Viscosity

Introduction:
                Viscosity is defined as the resistance offered to flow of liquid. The unit of viscosity is taken as poise. Also the viscosity is the force in dyne which is required to move one square centimeter layer of the liquid with a velocity of 1cm/sec to another parallel layer of the liquid.

 

Symbol for the viscosity:
The magnitude of viscous drag force for a fluid film between two parallel surfaces can be calculated as:

This equation is defining dynamic viscosity which is denoted by η. The dimensions of η are M/LT.
                Viscosity is defined functionally as Newton defined it, “it is the resistance arising from lack of slipperiness in a fluid.”

• This definition applies to values of h greater than 50 times the dimension of the molecules in the fluid.
• Very thin films have higher viscosities, and films of the order of 5 nm thick begin to show properties of the solid.
• Viscosity of oil is measures its resistance to flow and is usually measured in terms of Saybolt Universal Seconds (SUS) which defined as the time required, in seconds, for a given quantity of oil flowing through a capillary tube under specified test conditions.
• Viscosity is usually expressed at two temperatures, 18oC (0oF) and 99oC (210oF).

 

Viscosity Index:

• The viscosity of oil is inversely affected by the temperature, higher the temperature lower is the viscosity.
• The variation of viscosity of oil with changes in temperature is measured by its Viscosity Index (V.I.).
• The oil is compared with two reference oils which have same viscosity at 99oC (210oF).

 

Cloud Point and Pour Point

 When the oil is started cooling, it will solidify at some temperature. This temperature is called cloud point. The pour point is that temperature just above which the oil does not flow under certain conditions.

Flash Point

 

The temperature at which the vapors of oil flash when subjected to a flame is known as the flash point of the oil. If the container is closed at the time of the test it is called closed flash point, and if it is open it is called open flash point.

Fire point

 

It is the temperature at which the oil when lit with flame, will burn steadily at least for 5 seconds. This is usually    11oChigher than open flash point and varies from 190oC to 290oC for the lubricants used for the internal combustion engines.

Carbon Residue

 

 Carbon residue is the quantity of the known mass sample of the oil, which on evaporation under specific conditions remains as carboneous residue.

Oiliness

 

The property of an oil to cling to the metal surfaces by molecular action and then to provide a very thin layer of lubricant under boundary lubrication conditions is called the oiliness or lubricity or film strength.

Color

 

This has no practical significance except that it is an indication of the degree of refining of the oil.