Filter analysis in the OELCHECK laboratory

Year of publication: 2013


After an initial optical appraisal of the sample fabric, the experienced OELCHECK tribologist will choose the analysis procedures. As filter residue cannot be used like oil samples with our test equipment, the devices are either re-equipped or modified procedures are used.

Table of contents

  1. PQ index - searching for iron
  2. The RDE determines additives and wear
  3. The ATR-FTIR recognises oil type, impurities and blending
  4. Microscopic particle counting

PQ index - searching for iron

The PQ index tells us about the content of magnetisable iron as a dimensionless numerical value. If this value is clearly higher than the index found in oil, insignificant wear does not appear with the iron-containing components of the aggregate from which the filter originates. The PQ index works independent of the particle size. The test principle uses the fact that iron abrasion interferes with the magnetic field. The quantity of all magnetised iron particles (in this case, rust particles are not magnetisable) in the filter residue is determined in a magnetic-inductive manner. The index, which is named after the Kittiwake "Particle Quantifier" testing equipment and called the PQ index for short, indicates the measurement result.

The RDE determines additives and wear

RDE stands for Rotating Disk Electrode. The filter residue becomes stimulated after wetting, where necessary, with pure "0-ppm control oil" directly on a "function cog". In the arc that forms during a high voltage of 10 kV between the residue-occupied graphite disk and a graphite rod electrode fitted over it at a distance of 5mm, all applied elements are agitated and made visible by the spectroscopy. With the determination of 27 elements come solid or sludge-like wear and contaminant particles and additive components which have been deposited on the filter cloth.

The ATR-FTIR recognises oil type, impurities and blending

The principle of FT-IR (Fourier Transform Infrared) spectroscopy is based on there being different molecules present in the lubricant which, because of their typical chemical structures, absorb infrared light to different degrees with certain wavelengths. Changes to the specimen can be compared to the fresh oil reference spectrum and depicted, calculated and interpreted as typical "peaks" with certain "wave numbers". For the study of filter residue, we use a special variant of infrared spectroscopy. The ATR infrared spectroscopy (attenuated total reflection) is based on a weakened ("attenuated") total reflection. It proved especially effective in the case of checks on opaque materials. The ATR-FTIR provides information about oil mixtures with foreign oil and impurities, such as water, from oil-moistened filter residues. Changes in the case of additive composition (additive sludge) must also be recognised. Through comparison with the deposited spectra of fresh oil, the procedure provides information reliably and quickly whether an unknown oil is a mineral oil, "bio-oil" or synthesis oil.

Microscopic particle counting

If the filter residue contains bigger particles which indicate a wear process or dust contamination, we recommend supplementary microscopic particle counting. For this purpose, the residue must be rinsed out of the fabric with a solvent mixture. After that, the mixture is filtered with a pore size from 0.45µm to 2.5µm so that the particles remain on a filter membrane. This membrane is examined under our special OLYMPUS microscope. In addition to the categorisation of size classes, the particle analysis also makes qualitative statements possible. It is a distinction between reflective, metallically bright, coloured or black particles. Fibres or deposits of lubricating grease are recorded and evaluated separately. Representatively chosen particles are measured two-dimensionally and categorised by their longest extension. This task is finished with a high-resolution CCD camera integrated into the microscope with image analysis software. Making use of polarised light, representatively enlarged photographs are drawn up, from which the experienced tribologist makes the selection for the laboratory report.

Also read Extract filter specimens correctly & What filter deposits reveal...