Viscosity

Oil Viscosity

Viscosity is the most important property of the oil and is fundamental to providing optimum film strength, with minimal frictional losses, preventing metal-to-metal contact, scuffing, microwelding and wear of sliding surfaces. Always referred to as a standard temperature - usually 40°C or 100°C - the viscosity of engine oil may fall due to fuel dilution, by topping up with an incorrect oil grade, or shear of polymer additives. Conversely, viscosity may increase due to excessive soot loading (insolubles content) or if the filtration system is not operating correctly. For all oils, ageing caused by oxidation and thermal degradation may lead to thickening and an increase in viscosity.

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Fuel Oil Viscosity

Understanding fuel oil viscosity is important as it determines the temperature for handling, the size of the centrifuges and the temperature at which the fuel is injected into the engine. It is well known that as the temperature of the fuel is increased, the viscosity is reduced.

The viscosity usually quoted for a residual fuel oil is the kinematic viscosity expressed in centiStokes (cSt) at a certain reference temperature. Although ISO 8217 provides a reference temperature of 100°C it is anticipated that this will be changed to 50°C at the next revision; a value widely used throughout the industry.  Some oil suppliers publish temperature/viscosity charts based on the average data of a large number of samples. However, estimations from the charts are not precise as the exact relationship depends on the source and composition of the fuel.

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Kittiwake Products for Viscosity Testing

• Viscometer – Kittiwake’s Unheated Viscometer is used in conjunction with the Kittiwake Oil Test Centre and is suitable for monitoring the viscosity of both lubricating and hydraulic oil.

• Heated Viscometer - Kittiwake’s Heated Viscometer is suitable for measuring the viscosity of both fuel and lube oil from a wide variety of applications including diesel engines, gas and aviation turbines, gear boxes and hydraulics.

• Viscotube - Kittiwake’s DIGI Viscometer (or Viscotube) uses a falling ball technique to measure the viscosity of oil in centistokes.