# Physical Properties

API Gravity - API gravity is a measure of the density of crude oil as compared to density of water or in other words, it is a measure of specific gravity of crude oil. It is defined as,
API gravity = 141.5 X (density of water at 600F/ density of oil at 600F ) – 131.5

Carbon Residue - Carbon residue for a fossil fuel can be defined as the tendency of that fuel to form carbon deposits at high temperature in an inert atmosphere. Carbon residue for a fuel is measured in weight percentage (wt %) or parts per million by weight (ppm wt).

Cetane Number - Cetane (n-hexadecane) ignites readily and burns smoothly in a diesel engine. This hydrocarbon is arbitrarily assigned the cetane number of 100. Other fuels are compared with cetane for autoignition quality and then assigned with a cetane number.

Compressibility Factor - For Ideal gases, Ideal gas equation states, PV = nRT. This equation can be modified for real gases, to be expressed as, PV = ZnRT, here Z is the compressibility factor for the given gas.

Compressibility factor for natural gas is determined using empirical compressibility charts and you can use this calculator to quickly estimate the same based on Brill and Beggs Z factor correlation. Also this table will give you an the compressibility factor for air based on the pressure and temperature conditions.

Distillation Range - Distillation range for a mixture of close boiling liquids (which have their individual boiling points at a given pressure close to each other) can be defined as the temperature range over which this mixture exists in vapor liquid equilibrium (observed as boiling), at atmospheric pressure.

Initial Boiling Point - For a multicomponent liquid mixture, boiling occurs over a range of temperature. For such mixtures, the initial boiling point at a given pressure is defined as the temperature value when the first gas bubble is formed from the liquid mixture.

Initial Dew Point - For a multicomponent gaseous mixture, condensation occurs over a range of temperature. For such mixtures, the initial dew point at a given pressure is defined as the temperature value when the first drop of liquid is formed from the gaseous mixture.

Pour Point - Pour point of a liquid can be defined as the lowest temperature at which the liquid begins to flow or can be poured. Alternatively, it is the minimum temperature, below which the liquid will seize to flow. This term is usually used for crude oils.

Salt Content in crude oil - Crude oil extracted from the geological sources is sometimes accompanied with salt content. This salt content is usually measured in Pounds of Salt per Thousand Barrels of oil (PTB).

Specific Heat Ratio or Adiabatic Index - Ratio of specific heat of a gas at constant pressure Cp to the specific heat of the same gas at constant volume Cv is known as Adiabatic Index (Cp/Cv). This ratio is found useful in a large number of equation for thermodynamic analysis of the gas.

Standard Air Density - Calculation of air density at standard temperature and pressure conditions.

Sulfur Content in fossil fuels - Sulfur content is usually measured in weight percentage (wt %) or in parts per million by weight (ppm wt). Crude oil containing less than 0.5 wt % of sulfur is considered to have low sulfur content and is known as ‘sweet crude oil’. Higher sulfur content than 0.5 wt % results in crude oil being known as ‘sour crude oil’.

Thermal Conductivities of metals - List of thermal conductivity values of a group of frequently used metals, measured in W/mK.

Dynamic/Absolute Viscosity and Kinematic Viscosity - Dynamic viscosity or Absolute viscosity is used to determine the absolute viscous force resisting the fluid flow. In some cases the ratio of viscous forces and inertial forces in a fluid flow is considered to be important.  Hence the ratio of viscous forces to inertial forces in the fluid is represented by (absolute viscosity / density). This ratio is known as kinematic viscosity (ν) of a fluid.