Viscosity is the most important physical property of a lubricant, and is defined as a fluid's resistance to flow. This in turn determines the thickness of the oil film that prevents contact between metal surfaces. Trending of viscosity data is important, as deviations from the norm may indicate base oil degradation, additive depletion or the use of an incorrect lubricant.

The viscosity index characterises the effect of temperature on an oil's viscosity and is of particular importance in applications where operating temperatures vary significantly

Water contamination is detrimental to any lubricant and can shorten the service life of a gearbox by accelerating wear. The Karl Fisher method for determining moisture content is recommended, as even small amounts (500ppm) of water contamination can contribute to micro-pitting whi ch is a recognised failure mod e in win d turbine gearboxes. Water can be damaging to both the oil and the component

The total acid number is a quantitative measure of acidic compounds in the oil that are generated as a result of oxidation, and the formation of acidic degradation by-products.

The ferrous debris monitor or PQ (Particle Quantifier) gives a measure of the total ferrous content of the oil sample and from this measurement the total amount of ferrous (iron) debris can be determined irrespective of the size of the particles.

Wind turbine manufacturers have increasingly focused on oil cleanliness, which has a huge impact on the lifetime of bearings and the performance of the gearbox. Particle counting involves measuring the cleanliness of the oil and can also be used to evaluate the effectiveness of lubricant filters. The numbers of particles per milliliter of oil are counted in a variety of size ranges starting at four micron and going up to 100 microns. The total number of particles greater than four, six and 14 microns is assessed and given range numbers that indicate the cleanliness of the oil according to the ISO 4406 method.

The spectrometer measures the concentration of wear metals s uch a s iron, oil additives like phosphorus and contaminants such as silicon in the oil.

The FTIR produces an IR spectrum that is often referred to a s the 'fingerprint' of t he oil, a s it contains specific features of the chemical composition of the oil. The IR spectrum can be used to identify types of additives, trend oxidation a nd nitration by-products that co uld fo rm as a result of high operating temperatures and thermal degradation caused by aeration/foaming.

An MPE is performed by filtering the oil through a five-microns membrane patch and examining any debris present under a microscope. The membrane patch is examined for wear, contamination and colour, and a matrix describing the concentration and size of debris is recorded.