Particle Counts: What Are They and Why Do They Matter

15 July 2022

AUTHORED BY: Reliability360

Contamination particles are a lot like children – the impact of each additional child is more of a multiplication factor instead of simple addition, especially when it comes to keeping the home clean. Much like the multiplication factor for how quickly a home requires a deep cleaning based on how many children you have, you can also determine how clean or dirty your oil is by multiplying the number of particles that you or your IoT monitoring system counts in your fluid, according to the cleanliness code developed by the International Organization for Standardization (ISO).

Table showing how ISO codes and cleanliness levels correlate with particle counts.
Cleanliness Level Correlation Table

Understanding the ISO Cleanliness Code

To fully understand the ISO 4406 solid contaminant standard, we need to go back to the beginning. This standard did not originate with ISO, but rather from the National Aerospace Standards (NAS) organization. During the 1960s, NAS attempted to bring order to the chaos of particle counts in aircraft hydraulic fluids. The result was the creation of NAS 1638.

The first version of this standard utilized an optical microscope to size solid particles. All the particles within 1 milliliter of oil would be categorized into five size ranges:

  • 5-15 microns
  • 15-25 microns
  • 25-50 microns
  • 50-100 microns
  • greater than 100 microns

A chart was used to classify the oil’s cleanliness with a range of 00 to 12, based on the number of particles in each size range. The lower the number, the cleaner the oil. Prior to this time, a coding system to quantify oil cleanliness had not been established. This method worked well and was largely accepted by the industry through the 1970s and ‘80s.

With improved particle filters, the invention of the automatic particle counter and the IoT fluid monitoring technology, and the push for ISO 9000, several other attempts were made to create industry standards for cleanliness levels in the ‘80s and ‘90s.

The improvements in particle filters tipped the scales of natural distribution. Filters became more efficient at removing larger particles, which made the larger size ranges less representative of the particle distribution in the oil. With this change in the natural distribution of particles, ISO decided it was no longer necessary to report the concentration of these larger particle ranges.

ISO 4406:87

Recognizing the importance of hydraulic oil cleanliness, the shortcomings of NAS 1638, and the deficiencies of other standards organizations, ISO aspired to create a standard that would more accurately reflect the concentration of particles. The organization also sought to make its classification code easier to understand while expanding the standard to all lubricating fluids so there would be one accepted standard across all industries.

The process began by reducing the number of categories from five to two with the potential for a third. They also used a scale that would count particles that were a specific size or larger. This was a direct move away from the previous particle range method. The representative particles sizes chosen were:

  • 2 (optional)
  • 5 microns
  • 15 microns

Unlike NAS 1638, which categorized all particle counts as a single number, ISO 4406 represented each size individually. An ISO 4406 code is always shown with the micron sizes listed from smallest to largest.

The second major modification involved changing the scale altogether. This brought about the creation of what would later be known as the Renard table. The classifications spanned from 0.9 to 30, with each doubling from the lowest to the highest acceptable value. This approach was intended to make each step more meaningful and impactful. It also allowed for a simple method of expressing very small and large particle counts with a single value.

For example, a cleanliness code might be something like 18/14. This would indicate that there were 1,301 to 2,500 particles larger than 5 microns and 81 to 160 particles larger than 15 microns. ISO later dropped the 0.9 code and started the chart at 1 because obtaining that level of cleanliness was highly unlikely and was considered unreasonable to include in the chart.

Table showing ranges for particles per milliliter and how these ranges align to ISO 4406 range numbers.

An ISO 4406 Chart

ISO 4406:99

In the 1990s, there was a push for industry to become ISO 9000 compliant. During this time, it was discovered that the current method for calibrating automatic particle counters (APCs) did not meet the requirements of the ISO 9000 standard. Previously, all APCs were calibrated according to ISO 4402 using Air Cleaner Fine Test Dust (ACFTD). During the process of becoming ISO 9000 compliant, it was determined that this calibration material was untraceable.

The exact quantity and size of the particles in the ACFTD were unknown, resulting in inaccurate calibrations. The company that produced the ACFTD calibration fluid also announced that it would no longer be manufacturing the fluid. This led to ISO 11171, which utilizes ISO Medium Test Dust (ISO MTD) from the National Institute of Standards and Technology (NIST). NIST employs a scanning electron microscope (SEM) to accurately measure the number and size of particles down to 1 micron.

In addition to the lack of control over the calibration material, it was also discovered that the particle sizes being reported weren’t the same. Depending on the type of APC that the laboratory used, there were inconsistencies when measuring the same size particles. What was a 5-micron particle in an optical APC (calibrated to ISO 4402) was a 6-micron particle using an SEM. This led to the creation of new calibration standards – the ISO 11171 for calibration and the ISO 4406:99 for reporting.

ISO was presented with a major decision to make: Should there be two standards based on the type of APC and method of calibration, or should the current standard be adjusted so it would be compatible with both calibration methods and APC types? It was decided that two standards would create confusion, so they chose to develop a compatible standard.

With subsequent accuracy improvements in microscope technologies, ISO realized that the reporting of particles smaller than 5 microns was becoming more reliable. In response, a third micron size option was added to the cleanliness code.

