Density vs Specific Gravity
Density and Specific Gravity are the two commonly used terms but often confused with each other. Particularly, in industries these terms are widely used to weigh different substance and to calculate the concentration of liquids. Temperature and pressure are very important in calculating the two.
Mass density, which is commonly called as density of a material is its mass per unit volume. Symbol used for density are p, where as its SI units are kilogram per cubic meter. Temperature and pressure are two factors, which affect the density, for instance, when we increase pressure the volume of object decreases, which enhances the density of that particular object. Similarly, if we increase the temperature of an object, its density decreases as the volume increases. Density of water is 1.0 g/ml, if the density of any object is less than water’s density; it will float on the surface of water and vice versa. If you increase the amount of a substance, it will increase the mass but will not affect the density.
Specific gravity is a relative value, so it has no units. It is in fact another term for relative density. It is the ratio of density of a substance to the density of a reference substance, which is always water for solid and liquid, for gas it is an equal volume of air or hydrogen. Temperature and pressure are important in measuring SG, as it is a measure at the standard pressure of 1 atm and 4 degree C temperature. Temperature can vary, in industries, as every industry has its own standards and needs. Specific gravity is used in industries to calculate the concentration of solutions, which are manufactured for specific purposes. Specific gravity of water at standard temperature and pressure is considered as one, so if the SG of a substance is five, it means it is five times dense than water. Similarly, if the SG of a substance is less than one, it will float on the surface of water.
Differences and Similarities
Density is an absolute terms and specific gravity is a relative term, so density has SI units but SG has no units for its expression.
As mass is involved in both cases, which is greatly affected by pressure and temperature, so Density and SG change with the change in temperature and pressure.
Density and SG of water is 1, which is used as reference for other fluids, i.e. if they have density and SG less than 1, it means they will float on its surface.
If we know the one, density or SG, we can easily calculate the other value. If reference substance is not mentioned, it means it is water for solid and liquid. In this situation, if the substance is fluid we can use hydrometer to calculate gravity and SG.
In some cases, we can express density in terms of specific gravity also, which is a dimensionless term.
Density and SG are the two terms, which help us in calculating the relative mass of a substance. Gravity being an absolute term has SI unit to express itself, whereas SG is dimensionless term. However, if we know one, we can easily calculate the other.