FINA Lubricants Viscosity

Viscosity

Glossary Description: A fluid’s resistance to flow with respect to temperature.

How is the test performed? The fluid is placed in a ‘viscometer’, (a calibrated capillary tube for precise flow measurement between two pre-marked points on the tube) and pre-heated to a given temperature in a ‘viscosity bath’ (which is usually oil-filled). After the oil reaches the temperature at which the viscosity is desired, gravity-influenced flow of the oil is initiated in the viscometer and timed between two calibrated points. This time becomes the determinant for the result.

Reporting units: “Centistoke” (cs., cSt) is presently the customary unit.

Accuracy/Sensitivity: +/- 3% of value is expected in most used lubes, slightly better for new lubes.

Technical overview: Shearing of a lube occurs when its molecules are split into yet smaller molecules. This can happen from two basic processes: heat & pressure from the system (this can affect even the lubes base stock, though more aptly applies to any viscosity-index improvers which may have been present); mechanical shearing, such as ring scraping against a cylinder wall, trapping lube molecules and cutting them. Viscosity Index is a measure of a lubes resistance to thinning as temperature rises, an important property, particularly in cold climates: one wants a LOW viscosity for pumpability, but then wants the lube to remain thick enough to provide film strength at operating temperature. Polymer-based “VI improvers” help accomplish this in motor oils, in particular. VI is of little use in used motor lube analysis; too many variables preclude a valid assessment.

What causes viscosity to change?

Upward

¨       Lube oxidation

¨       Foaming/Pump Cavitation

¨       Emulsion with water

¨       Wrong fill or make-up lube (higher viscosity than recommended)

¨       Soot or Solids contamination

Downward

¨       Fuel or solvent contamination

¨       Molecular shearing (see above)

¨       Non-emulsified water contamination

¨       Wrong fill or make-up lube (lower viscosity than recommended)

¨       Refrigerant (air conditioning systems)

Suggested application: Viscosity is recommended for any application.

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