- Buyer's Guide
"I would like to perform optical particle analysis. Do you have any suggestions or recommendations for this? I have some particles and a microscope, but I am looking for information that describes this oil analysis method so I can try it for myself."
Implementing a procedure to characterize contaminant particles found in lubricants and becoming proficient in this technique will require considerable study and practice. First, you should set goals for the analysis, such as determining the particle size, metal type and wear failure modes. This information can be obtained from a particle characterization sample such as a ferrogram, filtergram or patch test.
With a ferrogram, wear particles are captured on a glass surface, which is placed in a magnetic field. For this reason, it has a bias toward ferromagnetic particles from iron or steel, but it also can capture non-ferrous particles. This is frequently referred to as analytical ferrography. Engines, most gearboxes, rolling-element bearings and most hydraulics are among the types of equipment that are most likely to produce magnetic wear particles.
A ferrogram is easy to heat-treat for identification purposes and has excellent transmission of bottom light. Unfortunately, the particles tend to pile up, and the preparation equipment is generally more expensive.
On the other hand, a filtergram has no bias toward ferrous particles and is easy to prepare. The necessary hardware is also less expensive. However, it is more difficult to distinguish metal types and to prepare two membranes (one ferrous and one non-ferrous). Special patches are also required for heat-treating, and the transmission of bottom light is not as effective. Equipment with critical non-ferrous frictional surfaces include worm gears, stainless-steel machinery and turbomachinery with bronze or Babbitt bearings.
A patch test, also called patch ferrography, can be used to collect all types of solid particles in oil, since the fluid is simply filtered through a patch or paper. With this method, there is no bias toward a ferromagnetic element or metallic debris.
After particle samples are taken and analyzed with a microscope, it is possible to heat the sample to observe color changes in the particles. This helps to identify the metal type. This procedure is typically run in ferrograms.
For reference, be sure to consult the corresponding ASTM methods (D7684-11 and D7690-11). Noria's oil analysis training courses can also provide more information about these techniques and the value of oil analysis.