What It All Means

ISO concluded that the concentration of smaller particles was of greater concern than larger ones. The organization determined that 4, 6, and 14-micron particles provided the best representation of the particles closest in thickness to a lubricant’s film. These particle sizes cause the most damage to moving surfaces and should be closely monitored.

You also need to consider the method used to calibrate the automatic particle counters, especially if it can’t distinguish bubbles from particles. This will rarely be presented in an oil analysis report, so you will need to contact your lab or original equipment manufacturer (OEM) to find out which ISO calibration standard they used. The most current calibration standard is ISO 11171.


While ISO 4406:99 has been widely accepted, there is still a significant number of industries and countries that use SAE AS4059. Several revisions have been made to this standard, and the most current revision of this standard is SAE AS4059F. The reporting of this standard utilizes the same table as the NAS 1638 standard shown below.

The most current revision has added optional reporting methods to include the ability to report on the specified cumulative particle size count instead of a single code that represents the cleanliness of the oil. You can identify if the code is specific to a cumulative particle size count by the suffix letter after the class. The letter will be between A-E and will tell you what cumulative particle size count category the code represents. For example, a Class 6B would indicate how many particles are greater than 5 microns (optical microscope) or 6 microns (most APCs).

Be careful, though. The AS4059 revisions may result in different cleanliness classes than those obtained with previous versions, such as those that specified the class without any letter suffix. These cleanliness classes with no suffix are based on particles greater than 6 microns, whereas classes based on the revisions are based on the number of particles in each of the size ranges except for the smallest, which is 4 microns.

AS4059 now permits contamination limits to be identified in a variety of ways:

  • Identical to NAS 1638
    • Example: AS4059 Class 8 is the same as NAS 1638 Class 8. The size of particles counted varies depending upon whether an optical microscope or light blockage APC is used.
  • Cumulative count above a specified size
    • Examples: AS4059 Class 8A, AS4059 Class 8B
  • Differential counts for various sizes
    • Examples: AS4059 Class 8B-D or Class 8A-D
  • Different classes for cumulative counts of particles greater than a particular size range
    • Example: 7B/6C/5D or 7B/4C.

There isn’t a right or wrong cleanliness standard to use when setting up a lubrication program. The important thing is that you follow a standard and use it correctly.

Table breaking down cleanliness classes for cumulative particle counts.

(1) Size range, optical microscope, based on
longest dimension as measured per ARP598 or APC
calibrated per ISO 4402:1991
(2) Size range, APC calibrated per
ISO 11171 or electron microscope, based on
projected area equivalent diameter
(3) Classes and contamination limits identical to NAS 1638

Setting Cleanliness Targets

Unlike most of the information in an oil analysis report, particle counts should not use the same trending methods as materials such as iron, silica, and copper. You will want to watch which direction your particle counts are moving, but this alone shouldn’t be used as a failure indicator. There are too many variables in play. The difference between codes can be one particle or several thousand particles. For this reason, it is recommended to set cleanliness targets and track particle counts or ISO codes relative to those targets or to use a true optical digital particle counter that can recognize different wear particles and distinguish bubbles from actual particles.


Both cautionary and critical alarm limits should be established. Ideally, the cautionary limit will be your target or one code above. The alarm limit should be two or three codes above your target. These limits will help make your decision-making process easier, such as if you’re considering whether to clean your oil with a filter cart or perform a full drain, flush, and fill.

Setting cleanliness targets for each lubricant offers several benefits. The main advantage is the extension of overall machine life, which can lower maintenance costs and increase production cycles. Case studies have shown particle contamination to be the leading cause of machine failures. Therefore, it only makes sense to monitor concentrations of these particles to control contamination.

Another benefit of setting cleanliness targets includes increasing the awareness and visibility of the efforts being made through various lubrication activities. Establishing a moderately aggressive target for your lubricants will require everyone to follow lubrication best practices. It can also create a common goal among maintenance and operations personnel for plants in which the lubrication duties are spread across multiple departments or divisions.

Since not all machines are treated equally, it will be important that cleanliness targets be set appropriately for each machine. For example, hydraulic and turbine systems are far more sensitive to solid contaminants than a gearbox or process pump. The oil’s viscosity will also play a significant role. The higher the viscosity, the more difficult it will be to remove particles.

Machine Environment

The machine’s environment can impact your cleanliness targets as well. A hydraulic system in a controlled environment, such as a pharmaceutical production area, may have a different target than the same hydraulic system inside a cement plant. More potential contaminants in the air can increase the need to maintain cleaner oils, which will require a more aggressive standard. A high-contaminant area may also demand a larger investment to help maintain the appropriate oil cleanliness levels.

Most OEMs provide general lubricant cleanliness levels for their machines to meet the warranty expectations. While these OEM targets make for a great starting point, they often do not take into consideration your specific environment or plant reliability goals. If your facility has a centralized lube room, set an overall cleanliness target for each oil type. When establishing these targets, begin with the most sensitive machine component using each oil and work through the same process for each machine.

Once you have set your cleanliness targets, take a stepped approach to cleaning your oils. Starting with a high-efficiency filter at a small micron level would be expensive. Instead, slowly progress to smaller micron filters over time. When you reach your goal, you can then maintain this cleanliness level with higher efficiency filters.

By better understanding the cleanliness code, setting appropriate targets, and closely monitoring your particle counts, you can determine how dirty or clean your oil is, which will help you achieve your overall reliability goals.