ISO cleanliness codes are the global standard for quantifying solid particle contamination in industrial fluids like oil, fuel, and hydraulics. This guide breaks down how ISO codes work, why they matter, how to interpret them, and how to achieve your target codes with proper filtration.
What Are ISO Cleanliness Codes?
Simply put, ISO Cleanliness Codes are a standardized way to measure and report how many solid particles are in a fluid, broken down by size. This measurement provides valuable insight that helps identify how clean or dirty your oil, fuel, or hydraulic fluid is.
The ISO 4406 standard classifies the number of solid particles in a milliliter of fluid at three size thresholds:
First number – Particles ≥ 4 µm
Second number – Particles ≥ 6 µm
Third number – Particles ≥ 14 µm
Example: 18/16/13
18 → 1,300–2,500 particles ≥ 4 µm/mL
16 → 320–640 particles ≥ 6 µm/mL
13 → 40–80 particles ≥ 14 µm/mL
Key point: Each code step represents a doubling of particles in that size range.
Why ISO Codes Matter
Even particles too small to see can cause serious damage inside your machinery. In hydraulic systems, particles get trapped in tight clearances between valve spools and bodies, causing sticking or sluggish movement. In rotating equipment, particles score bearing surfaces and lead to premature failure. Over time, even a small increase in contamination can have a compounding effect on wear and heat generation.
It is because of this fact that cleanliness codes directly impact component life. Studies have shown that improving fluid’s ISO code by just one step can extend the life of pumps, valves, and bearings by 50% or more. That means lowering a code from 18/16/13 to 17/15/12 isn’t just a small improvement – it’s a significant, measurable reduction in the rate of wear and cost of operations & maintenance.
How are ISO Codes Measured?
To determine an ISO cleanliness code, a calibrated particle counter is used to identify the number of particles per milliliter in a fluid sample. The counter must be calibrated to the ISO 11171 standard to ensure the results are consistent and traceable across labs & equipment. Particle counters detect particles at multiple size thresholds – typically 4, 6, and 14 microns for ISO 4406 reporting.
There are two main testing approaches:
- Inline Monitoring: A particle counter is installed in the system for real-time measurement during operation
- Bottle Sampling: A sample is collected and analyzed on a portable counter or in a lab.
Inline testing is ideal for continuous contamination monitoring, while bottle sampling is often used for routine checks & post-service verification.
ISO Code Chart Reference
The ISO 4406 chart provides the particle count ranges that correspond to each code number. A partial reference looks like this:
For example, if your lab results come back as 17/15/12, that means the fluid tested contains 640-1,300 particles ≥ 4 µm, 40–80 particles ≥ 6 µm, and 5–10 particles ≥ 14 µm. Interpreting these numbers correctly is critical for setting contamination control priorities and choosing the right filtration approach.
Target ISO Codes by Equipment Type
Not all equipment requires the same level of cleanliness. Systems with tighter tolerances or high pressures often require cleaner fluids due to their greater sensitivity to contamination. Manufacturers often publish recommended or maximum allowable ISO codes for their machinery.
Here are common targets used in the field:
Hydraulic Systems: 17/15/12 or cleaner
High-Pressure Hydraulics: 15/13/10
Turbines and Lube Oil Systems: 16/14/11
Following OEM cleanliness targets isn’t just best practice – it can also prevent warranty disputes and prove that your maintenance program meets manufacturer requirements.
Achieving Target Cleanliness Codes
If testing shows that your fluid’s ISO code is higher (dirtier) than the target, you’ll need to remove particulate contamination. The method which is used to do so depends on how severe the contamination is and whether or not the system can be shut down.
Primary methods for reducing ISO codes:
- Offline Filtration (kidney loop): Circulates fluid through fine filters without taking equipment out of service.
- High-Velocity Flushing: Cleans systems aggressively during commissioning, repairs, or turnarounds.
- Filter Carts: Portable units for cleaning small reservoirs or topping off fluids with clean oil
- Upgrading Filter Elements: Switching to finer micron ratings or higher Beta Ratio filters to capture more particles per pass.
Supporting practices like sealing reservoirs, using desiccant breathers, and clean handling procedures are just as important for preventing recontamination after cleaning.
Role in Oil Analysis Programs
ISO cleanliness codes are most valuable when tracked over time. A single measurement is a snapshot, whereas consistent testing over time shows the health of a system. By logging ISO codes regularly, you can see gradual contamination buildup, sudden spikes from component failure, or the impact of maintenance interventions.
In a predictive maintenance program, ISO codes can act as an early warning system. For example, a sudden jump in the ≥ 14 µm category might indicate bearing wear or seal failure. Tracking cleanliness codes alongside other oil analysis parameters, like moisture content and acid number, gives a complete picture of fluid and component health.
How PFP Helps Achieve Target Codes
PFP provides the services and equipment needed to bring fluids into compliance with target ISO codes quickly and efficiently. Our fleet of rental filtration equipment includes high-velocity flushing skids, portable filtration units, and filter carts that can be deployed nationwide for planned maintenance or emergency response.
In addition to cleaning fluids, we provide on-site particle count testing and documented before-and-after ISO cleanliness reports. This proof of performance ensures you can demonstrate compliance to OEM requirements and internal reliability standards.
Give us a call at 888-679-6645 to discuss your contamination control needs.
