Oil analysis is a regular predictive maintenance technique practiced by most lubrication engineers and maintenance professionals to keep their machines healthy and working. As the name suggests, oil analysis refers to analysis of oil on-field or in-lab to determine oil properties, the presence of suspended contaminants and wear debris, and in some cases to find out additive chemistry which helps in developing a better lubricant formulation.
Before we move towards oil analysis techniques let’s first understand the importance of oil sampling. This is a very important step before oil analysis. A wrong oil sample will lead to inaccurate oil analysis results. Some of the key points to note during oil sampling are:
• Always take the oil sample when the machine is running
• Always take the sample from the sample valve
• In absence of sample valve, always take the sample from any port before oil filter
• Oil samples should be collected in dedicated clean, dry sample bottles and should be properly labelled
There are different kinds of oil analysis techniques available today. Their usage depends upon different needs or requirements. Here, we will be listing some conventional and advanced oil analysis techniques.
Common on-site oil analysis tests
Visual analysis or visual inspection refers to analysing certain oil parameters via visual eye. Visual inspection of oil for clarity, colour, water in oil are considered some major pre-tests before deciding to send a sample for lab testing. Also, a foul smell from oil can be taken into account as oil degradation.
Crackle test is a test used to determine presence of water in oil. It’s an effective screening test. Water in oil can be very detrimental for machine parts as it promotes corrosion. As mentioned earlier, crackle test is just a screening test, further analysis has to be done via Karl Fischer water titration test (ASTM D6304) or infrared spectroscopy to get an exact quantity (ppm) of water in oil.
Particle counting is considered as one of the most important oil analysis tests for used oil. Here, a used oil sample is passed through a filter membrane. Particles larger in size will be unable to pass through the membrane. High coagulation or blockage of membrane signifies abnormal wear of machine parts. A patch test is a kind of particle counting technique used on-site by professionals.
Common in-lab oil analysis tests
A viscometer is a device used to measure the viscosity of oil. Viscosity is the most important property of oil and refers to measure of resistance of fluid to shear and tensile stresses. A viscometer is also used to determine viscosity index of an oil. Viscosity index refers to measure of change in oil viscosity with change in temperature.
A rheometer is a device used to measure the way in which a fluid responds when force is applied on it. In our case, this fluid corresponds to used oil.
A foam test is conducted to find oils foaming tendency. Air at high pressure is diffused into a small amount of oil sample and then after five minutes, the amount of foam formed is measured.
Advanced in-lab oil analysis tests
Fourier transform infrared spectroscopy (FTIR) is used to find the presence of impurities in used oil. Every molecule absorbs or emits infrared at different wavelengths. FTIR interferogram represents these wavelengths in the form of peaks and valleys. Further analysis of these waveform helps in detecting exact compounds. Additives, contaminants and products of degradation also leave a specific signature and hence can be easily identified. Oxidation of oil due to working environment is a common problem encountered by many professionals. Nitration or presence of nitric acid in oil speeds up oxidation of oil. FTIR also helps in computing levels of oxidation and nitration in oil. To evaluate a used oil it is necessary to first obtain FTIR analysis of the same oil when it is fresh or new. This will help in comparing condition of oil before and after its use.
Inductively coupled plasma (ICP) is another type of spectroscopy technique that uses inductively coupled plasma to produce excited atoms and ions causing them to emit light at a particular wavelength. ICP analysis is a fast, sensitive and high precision analysis used to detect fine or ultra-fine metals and non-metals or wear debris in oil sample. It can be also used to identify presence of oil additives and contaminants.
X-ray fluorescence (XRF) spectroscopy is a very powerful tool used to find the elemental composition of materials. With regards to oil analysis, the XRF is used to find sulphur content in oil. High levels of sulphur can promote rusting of machine parts.
Analytical ferrography is a technique used to separate ferrous contaminants and ferrous wear debris from used oil. The ysed oil is poured down over a special glass slide called a ferrogram. Below the ferrogram a powerful magnet is placed, which attracts ferrous particles out of the oil. These adhered ferrous particles are then further analysed by a microscope to determine particle type, size, shape and concentration.
Calorimetric analysis is a technique used to measure insoluble materials in oil that can lead to the formation of varnish. The technique is mainly used mainly for turbine oil analysis.
Complementary in-lab oil analysis tests
A total acid number (TAN) test is conducted to check any levels of acid build-up inside the lubricant. Lubricating oils generally have antioxidant additives to suppress corrosion. Depletion of these agents with time can result in the formation of acids that can corrode machine parts. Titration and field test kits are used to find TAN levels.
A total base number (TBN) test is conducted to check the levels of alkalinity additives inside a lubricant. These additives are generally mixed in engine oils to neutralise acidic byproducts of combustion. A low TBN can lead to corrosion, sludge and varnish of machine parts. Titration and field test kits are used to find TBN levels.
Soot index is a measurement of extent to which oil has been contaminated by soot. Soot is a fine black powdery substance produced due to incomplete combustion of hydrocarbons. Any spectroscopy can be used to compute soot index.
Harshvardhan Singh is an automotive engineer from India with expertise in tribology and associated fields. He can be contacted at firstname.lastname@example.org