- The number and size of particles.
- The types and quantity of contaminants present.
- The condition of the additives in the oil.
- Changes to oil chemistry caused by the working environment.
- The amount of water present.
- The viscosity of the oil (slipperiness).
It is not necessary to do all tests on all oils in all situations. The selection of the type of analysis depends on the oil and where it is used. The oil used in combustion engines, gearboxes, hydraulic systems, and gas turbines is not the same and the conditions under which it operates are different in each situation. For example, soot would be present in internal combustion engines but it would not be present in gearboxes. There is no value in paying money to measure the amount of soot in a gearbox. But the amount of soot in the oil of a diesel engine is of critical importance.
Testing laboratories are required to follow internationally recognized procedures when measuring oil contamination. Equipment used to measure contaminants also needs to be calibrated to recognized international standards. However, just as there are clean and dirty maintenance shops, there are clean and dirty laboratories. Results from laboratories without good calibration procedures and sample hygiene practices or from people who do not fully understand the equipment and procedures should not be trusted.
Not all solid particle counting laboratory equipment can count particles down to very fine sizes. Results from these laboratories would give false figures showing less contamination at low micron sizes than was actually present. Some laboratories use equipment and methods that do not count particles larger than 100 micron. Results from these laboratories would show incorrect large particle counts. In the future, these large particles would be smashed up and broken down, and the resulting smaller particles would quickly contaminate the oil.
If the sample itself is too heavily contaminated, optical counting methods cannot be used because the light emitted by the analyzer will not pass Solid Particle Oil Film Thickness Fig. 1. Solid particles in bearing oil film. 16 LUBRICATION & FLUID POWER/JANUARY-FEBRUARY 2004 through the sample in the same way the equipment was calibrated to receive. Optical counters can mistakenly count water droplets as solid particles. At times it can be necessary to confirm laboratory results by alternate means to prove the results are reliable.