ISO 4406 Oil Cleanliness Code: How to Read It and What Target to Set

The ISO 4406 oil cleanliness code is written as three numbers, for example 18/16/13. These are range numbers for particles ≥4, ≥6 and ≥14 µm(c) per 1 ml of sample: the higher the number, the dirtier the oil, and each step up means roughly twice as many particles. The target code is chosen by the most sensitive component in the system and the manufacturer's requirements.

How the ISO 4406 code works

The code consists of three scale numbers separated by slashes:

Position in the code	Particles counted
First number	all particles ≥4 µm(c) per 1 ml
Second number	all particles ≥6 µm(c) per 1 ml
Third number	all particles ≥14 µm(c) per 1 ml

The count is cumulative: a 20 µm particle is included in all three numbers, because it is larger than 4, 6 and 14 µm. The (c) suffix means the sizes come from an automatic counter calibrated to ISO 11171.

The first edition of the standard reported the code at two sizes, ≥5 and ≥15 µm. The 4, 6 and 14 µm(c) sizes came with the new calibration of automatic counters; according to Ayalytical (supplier of PAMAS counters), 6 and 14 µm(c) correspond to the former 5 and 15 µm. Microscope counting still uses two sizes, 5 and 15 µm, with a dash in place of the first number: for example, -/18/13.

Scale number table

The scale number is determined by the number of particles per 1 ml. Each step is roughly double the previous one (a Renard series). The ranges below are from the Donaldson table, converted to 1 ml (the original table gives values per 100 ml).

Scale number	Particles per 1 ml, more than	up to and including
24	80,000	160,000
23	40,000	80,000
22	20,000	40,000
21	10,000	20,000
20	5,000	10,000
19	2,500	5,000
18	1,300	2,500
17	640	1,300
16	320	640
15	160	320
14	80	160
13	40	80
12	20	40
11	10	20
10	5	10
9	2.5	5
8	1.3	2.5

How to read the code: an example

Take the result 18/16/13:

1. 18: more than 1,300 and no more than 2,500 particles ≥4 µm(c) per 1 ml.
2. 16: more than 320 and no more than 640 particles ≥6 µm(c).
3. 13: more than 40 and no more than 80 particles ≥14 µm(c).

If the next sample reads 16/14/11, the oil is about four times cleaner at each size: two steps, each a factor of two. The reverse is also true: one step up means roughly twice the contamination.

What the code does not show

Noria (Machinery Lubrication magazine) warns that too much weight is put on the ISO code and that it should play only a secondary role in evaluating a sample:

• The code coarsens the trend. Within one step the particle count can double without changing the number. If the target is 19/17/14, the previous sample was 18/16/13 and the new one is 19/17/14, the target is formally met, although particle ingress may have risen two- to fourfold.
• You need the raw counts. Laboratories often report counts at more sizes, for example >4, 6, 10, 14, 21, 38, 70 and 100 µm. The raw data is what lets you cross-check the particle count against elemental analysis and ferrous content.
• The code says nothing about what the particles are: dust, wear metal or soft oil degradation products. Other tests are needed for that; see Oil Analysis and Tribology: the Third Pillar of Reliability.

What cleanliness target you need

Look for the target first in the equipment manufacturer's documentation. If there is none, use a table of recommended values. Below is an extract from the table published by ExxonMobil in its Mobil technical topic on oil cleanliness (petroleum-based fluids, ISO 4406:1999, sizes 4/6/14 µm(c)).

Component	Pressure below 140 bar	140–212 bar	Above 212 bar
Ball bearing	15/13/10	-	-
Roller bearing	16/14/11	-	-
Journal bearing (high and low speed)	17/15/12	-	-
Industrial gearbox	18/16/13	-	-
Fixed gear pump	20/18/15	19/17/14	-
Variable piston pump	18/16/13	17/15/12	16/14/11
Proportional directional valve	17/15/12	17/15/12	16/14/11
Servo valve	16/14/11	16/14/11	15/13/10
Cylinder	17/15/12	16/14/11	15/13/10

How to use the table, per the same source:

1. Pick the most sensitive component. If one reservoir feeds several systems, the target is set by the most demanding consumer.
2. For non-petroleum and water-based fluids (for example water glycols), use a code one step lower than the table.
3. Go two steps lower if the equipment has frequent cold starts, excessive shock or vibration, or the component is critical to system reliability.

In Kazakhstan the second and third conditions are common: excavator hydraulics and mining plant drive gearboxes start in frost in winter and run under shock loads. Sampling and laboratory analysis practice for such units is covered in Laboratory Oil Analysis for Gearboxes and Hydraulics at Mining Plants.

How cleanliness is measured

Method	How it works	Notes
Automatic optical counter, ISO 11500	The sample passes through a capillary in a laser or white light beam; each particle casts a shadow on a photocell, and the drop in signal gives its size	The most widely used method, fully automated, fast results
Counter with sample dilution, ASTM D7647	Same optics, but the sample is first diluted with solvent	Removes some soft particles and counts hard ones; slower than ISO 11500
Microscope, ISO 4407	The sample is filtered through a membrane and particles are counted under an optical microscope	The original method; very time-consuming, rarely used today
Online monitor	A sensor in a bypass of the lubrication line counts particles continuously	Real-time trend without taking samples

Automatic counters are calibrated to ISO 11171. About 100 ml of sample passes through a laboratory counter, and the result is converted to 1 ml.

Whatever the method, the sample must be representative. As ExxonMobil stresses, incorrect sampling changes the cleanliness of the oil in the bottle, skews the result and leads to wrong conclusions.

Laboratory or on-site instrument. With an external laboratory you have to ship the sample and wait for the result. On-site analysis removes that wait: oil can be checked right after an oil change, filtration or repair, and a decision made immediately.

Which instrument to choose

• Comprehensive on-site oil analysis: Spectro MiniLab 153 (AMETEK Spectro Scientific). According to the manufacturer, it runs four tests in under 15 minutes: elemental analysis (ASTM D6595), particle count with ISO codes and ferrous particles (ASTM D7596, LaserNet Fines module), viscosity (ASTM D8092) and oil chemistry (ASTM D7889). Reports and trends are kept in TruVu 360 software.
• Other oil analysis instruments: see the oil analysis section.

KEG TRK supplies this equipment in Kazakhstan and helps select it for the task. Price on request.

Sources

• Donaldson. Understanding ISO Cleanliness Codes: https://www.donaldson.com/en/resources/technical-articles/understanding-iso-cleanliness-codes/
• Noria Corporation, Machinery Lubrication. What Is the ISO Cleanliness Code?: https://www.machinerylubrication.com/Read/28979/iso-cleanliness-code
• ExxonMobil. Mobil Technical Topic: How to ensure proper oil cleanliness in lubrication systems: https://www.mobil.com/lubricants/-/media/project/wep/mobil/mobil-row-us-1/pdf/technical-topic-how-to-ensure-proper-oil-cleanliness-in-lubrication-systems.pdf
• Ayalytical (PAMAS). ISO 4406: https://ayalytical.com/iso-4406/
• AMETEK Spectro Scientific. MiniLab 153: https://www.spectrosci.com/product/minilab-153