Spectrometry

A spectrometer is an instrument with which one can measure the quantities and types of metallic elements in a sample of oil. The operating principle is as follows. A diluted oil sample is pulverised by an inert gas to form an aerosol, which is magnetically induced to form a plasma at a temperature of about 9000°C. As a result of this high temperature the metal ions take on energy, and release new energy in the form of photons. In this way, a spectrum with different wavelengths is created for each metallic element. The intensities of the emissions are measurable for each such element by virtue of its very specific wavelength, calculated in number of ppm (parts per million). An ICP spectrometer can detect the very small metal particles in suspension in the oil, i.e. with a size between 0 and 3 microns.

Those small particles are a good indication of general wear, except in cases of sudden metallic rupture, where there will be relatively more large particles liberated (50 microns and more). The human eye can detect particles of a size starting from 50 microns, which allows them to be visualized using more conventional means. Thus, complementary analysis of such larger particles can be done by spectrometry (after acid attack), by ferrography (or related systems) or by optical or electronic microscopy.

Viscosity (Engine oils)

In the early days of the IC engine there were only monograde oils (e.g., SAE 20, SAE 30, SAE 50). By putting an additive into these oils, called a VI improver, multigrade oils were created. The VI (viscosity index) improver is a flexible molecule, rolled up like a ball at low temperature and stretched out like a string at high temperatures. This allows the oil to remain viscous at high temperatures. One can recognize multigrade oils as being represented by two figures. The first figure, followed by the suffix ‘W’, stands for the viscosity calss at low temperature (W = winter). The second figure is the SAE class at working temperature. Thus, for example, ‘SAE 20W-50’ means that the viscosity of the oil at low temperature corresponds with a SAE 20W, and the oil viscosity at 100°C corresponds with a SAE 50. The table below gives some data on viscosities.

The viscosity of used engine oil is mostly measured at 100°C, and can drop for reasons of fuel dilution, and/or shearing of the VI improver. Viscosity can increase as a result of heavy contamination of the oil by soots, and/or oxidation of the oil.

Viscosity (Industrial oils)

The viscosity of industrial oils, by contrast, is mostly measured at 40°C, and must correspond with the ISO table below, i.e., the viscosity of an ISO class oil must be within the minimum and maximum for that class. (Moves are in hand to make the viscosity class statement contain more data, to reflect changes in the oil in use.)

The viscosity can be decreased by adding a more fluid oil, or as a result of high water content, of by shearing of the VI-improver. The viscosity can be increased by adding a more viscous oil, and by oil oxidation (e.g. as a result of overheating).





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Base Number (BN)

The alkalinity of an oil is measured by titration through an acid, and expressed in mg KOH/g. The comparison between the TBN volume of the fresh oil and that of the used oil allows the determination to be made of whether the used oil is still capable of neutralizing acid residues. These acids are produced by combustion (sulphur in fuel) and oxidation of the oil and oil additives. When the oil is in service too long, the TBN will drop significantly.

Too low a TBN volume can be due to: heavy oxidation of the oil, when the oil has been in service for too long, of the oil level was insufficient, or due to a defective cooling system, producing overheating; use of a fuel containing a high sulphur content; use of an inappropriate lubricant; or contamination of the oil by fuel or water.

Particle Counting

This is an especially useful test for a hydraulic system with high sensitivity (e.g., servo-valves). Insuch a text, a certain quantity of hydraulic oil flows through a sensor, where all the insoluble material in the oil is detected and counted using the principle of light absorption. The particles counted are classified cumulatively:

>5µ;>15µ;>25µ;>50µ;>100µ.

Or differentially:

>5-15µ;>15-25µ;>25-50µ;>50-100µ;>100µ.

The results of particle counting can be expressed according to either ISO 4406 or NAS 1638. According to ISO 4406, the results are expressed cumulatively, and the ISO classification is deduced from the two first classes, >5µ and >15µ.

According to NAS 1638, the results are expressed differentially, in five classes. In each class one can get a NAS quotation, and the NAS code is the figure given to the first class.

Microscopy

After filtration of a certain amount of oil through a cellulose filter (of 0.8µ), the filter is examined under an optical microscope (magnitude 100x, 200x), and one is able to distinguish:

white or brilliant metal particles (demonstrating recent wear)
black metal particles (already oxidized)
rust particles
silt (i.e., very small particles below 5µ, responsible for erosive wear)
silica (sand, dust)
polymers (from oil additives)
welds
paint flakes
other impurities (fibre, plastics, and so on)

Dilution

Dilution of a use engine oil can be measured precisely by gas chromatography (GC) or by Fourier Transform Infrared spectroscopy (FTIR). More common is the use of the SETA-FLASH tester, where the flash point of oil is tested by a certain temperature. When a flashpoint is detected, the dilution is heavy (more than 4%), when not, the dilution is acceptable (less than 4%).

It is evident that heavy dilution of the oil is unfavourable for the engine, since it involves a lower viscosity and reduces the resistance of the oil film. The principal causes of dilution are a defective fuel injection system, a defective air inlet (obstructed air filter), incomplete combustion due to too low a working temperature, and badly regulated valves, or insufficient compression.

Water detection

The water-content of the oil is usually measured by the Aquatest or a Karl Fisher apparatus. The possible causes of water introduction include (a) condensation, due to too low a working temperature, defective crankcase ventilation, ‘stop and go’ in-service usage, and obstruction of the exhaust system; or (b) infiltration, due to leakage at the cylinder head gasket, or damage of the engine block.

Cooling water contains most often an anti-freeze based on glycol. Therefore a glycol test should be performed when water infiltration is suspected. The inhibitor in the anti-freeze agent is usually a sodium borate type.

Acid Number (AN)

The acidity of the oil is measured by titration through a base, and expressed in mg KOH/g. The figure below shows this graphically, showing the evolution of TAN as a function of time.