the SAE A6 Aerospace panel developed AS40596 for aircraft hydraulic systems. This subsequently became de facto ISO 11218. Few aircraft companies adopted this standard, and very few industrial companies even knew it existed despite the fact that it overcame the deficiencies of NAS 1638.

Changes To APC Calibration Method

The introduction of the APC during the 1960s revolutionized the measurement of the size distribution of dirt particles in a hydraulic system. The accuracy, repeatability and speed of analysis allowed it to be used extensively for research into the effects of dirt on components and systems. Oklahoma State University, under the direction of Prof. Ernest C. Fitch, was the main force behind the introduction of ISO 44027 - the method for calibrating APCs.

This was based upon the size distribution of A.C. Fine Test Dust (ACFTD), a silica-based test dust used in the automotive and hydraulics industry. The size distribution was derived in 1964 using the optical microscope, so in effect, the APC was set-up to record identical numbers of ACFTD particles as the microscope. A praiseworthy step because it should give identical results - in theory.

The notice of termination of the supply of ACFTD in 1992 prompted a need for a replacement. It also gave the overseeing-ISO committee the opportunity to obtain traceability for the particle size distribution (PSD) of the calibration material. The lack of traceability was a problem in the 1990s as more companies endeavored to achieve ISO 9000-type quality systems.

It was also embarrassing for laboratories involved with contamination testing which often had to explain why there was no traceability and control over the dust.

ISO Medium Test Dust (ISO MTD) was the selected replacement and by the National Institute of Standards and Technology (NIST), who also certified the PSD. The NIST used a Scanning Electron Microscope (SEM) with an Image Analysis software package to precisely identify the size and numbers of particles down to 1 µm.

As explained in Leonard Bench’s article, “How the New ISO Particle Count Standard will Affect You,” published in the May-June 2000 issue of Practicing Oil Analysis magazine, the new size distribution differed from that in ISO 4402 (size for size), and showed that the earlier data greatly underestimated the numbers of particles less than about 7 µm.

The net result was that if the APC was set to the same size (for example, >2 µm, >5 µm, etc.), the APC with the new calibration would record more small particles than one calibrated to ISO 4402 on the same sample. This is a complex issue and is fully described in ISO TRI63868. Because this situation could cause an uproar within the industry when companies would have to change specifications, the ISO committee decided to take the road of least resistance and selected new size descriptors that would give the same particle counts while retaining integer sizes.

The term “µm (c)” is used to differentiate data obtained using the two methods. The only major difference in particle counts is likely to be at the 4 µm(c)-size, which effectively has no previous equivalent; but as few organizations have cleanliness specification based upon the size, the change would be minimal.

The Replacement For NAS 1638

AS4059, which was developed in 1988, has undergone four revisions since and is now (at the time this article was written) at issue “D”. The author considers AS4059 to be a significant advance over NAS 1638 because it offers many benefits. The most significant changes are:

  • Uses ISO 11171 calibration for APCs, leading to increased precision, and improved repeatability and reproducibility,

  • Presents data in terms of cumulative counts (>X µm; >Y µm) rather than interval mode (X-Y µm); this is how particle counters present the data,

  • Introduces a cleaner class - 000,

  • Extends the size range to smaller sizes (>4 µm(c) for increased sensitivity,

  • Allows flexibility in cleanliness specification,

  • Gives information on how to apply it,

  • Gives the background to the changes, and

  • Gives guidance for sample bottle cleanliness and sampling.

The basis of the code is seen in Table 3.

Click here to see Table 3.

Notice that the same particle numbers are used in AS4059 as NAS 1638 (to verify this try subtracting the >15 from >5 µm numbers and comparing it with the 5 to 15 µm in NAS 1638). In the transition period, the standard applies to both methods of calibration of APCs (ISO 4402 and 11171).

Many are specified for analyzing oil samples and presenting data to NAS 1638 format for existing designs. The reasoning behind this is not entirely clear, but thought to be to separate old and new APC data. APC users are recommended to switch to the new standards.

In Table 3 it can be seen that the particle counts are defined by a number (relating to the quantity of particles) and a letter (relating to the size). This concept gives AS4059 much more flexibility than was possible with NAS 1638 because it is often the control of the cleanliness level that specifiers require rather than a fixed distribution; for example, they can specify improved control over critical sizes or relax controls over those that are not. Equally, sizes that are not critical can be omitted. There are three reporting options:

  • Specifying at single sizes - AS4059 6B which means that only the >5 µm/>6 µm(c) is subject to a cleanliness specification.

  • Specifying for specific sizes - AS4059 6B/ 6C/ 5D/ 4E/ 4F, note that 1 µm/4 µm(c) has no specification and tighter controls are required over the larger sizes.

  • Specifying cleanliness over the complete size range - AS 4059 6A through F.

Now What?

Is NAS 1638 dead? Well . . . not quite. NAS 1638 can be used for existing systems where correlation with earlier data is necessary. However, users are encouraged to change to AS4059 as soon as possible so that everyone is speaking the same language.

Current specifications, like those based on a single number, for instance NAS 1638 Class 6, can be swapped to AS4059 Class 6B through F to provide the same control over the 5 to 100 µm-size range, or perhaps even updated to reflect current and modern requirements. Although the current version of AS4059 allows two methods of calibrating APCs, the use of ISO 11171 is recommended as it gives improved precision.

One aspect of the change, which may be a little unclear, is that of component cleanliness - the scope of the original NAS 1638 document. As NAS 1638 is inactive for new components/systems, and AS4059 was developed for hydraulic systems, how is the cleanliness of components defined? The Society of Automotive Engineers (SAE) Committee has been made aware of this and is deliberating. The author recommends the use of AS4059 system for commonality using sizes B to F, and relating the particle counts to 100 mL of the components wetted volume.

Another aspect that is giving rise for concern is the statement in clause 6.1.3 that forbids the use of APCs with NAS 1638. This is in spite of cleanliness specifications being written around them. The reason for this is not entirely clear but it is thought to be the need to separate old and new APC data. APC users are recommended to switch to the new AS4059 standard.


  1. NAS 1638 “Cleanliness requirements of parts used in Hydraulic systems,” Aerospace Industries of America, Washington D.C., USA, 2001.
  2. ISO 11171 “Hydraulic fluid power - Calibration of automatic particle counters for liquids,” International Organization for Standardization, Geneva, Switzerland, 1999.
  3. ARP 598 “The determination of particulate contamination in liquids by the particle count method,” SAE, Warrendale, Pa., USA, December 1986.
  4. Day, M.J., and Rinkinen, J. “Contaminant monitoring of hydraulic systems - the need for reliable data,” Presented at 10th International Congress and Exhibition on Condition Monitoring and Diagnostic Engineering Management, Helsinki, June 1997.
  5. ISO 4406 “Hydraulic fluid power - Fluids - Method for coding level of contamination by solid particles,” International Organization for Standardization, Geneva, Switzerland, 1999.
  6. AS4059 “Aerospace fluid power - cleanliness classification for hydraulic fluids,” SAE, Warrendale, Pa., USA, April 2001.
  7. ISO 4402 “Hydraulic fluid power - Calibration of automatic count instruments for particles suspended in liquids - Method using classified AC Fine Test Dust,” International Organization for Standardization, Geneva, Switzerland, 1991.
  8. ISO TR16386 “Hydraulic fluid power - Impact of changes in ISO fluid power particle counter, contamination control and filter test standards,” International Organization for Standardization, Geneva, Switzerland, 1999.


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