🔍 Particle cleanliness class in hydraulic oils

Petrochemistry PN-ISO 4406

Overview

STATUS (as of 06.11.2026). According to the PKN catalogue card (read on 06.11.2026) PN-ISO 4406:2005 is a Polish version published on 17.03.2005 (12 pages), implementing ISO 4406:1999 [IDT] and replacing PN-ISO 4406:1994, with no withdrawal note. The later ISO editions (2017, 2021) have not been adopted into the PN. The current international edition is ISO 4406:2021 (fourth edition, 07.01.2021, confirmed after review), which superseded ISO 4406:2017; according to the foreword the main change is that particle contamination monitors (PCM) may also be used to determine the code. The description is based on the preview of ISO 4406:2021 (foreword, introduction, Clauses 1–4.4.5 with the complete Table 1); we did not have subclauses 4.4.6–5 or Annex A.

WHAT THE STANDARD IS. ISO 4406 is not a particle counting method but a way of expressing the result: it specifies the code used to define the quantity of solid particles in the fluid of a given hydraulic system. The counting itself is carried out according to ISO 11500 (automatic counter using the light extinction principle) or another recognized method, with a counter calibrated according to ISO 11171 or a PCM calibrated according to ISO 21018-3 or ISO 21018-4; microscope counting — according to ISO 4407.

CODE. With automatic counters and monitors the code consists of three scale numbers: the first corresponds to the number of particles ≥ 4 µm(c), the second ≥ 6 µm(c) and the third ≥ 14 µm(c) per millilitre of fluid; the numbers are written one after another, separated by slashes. Example from the standard: code 22/18/13 means more than 20 000 up to 40 000 particles ≥ 4 µm(c), more than 1 300 up to 2 500 particles ≥ 6 µm(c) and more than 40 up to 80 particles ≥ 14 µm(c) per millilitre. The designation µm(c) means that the size was measured with an instrument calibrated according to ISO 11171 (or a monitor according to ISO 21018-3/-4); the sizes ≥ 6 µm(c) and ≥ 14 µm(c) correspond to the former 5 µm and 15 µm of the first edition. With microscope counting the code has two numbers — for particles ≥ 5 µm and ≥ 15 µm; the microscope measures the longest dimension of a particle, whereas an automatic counter derives the size of an equivalent particle from the cross-sectional area, so the results of the two techniques usually differ.

SCALE. Table 1 of the standard assigns numbers from 0 (up to 0.01 particles per millilitre) to 28 (more than 1 300 000 up to 2 500 000) and "> 28" (more than 2 500 000), with a step ratio of generally two. Reproducibility below number 8 depends on the actual number of particles counted — the raw count should be more than 20 particles.

SIGNIFICANCE. According to the introduction of the standard, solid particle contaminant is always present in hydraulic fluid and can cause serious problems, and most manufacturers of hydraulic components require a certain cleanliness level according to ISO 4406 for warranty purposes, proper function and a long service life. The result is affected by sampling, counting accuracy and container cleanliness — the sample has to be representative of the fluid circulating in the system.

WHAT WE DO NOT GIVE HERE. The standard does not give required cleanliness levels for types of pumps, valves or servo drives — these are set by component manufacturers; we have therefore removed such lists, as well as figures without a source (share of failures caused by contamination, sample volumes, measurement times, counter background limits), brands, models and prices of instruments, and the attribution of sample dilution to ISO 4402. The counting procedure (ISO 11500, ISO 4407) and subclauses 4.4.6–5 of the standard are not discussed here.

Method principle

The number of solid particles per millilitre of fluid, determined with an automatic counter (according to ISO 11500, instrument calibrated according to ISO 11171), a contamination monitor (ISO 21018-3 or ISO 21018-4) or a microscope (ISO 4407), is converted into scale numbers from Table 1 of the standard. Each number covers a range of particle numbers, and an increase by one number generally means twice as many particles. For counters and monitors three numbers are written — for particles ≥ 4 µm(c), ≥ 6 µm(c) and ≥ 14 µm(c), e.g. 22/18/13; for the microscope two — for particles ≥ 5 µm and ≥ 15 µm.

Applications

• Recording and comparing the cleanliness level of fluids in hydraulic fluid power systems
• Checking the compliance of a fluid with the cleanliness level required by the manufacturer of hydraulic components (warranty, proper function)
• Reporting results from automatic counters (ISO 11500), contamination monitors (ISO 21018-3, ISO 21018-4) and microscope counting (ISO 4407)

Key parameters

Standard

Standard number

PN-ISO 4406:2005 (Polish version; adopts ISO 4406:1999 [IDT])

Title (PL)

Napędy i sterowania hydrauliczne — Ciecze robocze — Metoda kodowania poziomu zanieczyszczeń w postaci cząstek stałych

Title (EN)

Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles

Step-by-step procedure

1. Sampling

   Take a sample representative of the fluid circulating in the system, into a clean container (where used); the result is affected by the way of sampling, counting accuracy and container cleanliness.

2. Particle counting

   Count the particles according to ISO 11500 or another recognized method with a counter calibrated according to ISO 11171, with a PCM calibrated according to ISO 21018-3 or ISO 21018-4, or with a microscope according to ISO 4407. Express the result as the number of particles per millilitre.

3. Allocation of scale numbers

   For a counter or monitor allocate from Table 1 a scale number to the number of particles ≥ 4 µm(c) (first), ≥ 6 µm(c) (second) and ≥ 14 µm(c) (third); for the microscope — to the number of particles ≥ 5 µm and ≥ 15 µm. For numbers below 8 check that more than 20 particles were counted.

4. Writing the code

   Write the numbers one after another, separated by slashes (e.g. 22/18/13), and state which technique was used for counting.

Required equipment and apparatus

Health and safety (OHS)

• The standard contains no safety requirements; it only cautions that particle counts are affected by sampling, counting accuracy and container cleanliness

Frequently asked questions