Density

DENSITY AND SPECIFIC GRAVITY

1. Density
The density () of a substance is defined as its mass per unit of volume.

In the International System of units (SI):
m in kg, V in m3   in kg/m3
Density is affected by both temperature and pressure
Generally, a liquid, a solid or a gas, expands with temperature (except water, which contracts when temperature varies from 0 °C to 4 °C, then expands above 4 °C).

For liquids and solids, which are non-compressible, only the temperature has to be taken into account
Density of distilled water at 4 °C = 1000 kg/m3 or 1 kg/dm3 or 1 g/cm3
Density of distilled water at 20 °C = 998.2 kg/m3
Density of copper (Cu) = 8 900 kg/m3
There are no simple formulas to calculate the value (given by tables)

For gases, conditions of pressure and temperature have to be taken into account:
(P,T)
Where
P: absolute pressure
T: absolute temperature
M: molar mass in kg/kmol
Z: super compressibility factor
R: gas constant = 8 314 J / kmol x kelvin

Two "Standard conditions" are used in the oil industry:
• ISO 5167:
101 325 Pa abs. (14.696 psia) and 15 °C (59 °F)
• AGA 3:
14.73 psia (101 560 Pa abs.) and 60 °F (15.56 °C)

Examples:
Density of air = 1.228 kg/sm3 (AGA)
Density of butane (C4H10) = 2.464 kg/sm3 (AGA)
Density of hydrogen (H2) = 0.0855 kg/sm3 (AGA)


2. Specific gravity
For solids and liquids, Specific Gravity is defined as the ratio of the density of the solid or liquid to that of water in the same conditions of temperature.

Where density of water at 4° C = 1000 kg/m3
The resulting quotient is a dimensionless number.
Examples:
Specific Gravity of mercury (Hg) at 20°C
Density of mercury at 20° C/density of water at 200C
13 545 kg/m3 / 998.2 kg/m3 = 13.56
SG of water at 4°C is 1
1000 kg/m3 / 1000 kg/m3

For gases, Specific Gravity (G) is defined as the ratio of the molar mass (MG) of a gas to that of the molar mass of dry air (MA)

Where molar mass of dry air = 28.965 kg/kmol
The resulting quotient is a dimensionless number.
Examples:
SG of butane (C4H10):
MG (C4H10)/28.965 = 58.124/28.965 = 2.007


3. API gravity (American Petroleum Institute)
An arbitrary scale expressing the relative density of liquid petroleum products
The scale is calibrated in degrees API, calculated by the following formula:
Degree API = – 131.5
Where SG = specific gravity at 15.6 °C
For a liquid with SG = 1:
Deg. API = – 131.5 = 10


4. Physical properties of gas compounds

Compound Formula Molar mass (kg/kmol) Specific Gravity
Hydrogen H2 2.016 0.0696
Water vapour H2O 18.015 0.622
Nitrogen N2 28.013 0.967
Carbon monoxide CO 28.011 0.967
Oxygen O2 31.999 1.105
Carbon dioxide CO2 44.001 1.519
Air - 28.965 1.000
Hydrogen sulphide H2S 34.080 1.177
Methane CH4 16.043 0.554
Ethane C2H6 30.070 1.038
Propane C3H8 44.097 1.522
Butane C4H10 58.124 2.007
Pentane C5H12 72.151 2.491
Hexane C6H14 86.178 2.975
Heptane C7H16 100.206 3.460
Octane C8H18 114.233 3.944