important as performing the test properly and it is important to note that only results from the same method should be compared. The particle count is an easy test to interpret, assuming the test has been correctly performed. This point is made because there are many factors which can negatively affect a particle count. An increasing count is an indication of an increased number of particles in the oil. Exception tests such as analytical ferrography or patch microscopy would then be used to qualify the type and source of the particle contamination.


Analytical microscopy is a technique used to qualify contaminants, including wear debris, in an oil sample. There are two commonly performed variations of this technique:


• analytical ferrography, • patch microscopy.


Analytical ferrography uses magnetic fields to separate ferrous debris according to particle size. As the name suggests, the technique is biased to ferrous particles but some nonferrous particles are typically deposited on the substrate either via entrapment or by magnetic effects imparted to them by impaction of ferrous particles.


Patch microscopy, on the other hand, does not demonstrate a bias to ferrous particles. All particles above the membrane pore size are presented on a piece of filter paper, the filtergram, for examination. However, patch microscopy does not have the size separation attributes of the ferrography, so particle deposition is random. A modification of the patch test can be performed to analyse both ferrous and nonferrous debris separately. A magnet is used to hold back magnetic particles while a filtergram of nonferrous debris is prepared. Then a filtergram of the remaining magnetic debris is made.


In deciding on the correct


microscopy test to perform, the analyst must make some judgment on the machine metallurgy and the nature of the contaminant being sought.


It is not a good idea to


perform ferrography on a worm and wheel gearbox where the majority of wear particles are likely to be cupric (thus nonmagnetic) in origin.


Similarly, if a wear situation


is suspected on a reduction gearbox with helical gear sets, then analytical ferrography will probably provide far better resolution than the patch.


It is worth mentioning


that for filtered oil systems, a ferrogram or patch which does not show abnormalities should be treated with suspicion. Assuming that there was a reason to proceed with the analytical microscopy in the first place, one would then expect to see problems. A good approach to filtered systems is to remove a section of the filter medium, wash it out in solvent and perform the microscopy on the extract. Each laboratory will have its own system for quantifying and reporting wear and contaminants in each of these tests.


Interpretation


is subjective and can be expensive to perform because it is labour intensive. Analytical microscopy is a powerful technique which should be used to confirm and qualify contamination and wear situations identified by the routine tests.


Infrared analysis is the second type of spectroscopy commonly found in a laboratory.


Unlike


elemental analysis, FTIR as it is known, provides information on compounds rather than elements found in an oil. FTIR measures several useful degradation parameters so is particularly useful in engine oil samples.


Infrared


analysis detects the presence of water and can also be used to identify oil base stocks. While the Inductively coupled plasma (ICP) spectroscopy measures emissions of radiation of specific wavelength in the visible and ultraviolet regions of the electromagnetic spectrum, infrared analysis measures the specific wavelengths of radiation in the infrared region. The various degradation by products and


contaminants found in the oil cause characteristic absorptions in specific regions of the infrared spectrum. The higher the level of contamination in the sample, the higher the degree of absorption in the characteristic region. A plot of absorbance, transmittance, or concentration versus wave frequency is generated during the analysis of an oil sample and is called the infrared spectrum. This spectrum is subsequently analysed by specialized oil analysis software that yields measurements most commonly for soot, oxidation, sulphates, nitrates and water. Other compounds, such as additives, fuel and glycol can also be measured but for these an accurate sample of the new oil is needed as a reference. If no such reference sample has been supplied then readings of the latter parameters should be regarded with suspicion. The basic types of contamination that must be dealt with are examined below.


These are: • solids, • moisture, • condition caused.


Soot and other Solid Contamination:


It is generally recognized, backed by numerous tests and studies over the last 40 years, that it is contamination generated in an engine that is responsible for the majority of normal wear, is within the 1 - 15 micron range. Also this small solid contamination contributes to accelerating Condition Caused Contaminants such as Oxidation, Nitration, Acid formation and more. Consequently, it is imperative that this contamination be removed from the system as fast as possible. The typical full flow filter cannot control 1-15 micron particles because of its porous design which is needed to supply the engine with a high flow rate of oil and UF filtration that is capable


The Report • March 2017 • Issue 79 | 47


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