This is the sixth part of a ten-part series on Hydraulics and Filtration and will discuss the common methods used for analyzing hydraulic fluids. Furthermore, the required cleanliness levels for various hydraulic components are reviewed.
Fluid analysis is one of the most essential steps in the maintenance of hydraulic systems. Results from fluid analysis help determine fluid composition and contamination levels, thus ensuring its adherence to the conditions provided by manufacturers. Visual examination, due to its obvious limits, can never be considered a reliable method for determining fluid conditions. Methodical fluid analysis is the proper way of gaining accurate information on fluid properties.
Fluid analysis usually involves:
- Determining fluid viscosity
- Determining number of particles in the fluid
- Determining the amount of water present in the fluid
- Spectroscopy, analyzing metals and additives content in the fluid
The most important aspect of fluid analysis consists of determining fluid particle count and contamination level. This can be done in one of the following ways:
- Based on differential pressure
- Through “Patch Test” using a microscope
- Using portable particle counters
- Through laboratory Analysis
Each of these methods are discussed below:
Differential Pressure
This is mostly an empirical method which is based on the assumption that when the pressure drop remains constant during a specific period of time, the fluid has reached the required level of cleanliness. This method is very limited and uncontrollable, and thus should only be used as a last resort.
Patch Test
This method consists of passing a fluid sample through a media patch and comparing the remaining particle patterns on the patch against standards such as ISO 4406. A cleanliness indicator is provided as the result of this procedure.
This method is rather time-consuming and requires great experience and precision, while there is a large margin of error.
Particle Counters
Portable particle counters can be used to measure particle numbers and sizes rapidly, conveniently and with great accuracy. The result are usually presented as detailed reports with graphics and tables. Due to their numerous advantages over other counting methods, these devices are increasingly popular.
Laboratory Analysis
Laboratory analysis is the most comprehensive fluid analysis method. Fluid analysis methods usually include identifying the number of particles, fluid viscosity, the water content, and metals and additives existing in the fluid. In addition to these, laboratory analysis also provides analytic charts and graphs, micrography and additional information on the fluid.
Component Cleanliness Levels
In order to reach optimum performance, Hydraulic equipment manufacturers should specify the required fluid cleanliness levels for their products. Table below shows some common hydraulic components and their commonly required ISO 4406 codes.
Required cleanliness levels for hydraulic components
| Component | Required Cleanliness Level (ISO 4406 Code) |
| Servo Control Valves | 17/14/11 |
| Proportional Valves | 18/15/12 |
| Vane Pumps and Piston Pumps | 19/16/13 |
| Directional and Pressure Control Valves | 19/16/13 |
| Gear Pumps | 20/17/14 |
| Flow Control Valves and Cylinders | 21/18/15 |
| New, unused fluid | 21/18/15 |
It should be noted, that the specifications and required conditions of hydraulic equipment are provided officially by the manufacturer. In addition to being crucial in determining the required filtration ratio of the system, these specifications are important in keeping the warranty valid by defining the boundary between standard and non-standard use of the system.