The fluid composition changes less often. Because of the constant changes in density, people have developed different ways to express density. For liquids, specific gravity refers to the density of a liquid at a certain temperature as compared to the density of water at a certain temperature. However, temperature values are often forgotten or assumed. This makes the density value uncertain and a poor substitute for actual density.
Standard conditions can vary but are usually near ambient conditions.
Gas specific gravity is usually sufficient for estimating fluid density. Users that require the highest accuracy should use densitometers or chromatographs.