Ablation – process of removal by erosion, melting, evaporation, or vaporization.

Abrasion – a wearing, grinding or rubbing away by friction, usually (but not always) involving the action of particles against or between surfaces.

Absolute (Dynamic) Viscosity – the ratio of shear stress to shear rate, representing a fluid's internal resistance to flow. Although the fundamental unit of absolute or dynamic viscosity is the poise, results are often expressed in centipoise (cP). 1 centipoise equals .01 poise.

Accumulator – an in-line vessel that stores fluid under pressure for later release; used in some larger hydraulic systems.

Accuracy – the closeness of a measured result to the actual (true) value.

Acid – any one of various hydrogen-containing molecules or ions capable of giving up a proton to a base, of accepting an unshared pair of electrons from a base, or of reacting with a base to form a salt. A more limited definition restricts the acid to a hydrogen-containing substance that contains a non-metallic radical and produces hydrogen ions when placed in solution.

Acid digestion – process of dissolving a sample in an acid matrix, usually accompanied by heating.

Acid treating – A refining process where unfinished petroleum products (gasoline, kerosene, lube oil stocks, etc.) are brought into contact with sulfuric acid to improve their color, odor, and other properties.

Acidity – specific to oil analysis, acidity denotes the presence of weakly and strongly acidic materials whose total concentration is usually defined in terms of the TAN (Total Acid Number).

Active metal – a metal ready to corrode, or being corroded. Contrast with noble metal.

Additives – An agent added to oils, fuels, and coolants to impart specific beneficial properties to the finished products. Additives create new fluid properties, enhance properties already present and reduce the rate at which undesirable changes take place in a fluid during service. Selected additives are:

	TYPE PURPOSE TYPICAL COMPOUNDS Dispersants, Detergents keep sludge, carbon and other deposit precursors suspended succinimides, neutral metallic sulfonates, phenates, polymeric detergents, amines Basic Metal Compounds neutralize acids, prevent corrosion overbased metallic sulfonates, phenates, carbonates Oxidation Inhibitors prevent or control oxidation of oil by forming oxygen-inactive compounds and / or scavenging free oxygen zinc dialkyldithiophosphates; aromatic amines, sulfurized products, hindered phenols Extreme Pressure (EP) Antiwear Additives reduce wear, prevent scuffing and seizing by forming protective surface films zinc dialkyldithiophosphates; tricresylphosphates, organic phosphates, chlorine and sulfur compounds Viscosity Index Improvers reduce the rate of viscosity change with temperature polyisobutylene, methacrylate polymers, olefin copolymers- may incorporate dispersants Rust Inhibitors prevent the formation of rust on metal surfaces by formation of surface film or neutralization of acids high base additives, sulfonates, phosphates, organic acids or esters, amines Corrosion Inhibitors prevent corrosive attack on alloy bearings or other metallic surfaces organically active sulfur, phosphorus, nitrogen compounds such as phosphites, metal salts of thiophosphoric acid, sulfurized waxes, terpenes Antifoamants reduce foam with surface tension modifiers that break up large surface bubbles, or small entrained bubble reducers silicones, organic polymers Friction Modifiers reduce or modify friction long chain polar compounds, (amides, phosphates, phosphites, acids, etc.), molybdenum disulfides Metal Deactivators form surface films so metal surfaces do not catalyze oil oxidation zinc dialkyldithiophosphates, metal phenates, organic nitrogen compounds Pour Point Depressants lower "freezing" point of oils assuring free flow at low temperatures low molecular weight methacrylate polymers Emulsifiers permit water-in-oil or oil-in-water emulsions by changing interfacial tension soaps of fatty acids, sulfonic or napthenic acids Demulsifiers lower emulsion stability heavy metal soaps, alkaline earth sulfonates Tackiness Agents increase adhesiveness of lubricant on metal surfaces; reduce runoff high molecular weight polymers, aluminum soaps of unsaturated fatty acids Oiliness Agents lower friction under near-boundary conditions high molecular weight polar compounds such as fatty oils, oxidized waxes or soaps

Adhesion – the force of molecular attraction exerted between two surfaces in contact.

Absorption – the process by which one substance draws itself into another substance. An example is a sponge picking up water, or on oil recovering gasoline from wet natural gas.

Adsorption – the adhesion of an extremely thin layer of molecules (gases, dissolved solids, or liquids) to the surfaces of solids or liquids with which the molecules are in contact. This process is utilized in a broad range of fluid and additive applications to concentrate one material on the surface of another.

