Hydraulic oil should be sampled and analysed regularly, either by an in-house laboratory or a specialist testing provider. The results reveal fluid cleanliness, ageing, wear trends and external contamination, helping maintenance teams assess hydraulic-system condition, identify developing faults and plan corrective action.
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Both branches are essential for understanding what is happening in the oil and in the hydraulic system.
Particle analysis determines how many particles are present in defined size classes. This is usually measured optically: the oil passes through a light beam, and the number and size of the shadows detected by a photocell determine the particle count in the sample.
For consistent comparison, oil cleanliness—or oil contamination, depending on the point of view—is classified according to ISO 4406. The standard assigns particle-count ranges to cleanliness code numbers, making results easy to compare and allowing maintenance teams to identify changes quickly.
As the table shows, the particle count doubles from one code range to the next. A reduction of one code number therefore means approximately half as many particles, while an increase of one code number means approximately twice as many.
ISO 4406 particle analysis uses three size ranges: >4 μm, >6 μm and >14 μm. This shows which particle-size range is improving or deteriorating and helps determine the appropriate corrective action.
Some laboratories also report additional ranges. For example, OELCHECK may include >21 μm, >38 μm and >70 μm, providing further clues about the composition and likely origin of particles in the oil.
| Particles per 100 mL: from | to | ISO code |
|---|---|---|
| 250,000,000 | - | >28 |
| 130,000,000 | 250,000,000 | 28 |
| 64,000,000 | 130,000,000 | 27 |
| 32,000,000 | 64,000,000 | 26 |
| 16,000,000 | 32,000,000 | 25 |
| 8,000,000 | 16,000,000 | 24 |
| 4,000,000 | 8,000,000 | 23 |
| 2,000,000 | 4,000,000 | 22 |
| 1,000,000 | 2,000,000 | 21 |
| 500,000 | 1,000,000 | 20 |
| 250,000 | 500,000 | 19 |
| 130,000 | 250,000 | 18 |
| 64,000 | 130,000 | 17 |
| 32,000 | 64,000 | 16 |
| 16,000 | 32,000 | 15 |
| 8,000 | 16,000 | 14 |
| 4,000 | 8,000 | 13 |
| 2,000 | 4,000 | 12 |
| 1,000 | 2,000 | 11 |
| 500 | 1,000 | 10 |
| 250 | 500 | 9 |
| 130 | 250 | 8 |
| 64 | 130 | 7 |
| 32 | 64 | 6 |
| 16 | 32 | 5 |
| 8 | 16 | 4 |
| 4 | 8 | 3 |
| 2 | 4 | 2 |
The key question is whether particle counts are decreasing or increasing. Falling counts indicate that filtration is working effectively. If the particle load increases, one or more of the following causes should be investigated:
Use an effective breather or air filter on the reservoir to keep moisture and particles out as air is drawn into the tank. If contaminants are already present in the oil, suitable filtration can remove the solid particles.
Conventional filters cannot remove water. Special water-absorbing filter media can capture free water, but their capacity is limited and they do not remove larger quantities of emulsified water.
Where the oil contains substantial free or emulsified water, a vacuum dehydration system can remove it efficiently and restore the fluid to a controlled moisture level.
In most oil analyses, the reported viscosity should correspond to the viscosity stated in the oil data sheet, typically within ±5%. A significant deviation indicates that the oil has undergone a fundamental change and the cause should be investigated.
A drop in viscosity is often caused by a lower-viscosity fluid being added to the reservoir. Correct handling procedures and staff training help prevent this simple but costly source of error.
The TAN, also called the neutralisation number, indicates the amount of acidic constituents in the oil. It is used to assess oil ageing and oxidation. Because fresh oils can have very different initial TAN values, every result must be compared with the oil’s original baseline.
If TAN increases, action is required. Acidic degradation products form as the oil ages, so oxidation must be slowed or stopped by controlling oil temperature, reducing water content and removing particles that act as oxidation catalysts.
Some oil analyses report a PQ index. This value indicates the concentration of ferromagnetic wear particles in the oil and helps assess the composition of the particle load and identify which components may be wearing.
Repairs caused by hydraulic-system failures are expensive because they combine repair costs, production downtime and consequential damage. Short hydraulic-oil service life is another significant operating cost. Oil analysis shows whether action is required, whether damage is developing and whether the existing hydraulic-oil filtration is performing effectively.
If hydraulic components fail frequently, the oil ages prematurely or contamination remains consistently high, inadequate filtration is often the underlying cause. The filtration supplied as standard with many hydraulic systems is not sufficient to maintain the required level of oil cleanliness over the long term.
If you want to improve filtration, ask FRIESS for advice on an electrostatic oil cleaning system. Before-and-after oil analysis provides objective evidence of the improvement achieved with FRIESS oil purification.
Reliable hydraulic systems depend on clean, particle-free oil and stable lubrication performance. Electrostatic hydraulic-oil cleaners remove fine and submicron contamination, helping extend oil and component life. The costs of replacement oil, waste-oil disposal and hydraulic-system failures can all be reduced substantially.
A comprehensive oil analysis establishes the condition of the hydraulic fluid and shows whether corrective action is required. Regular measurements before and after the oil cleaning system is installed also make the economic benefit measurable.
