Hydraulic Fluid Cleanliness Standards

The cleanliness of hydraulic fluids needs to be monitored for maintaining the components of hydraulic systems at a satisfactory level. Many national and international organizations such as ISO, SAE, National Aerospace Standards (NAS), etc., have developed standards for specifying the particle size classification and contamination concentration levels in hydraulic fluids. All standards specify the contamination level in counts per fluid volume and provide easy methods for converting the particle counts into limits that are simple to interpret. Here is an overview of these standards.

The cleanliness classes are based on particle size [differential (e.g. 5 – 15 µm) or cumulative (e.g.>6 µm)], number, and distribution. Before proceeding further, the knowledge of the following terms and definitions would be of great help in understanding various cleanliness standards.

Methods of particle counting

There are two basic methods of counting particles in hydraulic fluids.

  1. By using an optical microscope where the longest dimension sizes the particles. [See Figure 1(a)]
  2. By using automatic particle counters (APCs) calibrated as per ISO 11171, where the particles are sized by the area rather than the length. [See Figure 1(b)]

Figure 1 | Sizing of particles

Particle size analysis

Several methods and instruments, based on different physical principles, are used to determine the size distribution of the particles suspended in a given sample of hydraulic fluid. The numbers of particles found in the different size ranges characterize this distribution. A particle to be analyzed can be sized by: (1) the longest dimension, as sized by a microscope, or (2) the equivalent projected area, as sized by APC calibrated as per ISO 11171.

The ISO System

In ISO 4406: 1999, particle counts are determined cumulatively, for particle sizes > 4 μm(c), > 6 μm(c), and > 14 μm(c) as per the size classification standard ISO 11171, using particle counters and allocated to measurement codes. In 1999, the previous version of the standard, i.e., ISO 4406:1987, was revised and the size ranges of the particle sizes redefined. The contamination code rating system as per ISO 4406: 1999 is given in Table 1.

The NAS System

The NAS 1638 was originally developed in 1964 to define classes for the contamination present within aircraft components. The application of this standard was extended to industrial hydraulic systems as no other standards existed at the time.

In the NAS 1638 classification, the code number refers to a maximum quantity of particles within a specific size class. The NAS system divides particles into five particle size ranges. A series of 14 classes were specified, covering very clean to very dirty levels. The method of counting the particles referenced the optical microscope method. The NAS code specifies a single code number based on the highest particle count in any of the size ranges. As the code number goes high, the degree of contamination increases for any size range. This standard is now considered obsolete. However, it is still widely used in old systems. The cleanliness codes as per NAS 1638 is given in Table 4.

The SAE System

The SAE aerospace standard AS4059 was developed in 1988 as a replacement/equivalent to the obsolete NAS 1638 format. Since then this standard has undergone six revisions and is now at issue ‘F’. This standard specifies contamination classes and levels of particulate contamination in hydraulic fluids. This standard offers two classifications. One classification, based on the microscopic counting, applies to those currently using NAS 1638 classes and desiring to maintain the methods/format. The other alternative, based on the automatic particle counting, applies to those using the methods of previous revisions of AS4059 and/or cumulative particle counts. The cleanliness classes for differential particle counts are given in Table 2, and the cleanliness classes for cumulative particle counts are given in Table 3.

Meaning of Index (c)

Particle size specifications usually contain the index(c), [Example: 4 μm(c)]. This notation is used to indicate that the calibration material used is certified and traceable to a national standard.

A Comparative Study

A comparative study of these standards in terms of the methods of particle counting, particle size classification, contamination concentration levels, and the applicability of these standards is undertaken in the following sections.

Method of Particle Counting

  • ISO 4406: 1999 uses the electron microscope counting method
  • SAE AS 4059 Rev F uses the optical counting method or electron microscope counting method
  • NAS 1638 uses the optical counting method

Particle Size Classification

  • ISO 11171 specifies the following three-dimensional cumulative sizes of particles:

>4 µm(c)

>6 µm(c)

>14 µm(c)

  • SAE AS 4059 specifies the following size ranges of particles for the optical counting method:

6 -14 μm(c)

14 -21 μm(c)

21 -38 μm(c)

38 -70 μm(c)

>70 μm(c)

  • SAE AS 4059 specifies the following cumulative sizes of particles for the automatic particle counting method:

> 4 μm(c)  (Code A)

> 6 μm(c)  (Code B)

> 14 μm(c)  (Code C)

> 21 μm(c)  (Code D)

> 38 μm(c)  (Code E)

> 70 μm(c)  (Code F)

  • NAS 1638 specifies the following differential sizes of particles:

5 – 15 µm

15 – 25 µm

25 – 50 µm

50 – 100 µm

> 100 µm

Contamination Concentration Levels

  • In ISO 4406: 1999 specifies the cleanness level of a given sample of fluid by a three-number range code representation, based on the cumulative numbers of particles of sizes greater than 4, 6, and 14 microns respectively, present in one millilitre of the fluid.
  • SAE AS 4059 specifies the cleanness level of a given sample of fluid by a single figure representing the maximum allowed cumulative particle counts (i.e. worst case), present in 100 ml of the fluid, for the designated particle sizes according to the particle counting method.
  • NAS 1638 specifies the cleanness level of a given sample of fluid by a single figure (from 0 to 12) representing the maximum allowed differential particle counts (i.e. worst case), present in 100 ml of the fluid, for the designated particle size ranges.

Applicability of the Standards

  • In ISO 4406: 1999 standard is widely used throughout the world for determining the hydraulic fluid cleanliness.
  • AS4059 class using differential particle count method applies to those currently using NAS 1638 classes and desiring to maintain the methods/format, and results equivalent to those specified in NAS 1638.

AS4059 class using cumulative particle counts applies to those using the methods of previous revisions of AS4059 and/or cumulative particle counts

  • The NAS 1638 cleanliness standard was developed for aerospace components in the US and is still widely used for industrial and aerospace fluid power applications. It may be noted that NAS 1638 has now been made inactive for new designs.

For information on a complete range of Paperback & Kindle eBooks on Pneumatics and Hydraulics, authored by Joji Parambath, please visit: https://jojibooks.com

Joji Parambath


Fluidsys Training Centre Pvt Ltd


Email: info@fluidsys.org


  1. Aerospace Fluid Power – Contamination Classification for Hydraulic Fluids AS4059F https://www.sae.org/standards/content/as4059f/
  2. Automatic particle counters for fluid contamination control by Noria Corporation
  3. Fluid Condition Handbook, SYSTEM CONDITIONS, MP FILTRI S.p.A. www.mpfiltri.com
  4. Hydraulic Fluids, Joji Parambath, https://jojibooks.com
  5. Lubricating Oil Laboratory (Swansea), Swansea
  6. Particle Measurement Technology in Practice. From Theory to Application, HYDAC Filtertechnik GmbH, Industriegebiet 66280 Sulzbach / Saar, Germany, www.hydac.com
  7. Swift-JB International, LLC is a division of Swift Filters, Inc

Note: The Tables of cleanliness standards are not included here. The complete article with the Tables can be downloaded by clicking the following link:

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