Aeration – combining or charging a fluid with a gas, as in oil being aerated with air.

Aerosol – a highly dispersed suspension of fine solid or liquid particles in a gas.

Aftercooling – the process of cooling compressed gases under constant pressure after the final stage of compression.

Agglomeration – the action of combining a number of small particles into a larger single mass, often the operating principle for additives which promote particle attraction and clumping.

Air entrainment – The incorporation of air (bubbles) as a dispersed phase in a liquid. Air entrainment may result from mechanical means or by sudden environmental changes. The presence of entrained air is usually obvious from the hazy, opaque, or bubbly physical appearance of the liquid while dissolved air can only be determined by specific testing.

Alkali – any substance having basic (as opposed to acidic) properties. A more limited definition restricts the alkali to hydroxides of ammonium, lithium, potassium and sodium.

Alloy steel – steel containing specified quantities of alloying elements added to effect changes in mechanical or physical properties.

Ambient temperature –temperature of the area or atmosphere around a process, (not the operating temperature of the process itself).

Anhydrous – free of water, especially water of crystallization (water present as an internal part of the structure of a salt crystal).

Aniline point – the minimum temperature for complete miscibility of equal volumes of aniline and the sample. A product of high aniline point will be Iow in aromatics and naphthenes and, therefore, high in paraffins. Aniline point is often specified for spray oils, cleaning solvents, and thinners, where effectiveness depends upon aromatic content. In conjunction with API gravity, the aniline point may be used to calculate the net heat of combustion for aviation fuels.

Annealing – a generic term denoting a treatment consisting of heating a metal to a suitable temperature and holding it there, followed by cooling at a suitable rate. This process is used primarily to soften metallic materials, but also to simultaneously produce desired changes in the metal’s microstructure or other properties. When the term is used by itself, full annealing is implied. When the treatment is applied only for the relief of stress, it is properly called stress-relieving or stress-relief annealing.

Anodizing – forming a conversion coating on a metal surface by anodic (electron-removing) oxidation; most frequently applied to aluminum.

Apparent viscosity – The ratio of shear stress to rate of shear of a non-Newtonian fluid such as lubricating grease, calculated from Poiseuille's equation and measured in poises. The apparent viscosity changes with changing rates of shear and temperature and must, therefore, be reported as the value at a given shear rate and temperature.

API engine service categories – gasoline and diesel engine oil quality levels established jointly by API, SAE, and ASTM, and sometimes called SAE or API/SAE categories; formerly called API Engine Service Classifications.

API gravity – a gravity scale established by the American Petroleum Institute and in general use in the petroleum industry, the unit being called "the API degree", which is represented by the equation:

API gravity, deg = (141.5/specific gravity at 60/60ºF)-131.5

API base stock – base fluid for automotive engine oils. The API (American Petroleum Institute) currently designates five classes of base stocks. These stock types are independent of crude source or processing. Instead, they are based on physical characteristics of the base stock, which are then related to formulated engine oil performance:

	GROUP CHARACTERISTICS Group I Contain <90% saturates and/or >0.03% sulfur, with viscosity index ³ 80 and <120. Commonly referred to as conventional, solvent refined base stocks. Group II Contain ³ 90% saturates and £ 0.03% sulfur, with viscosity index ³ 80 and <120. Commonly referred to as severely hydrotreated and/or hydrocracked base stocks. Group III Contain ³ 90% saturates, with £ 0.03% sulfur, with viscosity index ³ 120. Commonly referred to as VHVI or XHVI base stocks. Group IV PAO's (polyalphaolefins) Group V All other base stocks not included in Groups I, II, III, or IV.   (where < stands for less than, £ stands for less than or equal to, ³ stands for greater than or equal to, and > stands for greater than)

Aromatic – derived from, or characterized by, the presence of the benzene ring.

Ash – a measure of the amount of inorganic material in lubricating oil. Determined by burning the oil and weighing the residue.

Asperities – microscopic projections on metal surfaces. Asperities result from normal finishing processes or from any other process or condition which changes the surface profile. Interference between opposing asperities in sliding or rolling wear is a source of friction, and can lead to metal welding and scoring. Ideally, the lubricating film between two moving surfaces should be thicker than the combined height of the opposing asperities.

Auto-ignition point – The temperature at which a substance ignites without other sources of energy. Also, associated with the spontaneous ignition and sustained combustion of part or all of the fuel-air mixture in an engine. The flame speed is many times greater than that associated with normal spark ignition, producing a characteristic "knocking" sound.