A Comparison of NAS 1638 and ISO 4406 Cleanliness Codes
The NAS system was originally developed in 1964 to define contamination classes for contamination contained in aircraft components. The application of this standard was extended to industrial hydraulic systems as nothing else existed at the time. It is still referred to in some industries, although the ISO 4406 cleanliness codes are more commonly found.
The NAS 1638 coding system defines the maximum numbers permitted of 100mL volume at various size intervals (differential counts) rather than using cumulative counts as in ISO 4406.
Although there is no guidance given in the standard on how to quote the levels, most industrial users quote a single code which is the highest recorded in all sizes. This convention is employed with laser particle size analyzers.
Contamination level classes according to NAS 1638 (January 1964)
The contamination classes are defined by a number (from 00 to 12) which indicates the maximum number of particles per 100 mL, counted on a differential basis, in a given size bracket. Note that the comparison to ISO 4406 is necessarily an approximation.
Maximum Contamination Limits (per 100 mL)
|Size range in microns|
|Approximate ISO 4406 Equivalent||NAS 1638 code||5-15 µm||15-25 µm||25-50 µm||50-100 µm||Over 100 µm|
ISO Revises Standards for Particle Counting and Reporting
Fluid contamination is a primary root cause of mechanical component wear and reduction in system reliability and performance. Because of this, most companies have established programs to monitor and control contamination. These programs include particle counting. Three new or revised ISO fluid power standards will have a major impact on such programs. The changes are primarily the result of a new method for calibrating liquid automatic particle counters (APCs) that is traceable to the National Institute of Standards and Technology (NIST), but unfortunately produces a substantially different calibration. This article presents an introduction to the new calibration method including the impact on resultant particle sizes, particle counts and contamination classes.
AUTOMATIC PARTICLE COUNTER CALIBRATION
APCs have generally replaced optical microscopy as the primary method for quantifying particulate contamination in fluid wetted systems. In the late 1960s a calibration procedure was developed to ensure that particle counts obtained with an APC agreed as closely as possible with counts obtained by optical microscopy. The method, using AC Fine Test Dust (ACFTD), ultimately became International Standard ISO 4402 , and is still in use today by most oil analysis laboratories around the world.
For some time, industry had been aware that when more sophisticated instruments such as scanning electron microscopes are used to analyze the ACFTD, a substantial increase in the numbers of particles (especially below 10 µm) are reported compared to the distribution based on optical microscopes, given in ISO 4402. Also, in 1992, AC Rochester discontinued manufacturing ACFTD. Therefore National Fluid Power Association (NFPA) and ISO standards committees began to develop a revised APC calibration method based on a new contaminant whose particle size distribution could be NIST traceable. ISO 12103-A3  Medium Test Dust (ISO MTD) was selected as the best candidate. In 1993, NIST undertook an effort to certify the particle size distribution of suspensions of ISO MTD as a reference material for APC calibration. This effort resulted in NIST Standard Reference Material SRM 2806 consisting of a suspension of ISO MTD in MIL-H-5606 hydraulic fluid. Their analyses show a significant difference in the particle size distribution of ISO MTD as measured with an electron microscope compared to previous results with an APC calibrated per ISO 4402 (see Figure 1).
Figure 1 shows significantly more particles at sizes below about 10 µm were observed by NIST than with the ACFTD calibration. The higher particle counts are a result of the enhanced sensitivity of scanning electron microscopy when compared to optical microscopy performed in the 1960s. At sizes larger than 10 µm, fewer particles were observed by NIST. This is primarily because NIST reported the projected area equivalent spherical diameter of particles, which is smaller than the longest chord dimension used to generate the published ACFTD size distribution. The two methods intersect around the 10 µm level.
ISO has updated the current APC calibration procedure to use the SRM 2806. The revised method was approved on December 9, 1999, as ISO/FDIS 11171 . This calibration procedure includes many other enhancements to ensure better resolution, accuracy, repeatability and reproducibility; however, the effect of the new ISO MTD dust and NIST counts will have the largest impact. Also, a new designation, µm(c) is recommended for reporting particle sizes using the new ISO 11171 procedure, with the “(c)” referring to “certified” sizes traceable to NIST, so that the results are easily distinguished from those obtained with ISO 4402.
