What are hydraulic fluid ISO cleanliness codes and how are they measured?

If you've been working on hydraulic systems for a long time, you'll quickly discover that the fluid within the system isn't just an instrument for transferring power, but it is the vital lifeblood of each component. Cylinders, valves, pumps, and actuators rely on clean fluid in order to perform reliably and meet their intended life span. But contamination, especially particulate pollution, is the most common reason for failures in hydraulic systems across the globe. This is the point at which ISO cleanliness codes come into play, a globally accepted, standardized method of detailing how clean or dirty the hydraulic fluid actually is.

Knowing ISO codes for cleanliness isn't only a technical task for engineers wearing lab coats. It's practical for anyone who operates or specifies hydraulic equipment. This article explains the meaning of codes and how they are determined and how reaching the correct cleanliness level could be the distinction between a system that is running for years and one that is failing ahead of schedule.

The problem of clean codes has been solved

Fluids used in hydraulics are not always completely free of contaminants. Even the freshest fluid from sealed drums contains some degree of contamination by particulates. When a system functions further, particles are introduced from the outside, are created by wear and tear of components, and then build up in time. The problem has been the same: the question of how to define the contamination in a manner that is comparable, consistent, and applicable across various kinds of equipment, industries, and geographical areas.

Before the standardization of codes was introduced, reporting of contamination was a bit sporadic. A lab could declare particles per milliliter, while another may employ gravimetric analysis in milligrams/liter, and neither could be translated into a decision about maintenance on the floor of the shop. ISO 4406 changed that by creating a Unicode system that converts data from raw particles to an easily readable, universally accessible format.

What exactly is ISO 4406? What does ISO 4406 actually mean?

ISO 4406 is the international standard to report the levels of particulate contaminants within hydraulic fluids and other lubrication products. The code is written in three numbers that are separated with forward slashes, for instance: 18/16/13.

Every number within the codes is the range of particles per milliliter fluid, but only at an exact threshold for particle size. The three sizes used to date are four micrometers (um), six um, and fourteen. In the code 18/16/13:

• (1) The number (18) is a sign of contamination of >=4 um(c) threshold
• 16). The number (16) is a sign of contamination above the 6 um(c) threshold
• The number 3 (13) is a sign of contamination of >14 um(c) threshold

The "(c)" designation refers to the calibration method used. "(c)" designates the method of calibration that is used, specifically, calibration using ISO middle test dust, which guarantees that particle counters in different labs produce similar results.

Each code number is the range of particle counts. The scale functions logarithmically. Every increment on the scale is roughly equivalent to doubling the amount of particles. To illustrate, a code of 14 refers to between 80 and 160 particles per milliliter. An 18 code means between 1,300 and 2,500 particles per milliliter. 22 is between 10,000 and 40,000 microliters of particles. The smaller the number, the less sluggish the fluid.

This scale of logarithms is designed. It lets the code span the entire variety of contamination levels, including ultra-clean servo systems to mobile devices that are heavily loaded—all while keeping the notation simple and easy to understand.

What is the best way to measure cleanliness?

There are two methods to determine particle contamination as well as assign the ISO clean code. These are automated particle counting and microscopic counting.

The Automatic Particle Counting (APC) is the most frequently employed method for contemporary hydraulic program maintenance. A sample of fluid is dragged by a laser or optical counter that determines the size of particles when they travel through a laser beam that is focused. The device counts particles at every size threshold and generates an amount of particles per milliliter, then maps these counts to ISO codes.

Automatic particle counting is speedy and repeatable. It is also capable of handling large quantities of samples. But it is also sensitive to water droplets, air bubbles, and the free water present in the fluid. These can be misinterpreted as particles and result in falsely high cleanliness codes. Proper sample preparation and degassing techniques are crucial for precise results.

Microscopic counting requires drawing a predetermined amount of liquid through a filter membrane and then inspecting the filter using an instrument that is calibrated and manually weighing particles by size. This process is slower and labor-intensive, but it gives direct evidence of the type of particle shape, morphology, and shape—data that could aid in identifying the source of contamination. Microscopic counting is commonly utilized to confirm the accuracy of automated particle counter results when the fluid is contaminated by air or water entrainment that can affect APC accuracy.

Both methods require proper sampling methods. An unrepresentative or infected sample could result in false results, regardless of how accurately the instrument is operating. The sampling ports should be placed in flow zones that are active—not drain or dead leg points. Additionally, the sampling line must be flushed thoroughly before the sample is taken. The samples should be collected in sealed, clean samples in ISO-certified bottles and then immediately analyzed to avoid particle settlement or contamination.

Cleanliness goals are set by system type

Different hydraulic systems do not require the same degree of cleanliness. The cleanliness requirement is mostly determined by the component that is most sensitive within the system since it sets the standard to determine how pure the fluid has to be.

Proportional and servo valves, due to their tight clearances inside (typically between 1 and 5 μm), need the purest fluid from any hydraulic part. Systems based around these valves generally aim for ISO cleanliness codes of 14/12/10 or better. The standard industrial systems using piston or gear pumps as well as the directional control valves usually work perfectly at 16/14/11. Mobile hydraulic equipment that is used in harsh fields might be able to handle 18/16/13, but cleaner is always the best option from a wear perspective. General-purpose systems using vane pumps or gear pumps may be able to handle 17/15/12.

Equipment manufacturers provide suggested cleanliness goals in their technical documents. It isn't a requirement in order to reach the recommended service life of the equipment.

What is the significance of this to system longevity?

The relation between cleanliness of fluids and component life is well established in the industry of fluid power. Field and research data have repeatedly shown that reductions in the ISO cleanliness standard by only two levels—which is cutting particle counts by about half—can increase the life of components significantly Some studies point to an increase of 50 percent or more in the lifespan of valves and pumps in the same operating conditions.

The particles cause damage via two major mechanisms. The second is abrasive wear, which is when hard particles that are trapped in the clearance spaces between surfaces cut and scratch the metal, gradually increasing the gap and reducing effectiveness. The other is fatigue wear, where the particles embedded into seals and bearing surfaces cause stress concentrations to create micro-cracks and eventually lead to seal spalling or failure.

Beyond the damage to components, contamination accelerates the degradation of the fluid itself. Metallic particles act as catalysts for the oxidation process, causing the breakdown of additives in fluids quicker and decreasing the service life for the petroleum.

Making ISO codes for cleanliness to work

For maintenance staff, ISO clean codes can provide an objective, quantitative reference for fluid state. The creation of a baseline cleanliness number for a system once it is brand new and functioning normally provides an ideal benchmark to which subsequent samples can be evaluated. An increase in codes—or dirtier fluid—is a sign that the filter may be damaged and a component might be producing abnormal wear debris, or an external contamination intrusion is taking place.

Regular oil sampling and monitoring of cleanliness codes as part of a predictive or preventive maintenance program permit issues to be identified before they become unexpected downtime or a catastrophic failure. The cost of taking a sample and analysis of particle counts is negligible when compared with the cost of a failing pump, a stuck valve or servo, or a shutdown that is not scheduled.

ISO cleaning codes serve as, in essence, a useful instrument for controlling one of the biggest aspects of hydraulic system performance. Understanding how they are read and how they are made and the specific targets that apply to your particular system will allow you to make informed choices and also to ensure that your hydraulic systems are functioning at their peak.