This technique is typically used for critical hydraulic and turbine systems which have very small clearances, e.g. servo valves. It can also be used to look for early signs of bearing wear or evaluating equipment filter performance.
There are three basic methods used, Automatic Optical Particle Counting (ISO 11500), Optical Microscopy (ISO 4407), and Pore Blockage Particle Counting (BS3406). In this article we will be discussing the Automatic Optical Particle Counting method as this is the most widely used. These come in laboratory scale units (example pictured below) and portable units for field use.
This laboratory method uses a laser beam to count and measure the particles as the oil passes through the beam.
There can be problems with dark fluids, water in the oil, air bubbles or a large number of silt particles (~2µm). Also samples with a viscosity greater than 320cSt may need pre-treatment. Therefore the area taking the samples and the testing laboratory must be aware of these potential issues and treat the sampling and testing accordingly, e.g. the above instrument uses a vacuum system to remove any air bubbles.
The most common method of reporting the particle count is covered by the global standard ISO 4406.
In this standard the particles are reported in 3 size categories:>= 4µm/>= 6µm/>= 14µm.
For each category a code is designated.
Process for making ISO Code Designation
1. The number of “particles” in the oil are counted and measured per ml of oil
2. The quantities measured are grouped by their particle sizes (>=4μm(c), >=6μm (c), >=14μm (c))
3. Each group is then assigned a range code as per the following table. Automatic particle counters will print out this code automatically.
Note: ISO 4406 is a classification system, not a test procedure
Some example cleanliness targets
Variable piston pump:18/16/13 filtration 6 microns, pressure < 2000psi
Servo Valve: 16/14/11 filtration 3 microns, pressure <2000psi
Gear Motor:20/18/15 filtration 25 microns, pressure <2000psi
Regardless of the test method used, it’s critical to take a relevant and representative sample. Incorrect sampling can introduce extraneous particles into the sample bottle which will lead to incorrect results and impact on maintenance decisions.
So when should you change the oil?
In the case of cleanliness you should consult your oil and/or equipment suppliers to ascertain the recommended cleanliness levels for the equipment being used. By testing systems regularly required cleanliness levels can be maintained and potential warning signs identified. This will help to extend equipment life, enhance productivity and reduce costs.