ISO has also developed a new procedure, ISO 11943 , for calibration and verification of online particle counting and dilution systems. ISO 11943 is applied to online particle counters used for contamination analysis of operating systems in the field as well as filter test systems.
REDEFINITION OF PARTICLE SIZES
Based on the size distribution of ISO MTD in Figure 1, for particle sizes below about 10 µm, the particle size determined by NIST for a given particle count is greater than the corresponding particle size per ISO 4402. The difference between the methods increases as the particle size decreases. For example, the particle count for 1 µm (ISO 4402 calibration) of about 2000 particles/µg corresponds to a particle size of 4.2 µm(c) (NIST size distribution). Thus, below about 10 µm, particle sizes with NIST (ISO 11171) calibration will appear higher than the old definition (ISO 4402), e.g., 2 µm is 4.6 µm(c) and 5 µm is 6.4 µm(c). Above about 10 µm, the opposite relationship exists; new particle sizes will appear lower than the old definition, e.g., 15 µm is about 13.6 µm(c). The relationship between particle sizes defined by the ISO 4402 and NIST size distributions for ISO MTD is shown in Table 1. Note that these changes will not impact particle counts obtained with optical microscopy, as these counting methods have not changed.
Because of possible differences between automatic particle counters and the accuracy of their original APC calibration, these relationships may vary slightly and must be determined for each APC to be used. It should also be noted that even though a particle size or count may change because of the new calibration, the actual contamination level in a system will not be influenced and will remain the same.
IMPACT OF NEW CALIBRATION ON PARTICLE COUNTS
Particle counts obtained with an APC calibrated with the new ISO 11171 procedure will differ from the corresponding particle counts obtained with the APC calibrated per ISO 4402 at any particular size. Users of particle count data must be made aware of the APC calibration method and how to interpret results when using the new calibration method. As a first approximation, historical automatic particle count data may be converted from ISO 4402 sizes to the new NIST sizes using Table 1.
The values in Table 2 illustrate the effect of the two calibration procedures on particle counts. As shown in the table, if one changes from ISO 4402 to the new ISO 11171 calibration without making adjustments to the sizes being monitored, significant differences arise in particle concentrations. For particle sizes smaller than about 10 µm, apparent increases in particle concentration will be reported which could prompt unnecessary action. The magnitude of the difference increases with decreasing particle size. For sizes larger than 10 µm, the reverse occurs and apparent decreases in concentration result from the new calibration. Failure to recognize this variation as the result of a change in calibration, rather than a change in contamination level, may lead to misinterpretation of particle count data and inappropriate action.
CHANGES TO THE ISO CODE
On December 2, 1999, ISO approved modifications to the ISO 4406 coding method based on the new calibration procedure. For APC counts, the revised procedure, ISO FDIS 4406.2 , uses three code numbers, corresponding to the concentrations of particles larger than 4 µm(c), 6 µm(c) and 14 µm(c) with the new calibration method. The new 6 µm(c) and 14 µm(c) sizes correspond closely to ISO 4402 sizes of 5 µm and 15 µm (see Table 1). These sizes were chosen so that no significant shift in code number occurs due to changes in the APC calibration method. For optical microscopy the calibration is unchanged and the two digits will remain the same as before at 5 µm and 15 µm. Thus, the second two digits of the actual code will be similar regardless of the calibration or measurement technique used.
In some instances, companies had previously adopted for their own use a 3-digit form of the ISO 4406 code with the third digit representing 2 µm. For these companies the change to three digits used in the new ISO 4406:1999 will be relatively easy because the second two digits will typically remain the same as before (only the particle sizes are adjusted). Because the first digit for the new code will represent 4 µm(c) rather than 4.6 µm(c) which more closely corresponds to the old 2 µm, an increase in the particle count and ISO code for this digit may be noticed. As a rule of thumb, specifications should be increased in the first digit by about 1 code level with the second two digits remaining unchanged. For example, a specification of 17/15/12 based on 2, 5, and 15 µm (ACFTD calibration) should be changed to ISO 18/15/12 based on 4, 6, and 14 µm(c) (NIST calibration).
