Relative density
Relative density (also known as specific gravity) is a measure of the density of a material. 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 denotes density.
Since water's density is 1.0 × 103 kg/m3 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). There are no units of measurement.
Water's density can also be measured as nearly one gram per cubic centimeter (at maximum density) in metric units. The relative density therefore has nearly the same value as density of the material expressed in grams per cubic centimeter, but without any units of measurement.
Relative density or specific gravity are often ambiguous terms. This quantity is often stated for a certain temperature. 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. But they are also often compared to water at a different 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 calculated from formula in 68th CRC Handbook of Chemistry and Physics 1987–1988.
| Density of water at 1 atm (101.325 kPa, 14.7 psi) (little or no difference at 100 kPa)
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Water is nearly incompressible. But it does compress a little; it takes pressures over about 400 kPa or 4 atmospheres before water can reach a density of 1000.000 kg/m³ at any temperature.
Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones. 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. Basically, density is defined as the mass of a sample divided by its volume. With an irregularly shaped rock, the volume can be very difficult to accurately measure. The most accurate way is to put it in a water-filled graduated cylinder and see how much water it displaces. It is also possible to simply suspend the sample from a spring scale and weigh it under water. Solving Isaac Newton's equations yields 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), and F is the force, measured in the same units, while the sample was submerged. Note that with this technique it is difficult to measure relative densities less than one, because in order to do so, the sign of F must change, requiring the measurement of the downward force needed to keep the sample underwater.
Another practical method uses three measurements. The mineral sample is weighed dry. 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. Subtracting the last reading from the sum of the first two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method works with scales that can't easily accommodate a suspended sample, and also allows for measurement of samples that are less dense than water. Surface tension of the water may keep a significant amount of water from overflowing, which is especially problematic for small objects being immersed. A workaround would be to use a water container with as small a mouth as possible.
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. (S= specific weight of the material/ specific weight of 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/m^3. Specific Gravity is unitless.
Specific gravity of Biogas
The density of biogas at 50% methane proportion is 1.227 kg/m3. Hence Specific gravity of Biogas is 1.227.
Methane properties
Liquid phase density (1.013 bar at boiling point) : 422.62 kg/m3 Gaseous phase density (1.013 bar and 15 °C (59 °F)) : 0.68 kg/m3
* 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 body effectively excretes these toxins via urination, and the role of the kidney is to concentrate as many toxins as it can into the least amount of urine to provide for a more efficient emission. 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; basically, specific gravity is measuring the concentration of solutes in the solution.
The body generates countless toxins every moment. In the kidneys, these toxins are dissolved in water so the body can filter them out through urination. A healthy kidney will use fewer fluids to eliminate these toxins to promote fluid concentration. In an unhealthy kidney, however, more water might be required to dissolve these toxins.
Such is the case in a person with renal failure. A person with this problem would drink more water to account for the excess water loss and his specific gravity would be lower. If the kidneys fail over an extended period of time, more water would be needed in order to concentrate the same amount of urine. Toxin levels in the body would rise, and ultimately, one could not keep up with the amount of water necessary to excrete the toxins. The rising toxin levels in the body do not increase the specific gravity in the urine because these toxins are not manifesting themselves in the urine which is still heavily diluted. The urine will have the same fixed gravity regardless of water intake.
Lowered specific gravity can also occur in diabetics that are lacking an anti-diuretic hormone. This hormone generally sends an appropriate amount of fluids into the bloodstream, and less water is available for urination. A lack of ADH would increase the water volume in the kidneys. A person with this issue could urinate up to fifteen or twenty liters a day with a low specific gravity. Another occurrence resulting in low specific gravity is when the kidney tubules are damaged and can no longer absorb water. Such an instance would also result in a higher water volume in urine.
A high specific gravity is most often indicative of dehydration. 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. Naturally, a lesser volume of liquid provided for urination with the same amount of toxins would result in a higher specific gravity- a higher density of the solutes. There are also other instances where the specific gravity might be raised. When the renal blood pressure is lowered, the artery must compensate with other fluids. 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. A pattern presented throughout the report indicates that when urine volume is increased, the specific gravity is lowered. This can be logically understood upon the cognitive awareness that when there is an identical amount of a solute in two solutions, the solution with a greater liquid will be less dense that that of the lesser liquid. 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.