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R&O oil -- Rust & Oxidation inhibited lubricating oil. A highly refined industrial lubricating oil formulated for long service in circulating systems, compressors, hydraulic systems and turbines. Commonly referred to as Turbine Oils

Rate of shear -- the difference between the velocities along the parallel faces of a fluid element divided by the distance between the faces.

Reducer -- a connector having a smaller line size at one end than the other.

Refraction -- the change of direction or speed of light as it passes from one medium to another.

Refractive index -- the ratio of the velocity of light at a specified wavelength in air to its velocity in a substances being examined.

Rerefining -- a process of reclaiming used lubricant oils and restoring them to a condition similar to that of virgin stocks by filtration, clay adsorption or more elaborate methods.

Reservoir -- a container for storage of liquid in a fluid power system.

Reservoir (sump) filter - a filter installed in a reservoir in series with a suction or return line.

Residual dirt capacity -- the dirt capacity remaining in a service loaded filter element after use, but before cleaning, measured under the same conditions as the dirt capacity of a new filter element.

Return line -- a location in a line conducting fluid from working device to reservoir.

Return Line Filtration -- filters located upstream of the reservoir but after fluid has passed through the system's output components (cylinders, motors, etc.).

Rheology -- the study of the deformation and flow in terms of stress, strain, temperature and time. The rheological properties of grease are commonly measured by penetration and apparent viscosity.

Ring lubrication -- a system of lubrication in which the lubricant is supplied to the bearing by an oil ring.

Rings -- circular metallic elements that ride in the grooves of a piston and provide compression sealing during combustion. Also used to spread oil for lubrication.

Ring sticking -- freezing of a piston ring in its groove in a piston engine or reciprocating compressor due to heavy deposits in the piston ring zone.

Roll-off cleanliness -- the fluid system contamination level at the time of release from an assembly or overhaul line. Fluid system life can be shortened significantly by full-load operation under a high fluid contamination condition for just a few hours. Contaminant implanted and generated during the break-in period can devastate critical components unless removed under controlled operating and high performance filtering conditions.

Roller bearing -- an antifriction bearing comprising rolling elements in the form of rollers.

Rotating pressure vessel oxidation test (RPVOT) -- measure of the oxidation stability and lubricating oils in separate tests: ASTM D 942 (grease), ASTM D 2272 (oil) and ASTM D 2112 (insulating oil). In all tests, the sample is placed in a container, or bomb, which is then charged with oxygen and pressurized; a constant elevated temperature is maintained. ASTM D2272 utilizes a rotating bomb, which is placed in a heated bath. Oxidation stability is expressed in terms of pressure drop in a given time period (D 942) or in terms of the amount of time that is required to achieve a specific pressure drop (D 2272, D 2112).

Rust prevention test -- a test for determining the ability of an oil to aid in preventing the rusting of ferrous parts in the presence of water.

SAE -- Society of Automotive Engineers

Sample preparation -- fluid factors that can enhance the accuracy of the particulate analysis. Such factors include particle dispersion, particle settling, and sample dilution.

Saponification -- the alkaline hydrolysis of fats to form soap; more generally, the hydrolysis of an ester by an alkali with the formation of an alcohol and a salt of the acid portion. Saponification is a method used to make soap-type grease thickeners.

Saturation level -- the amount of water that can dissolve in a fluid.

Saybolt Universal Viscosity (SUV) or Saybolt Universal Seconds, (SUS) -- the time in seconds required for 60 cubic centimeters of a fluid to flow through the orifice of the Standard Saybolt Universal Viscometer at a given temperature under specified conditions. (ASTM Designation D 88.)

Scuffing -- abnormal engine wear due to localized welding and fracture. It can be prevented through the use of antiwear, extreme-pressure and friction modifier additives.

Scuffing particles -- large twisted and discolored metallic particles resulting from adhesive wear due to complete lubricant film breakdown.

Semisolid -- any substance having the attributes of both a solid and a liquid. Similar to semiliquid but being more closely related to a solid than a liquid. More generally, any substance in which the force required to produce a deformation depends both on the magnitude and on the rate of the deformation.

Shear rate -- rate at which adjacent layers of fluid move with respect to each other, usually expressed as reciprocal seconds.

Shear stress -- frictional force overcome in sliding one "layer" of fluid along another, as in any fluid flow. The shear stress of a petroleum oil or other Newtonian fluid at a given temperature varies directly with shear rate (velocity). The ratio between shear stress and shear rate is constant; this ratio is termed viscosity of a Newtonian fluid, the greater the shear stress as a function of rate of shear. In a non-Newtonian fluid -- such as a grease or a polymer-containing oil (e.g. multi-grade oil) -- shear stress is not proportional to the rate of shear. A non-Newtonian fluid may be said to have an apparent viscosity, a viscosity that holds only for the shear rate (and temperature) at which the viscosity is determined.

SI -- Systeme International, International System of Units.

Silicon (Si) -- element number 14 on the periodic table of the elements. In the context of oil analysis silicon can be present as an oil contaminant in various sampled components and the origins can be from such sources as: dirt, oil additives, and coolant just to name a few.

Silicone -- generic term for a family of relatively inert liquid organosiloxane polymers used as synthetic lubricants. The properties of silicone oils include very high viscosity index, good high temperature oxidation stability, good hydrolytic stability and low volatility. Silicones generally have poor lubricity.

Silt -- contaminant particles 5 µm and less in size.

Silting -- a failure generally associated with a valve which movements are restricted due to small particles that have wedged in between critical clearances (e.g., the spool and bore.)

Silver (Ag) -- element number 47 on the periodic table of the elements. In the context of oil analysis silver can be present as a wear metal in various sampled components and the origins can be from such sources as: bearings, oil coolers, and bushings just to name a few.