Viscosity, Specific gravity & API Gravity

Viscosity:
A Viscosity is a measure of the resistance of a fluid which is being deformed by either shear, stress or tensile stress. For (fluids only), viscosity is the measure of "thickness" or "internal friction". 


Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. in short, the less viscous the fluid is, the greater its ease of movement (fluidity) will be.


Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.  All real fluids  have some resistance to stress and therefore are viscous, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.

Specific gravity:

Specific gravity is the ratio of the density (mass of a unit volume) of a substance to the density (mass of the same unit volume) of a reference substance. 
Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. 
The reference substance is usually water for liquids or air for gases
Temperature and pressure must be specified for both the sample and the reference. 
Pressure is nearly always 1 atm equal to 101.325 kPa.  


API Gravity:

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. API gravity is thus an inverse measure of the relative density of a petroleum liquid and the density of water, but it is used to compare the relative densities of petroleum liquids. For example, if one petroleum liquid floats on another and is therefore less dense, it has a greater API gravity. Although mathematically, API gravity has no units (see the formula below), it is nevertheless referred to as being in "degrees". API gravity is graduated in degrees on a hydrometer instrument. The API scale was designed so that most values would fall between 10 and 70 API gravity degrees.

The formula to obtain API gravity of petroleum liquids, from specific gravity (SG), is:

Direct Measurement of API gravity (Hydrometer method) = This method is detailed in ASTM D287.

• Crude oil is classified as light, medium or heavy, according to its measured API gravity.
• Light crude oil is defined as having an API gravity higher than 31.1 °API. (less than 870 kg/m3)
• Medium oil is defined as having an API gravity between 22.3 °API and 31.1 °API. (870 to 920 kg/m3)
• Heavy oil is defined as having an API gravity below 22.3 °API. (920 to 1000 kg/m3)
• Extra heavy oil is defined with API gravity below 10.0 °API. (greater than 1000 kg/m3)