relative density - Specific Gravity of water, Specific gravity of Biogas, Methane properties, The kidneys and specific gravity

Density measured relative to some standard, typically water at 20°C; symbol d, expressed as a pure number; measured using hydrometers; formerly called specific gravity. For example, for alcohol, density ? = 789 kg/m³, d = 0·791.

It is dimensionless, equal to the density of the material divided by some reference density (most often the density of water, but sometimes the air when comparing to gases):

where

ρ denotes density.

Since water's density is 1.0 × 10 in SI units, the relative density of a material is approximately the density of the material measured in kg/m3 divided by 1000 (the density of water).

Water's density can also be measured as nearly one gram per cubic centimeter (at maximum density) in non-SI units. Sometimes when this is done, it is a comparison of the density of the commodity being measured at that temperature, with the density of water at the same temperature.

Relative density is often expressed in forms similar to this:

relative density: or specific gravity:

The superscripts indicate the temperature at which the density of the material is measured, and the subscripts indicate the temperature of the water to which it is compared.

Density of water at 1 atm (101.325 kPa, 14.7 psi) Temperature Density Celsius Fahrenheit kg/m3 0 °C 32 °F 999.8425 4.0 °C 39.2 °F 999.9750 15 °C 59 °F 999.1026 20 °C 68 °F 998.2071 25 °C 77 °F 998.0479 37.0 °C 98.6 °F 993.3316 100 °C 212 °F 958.3665

Water is nearly incompressible. The reason that relative density is measured in terms of the density of water is because that is the easiest way to measure it in the field. The following formula for measuring specific gravity:

where

G is the relative density, W is the weight of the sample (measured in pounds-force, newtons, or some other unit of force), F is the force, measured in the same units, while the sample was submerged. Then a container filled to the brim with water is weighed, and weighed again with the sample immersed, after the displaced water has overflowed and been removed. Surface tension of the water may keep a significant amount of water from overflowing, which is especially problematic for small objects being immersed.

Specific Gravity of water

The specific gravity is defined as the ratio of specific weight of the material to the specific weight of distilled water. This implies that if the specific gravity is approximately equal to 1.000, then the specific weight of the material is close to the specific weight of water. If the specific gravity is large this means that the specific weight of the material is much larger than the specific weight of water and if the specific gravity is small this implies that the specific weight of the material is much smaller than the specific weight of water. The specific weight of a gas is generally defined by comparing the specific gravity of air at a temperature of 20 degrees Celsius and a pressure of 101.325 kPa absolute, where the density is 1.205 kg/m3.

Specific gravity of Biogas

The density of biogas at 50% methane proportion is 1.227 kg/m3.

Methane properties

Liquid phase density (1.013 bar at boiling point): 422.62 kg/m

Critical temperature  : -82.7 °C Critical pressure  : 45.96 bar

The kidneys and specific gravity

The role of the kidneys in the human is to aid the body in its riddance of bodily toxins. The specific gravity of urine is the measurement of density of these minerals and toxins in the urine in relation to the density of the water; A person with this problem would drink more water to account for the excess water loss and his specific gravity would be lower. Another occurrence resulting in low specific gravity is when the kidney tubules are damaged and can no longer absorb water. If a person has gone without water for a day, his water level in his blood is lowered, and his brain signals the release of an anti-diuretic hormone which redirects water from urine into the bloodstream. Water is reabsorbed into the bloodstream to balance out the volume of blood and the volume of water in urine is subsequently lowered. As water is also used to control body temperature, when the body temperature goes up, less water is in the kidneys as it is used to aid in perspiration.

When testing for specific gravity, one should be aware that enzymes or dyes used in diagnostic tests can increase specific gravity. As stated before, specific gravity measures the concentration levels of the solute in the solution, ergo the solution of greater volume has a lower specific gravity.