Figure 2 reflects the impact on the ISO 4406 code for automatic particle counts conducted on two typical fluid samples. Note that for the new NIST calibration per ISO 11171, the cleanliness classes are determined by particle counts > 4, 6, and 14 µm(c). In each case, the second two digits of the code are nearly identical to the previous ISO 4406 2-digit code. Indeed, differences only result when observed particle concentrations correspond to transitions between two code numbers. As a result of the change in size classes with the revised code, the ISO code designation for a particular sample will not change appreciably, regardless of whether the new or old calibration method is used. This permits equipment users and others who specify cleanliness levels in terms of an ISO code to maintain their historical databases and their existing cleanliness specifications without change.
Changes In NAS 1638 - In order to implement the new NIST calibration with minimal impact on NAS cleanliness classes, it has been proposed to revise the reference particle size ranges in NAS 1638  based on the particle size relationship in Table 1; therefore, the actual particle sizes measured would be similar regardless of the method of calibration.
As an example, 5 µm per ISO 4402 calibration is replaced with 6 µm(c) per NIST calibration. Because optical microscopic particle counting procedures are not being changed, no changes are required in the cleanliness standards provided the size ranges counted correspond to the original size ranges set by the standards. In addition to the change in particle size, it has also been proposed that NAS 1638 be changed to utilize cumulative particle counts.
Table 3 shows the changes proposed in the NAS 1638 cleanliness classes for APC analysis. It should be noted that, because the first size range has been extended to 1 µm or 4 µm(c), new contamination limits are included.
Three new or revised ISO standards will have an impact on particle counting and reporting standards - ISO 11171, ISO 11943 and ISO 4406. The new APC calibration method, ISO 11171, utilizes NIST certified calibration samples, instead of obsolete ACFTD. ISO 11943 for online APC calibration is based on the revised calibration method and validates the entire on-line particle counting system, including a dilution system when used. ISO 4406, the revised solid contamination level code, uses the new calibration method and adds a third size, 4 µm(c), to the code for APC results. Sizes for the revised code were chosen to allow users of the code to retain their historical data base and leave cleanliness level specifications based on the code unchanged.
The adoption and implementation of these new and revised international standards will cause the following to occur:
- Apparent particle sizes when using an APC calibrated per ISO 11171 will change;
- Particle counts when using an APC calibrated with ISO 11171 will change;
- Cleanliness codes will change unless APC reported particle sizes are adjusted in accordance with the revised ISO 4406:1999 or the proposed revised NAS 1638.
The projected changes in each of these parameters have been discussed in this paper. Users should always keep in mind that any reported changes are an artifact of the methods utilized and actual contamination levels will remain the same as before.
1) ISO 4402, “Hydraulic fluid power - Calibration of automatic-count instruments for particles suspended in liquids - Method using classified AC Fine Test Dust contaminant,” 1991.
2) ISO 12103, “Road vehicles - Test dust for filter evaluation - Part 1: Arizona test dust,” 1997.
3) ISO FDIS 11171, “Hydraulic fluid power – Calibration of liquid automatic particle counters,” 1999.
4) ISO 11943, “Hydraulic fluid power - on-line liquid automatic particle-counting systems - Methods of calibration and validation,” 1999.
5) ISO FDIS 4406.2, “Hydraulic fluid power - Fluids - Code for defining the level of contamination of solid particles,” 1999.
6) NAS 1638, “Cleanliness Requirements of Parts Used in Hydraulics Systems,” 1992.
Dr. Leonard Bensch
Vice President, Pall Corporation
25 Harbor Park Drive
Port Washington, NY 11050
Practicing Oil Analysis (5/2000)