Specific volume is a term, usually applied to gases, that indicates the amount of volume occupied by a certain mass of the gas. Specific volume is an intensive property, which means that it is not determined by the total amount of gas. It can vary according to other factors such as pressure and temperature. It is also reciprocal to density, meaning that it is inversely proportional to the mass per volume formula that is used to determine density. All other factors remaining constant, when specific volume increases, density decreases, and vice versa.
The most common way to express specific volume is by the mathematical expression v = Volume/Mass. The volume and mass of the gas in question must of course be known in order to use this simple equation. It may be possible, however, to derive this value from other data and from known values for other properties of certain gases. If the density of a given volume of gas is known, the specific volume may be found by applying the formula v = 1/r, where r is the density.
Pressure and temperature have an effect on the volume of a given mass of gas. For this reason, specific volume is a physical property which can change for any gas depending on other factors. Assuming other conditions remain constant, increases in temperature cause gases to expand, reducing specific volume while increases in pressure compress gases, resulting in an increase in specific volume. The rates at which a particular gas expands or contracts according to changes in temperature and pressure are different for each gas.
Units for describing specific volume are most commonly the International Standard (SI) system of units or the US customary system, which is analogous to the old British Imperial system. The SI units for specific volume are cubic meters per kilogram while it is expressed as cubic inches or feet per pound in the other systems. It is of course possible to use other scales of units within either system, such as cubic meters per metric ton or cubic yards per American ton. Smaller units can be used as well.