How to interpret hydraulic oil particle count reports and act on them?

A report on the particle count of hydraulic oil will tell you the number of solid particles in a milliliter of fluid, as measured over certain sizes. The results are reported in the form of an ISO 4406/2021 cleanliness code, which is a three-digit rating such as 18/16/13. Each number indicates the level of contamination at three thresholds for particle size, which are >=4 um(c) and greater than 6 um(c) and >14 um(c). Understanding this report in the right way and acting upon it is among the most effective methods to stop the failure of hydraulic systems before they occur.

Why are reports on particle count important?

Hydraulic fluid is the vital component of every hydraulic system, but it's also the most frequently cited reason for failure. Research across the power fluid industry has consistently shown that particulate contamination is responsible for more than 70% of failures to hydraulic components. Cylinders, valves, pumps, and actuators all come with very tight clearances inside, which are usually between 1 and 25 microns, and even particles that are invisible to the naked eye can cause increased wear as well as valve sticking, seal degradation, and seizures of the component.

The report on particle counts provides you a precise measurement of the actual contents of your fluid. Without it, you're making assumptions. With it, you are able to base maintenance decision-making on information, which can prolong the life of your components while reducing downtime that is not planned and making sure that you avoid expensive repairs.

Knowing how to read the ISO 4406 cleanliness code

The ISO 4406 standard is the world-wide standard for reporting the cleanliness of hydraulic fluids. Each particle count report you receive will convert the raw count into this standard.

The code is composed of three numbers that are separated by slashes. Every number represents a range code, not the actual particle count, which corresponds to a specified range number of particles per milliliter.

• The first number refers to particles with a diameter greater than 4 um(c)
• The second number refers to particles greater than 6 um(c)
• The third number is a representation of particles greater than 14 μm (c).

Every range code will double its particle count by the code before it. Thus, a change from code 16 to 17 indicates that the level of contamination has increased by two times the contamination level, while an increase from 16 to 18 means that the contamination level has quadrupled. This scale of logarithms is significant because small changes in the code numbers represent huge changes in the level of contamination.

A report indicating 19/17/14 means the following:

• >=4 um(c): 250,000-500,000 particles/mL (code 19)
• >=6 um(c): 64,000-130,000 particles/mL (code 17)
• >=14 um(c): 8,000-16,000 particles/mL (code 14)

This would be a heavy contamination of the fluids used in most hydraulic systems.

Cleanliness levels to be targeted depending on the type of component

Different components within the hydraulic system are different in their susceptibility to contamination, depending on their clearances within the system and pressures at which they operate. Manufacturers typically provide the required level of cleanliness. Always adhere to the highest specifications for your system.

General guidelines:

In the event that your count of particles indicates an overall cleanliness score that is lower (higher) than the goal for the component that is most sensitive to your system, it is at risk.

The full report: going beyond that of the ISO code

A comprehensive report on particle count from an oil analysis laboratory has more than simply the ISO code. Knowing the details of each section can help to determine the root for contamination and not only the severity.

The distribution of particle size illustrates how contamination is dispersed across a range of sizes. A sudden spike in particle size at around the 14 μm+ level often suggests wear of metals from cylinders or pumps. A high level of particles in the 4-6 μm range with a small amount of larger particles is usually indicative of ingressed environmental pollution—dust, airborne particles, and other airborne particles that have entered through seals or breathers.

A trend in particle count over time is typically more valuable than one reading. The majority of laboratories have the historical data of previous samples. The gradual rise in the amount of contamination over many sampling intervals suggests that the filtration system you are using is losing ground—whether due to filters becoming overwhelmed, bypassing, or a new contamination source having popped up.

The ferrous particle count (PQ Index) determines the mass of iron-based particles present in the fluid. The presence of a high PQ index along with high ISO codes indicates active wear on the metal, not dirt that has gotten in. This is an obvious sign of deterioration in the component.

The amount of water content is typically included in particle counts because the presence of water can dramatically increase the damage caused by particles and encourages corrosion. If you find a reading of more than 0.1 percent (1000 parts per million) of mineral hydraulic fluids, it is alarming and that requires immediate attention.

Responding to the results An underlying decision-making framework

A report can only be effective if it triggers the correct response. Here's a useful decision-making structure:

When the value is equal to or above target, there is no immediate action to take. Keep your regular sampling interval (typically every 500 hours of operation or every 3 months). Verify that the condition of your filter is normal and that the breathers are in good condition.

If the score is less than 2 or 3 points higher than what is required: This indicates a yellow flag. Make sure to replace and check filters if they're in the range of 20 percent of their service lifespan. Inspect fill-point seals and the breather for any contamination that has entered the seal. Re-test after 100-200 hours to determine if the contamination is increasing or stabilizing. Do not prolong the time between draining oil.

If the value is greater than 3 points higher than what is required: This is an alarming red flag that requires prompt action. Complete a system flush with a kidney-loop filter with a flow of at least 3-5 times the capacity of the reservoir per hour. Replace all filters in the system. Check for contamination at the ingress points—rod seals, breather caps, and reservoir access covers—and the integrity of the fill port. Repeat the test within 4 to 8 hours after the kidney loop operation to ensure the flush's effectiveness before reintroducing the machine to its full capacity.

If the ferrous content is high along with an elevated ISO code: Pull the machine out of service. Examine the motor and pump for wear. Make sure to stop operating when metal is generating the fluid. The pollution will accelerate since wear particles damage other surfaces.

Common errors in reporting to avoid

Many errors can cause reports on particle count to provide inaccurate results. The technique used to collect samples is just as important as the analysis performed in the lab. The samples collected from above the reservoir, rather than from an actual return line, will exhibit artificially low levels of contamination. The samples taken prior to the start-up phase before the system is at operating temperature may miss particles inside cooler pocket fluids. Always take samples from a pressurized return line when the system is operating at a normal operating temperature. This is done with a specially designed sampling valve.

Equipment for sampling that is contaminated—dirty sample bottles, tubing that is not properly cleaned, or handling it without gloves—is a common cause of false high readings, which can lead to excessive maintenance. Only use clean bottles that have been pre-certified and supplied from your lab.

What is the best time to collect oil samples to conduct analysis of the particle count?

For hydraulic systems that are critical, you should take samples every 250-500 operating hours or every month, whichever is first. For less critical systems, a quarterly sample is generally sufficient.

Do I understand the results of a particle count without the aid of a laboratory?

Portable particle counters are able to be used for testing on-site and are becoming popular in predictive maintenance programs. However, laboratory tests provide additional information (ferrous content, water content, and viscosity) that portable counters can't be able to measure.

What's the difference between automated particle counters and counting with a microscope?

Automated particle counters (used by the majority of laboratories) utilize the blockage of light by lasers to measure and size particles with high speed. Countering with a microscope is slower but allows for visual identification of particle kinds (metallic, fibrous, and crystalline). For regular oil analyses, automated counting is sufficient.

Does an extremely low particle count mean the oil is safe to continue using?

Not necessarily. Particle count is a measure of the amount of solid contamination. There is a chance that oil may need to be replaced due to oxidation, viscosity degradation, depletion of additives, or water contamination—things that a complete analysis of oil, not just the count of particles, will uncover.

Particle count reports are among the most effective and underutilized tools for the maintenance of hydraulics. Being able to accurately read them and then respond in a systematic manner is the key difference between a reactive culture of maintenance and one that is truly predictive. The value of data is only realized in the event that it triggers the actions.