The terms Specific Gravity and Relative Density have been used for a number of years. Yet there seems to be some confusion over what exactly they mean. Specific Gravity is formally defined as the ratio of gas density to air density when both are at standard conditions of 0 Degree C and 760 mm. Over the years the definition evolved to become the ratio of gas density to air density at the same temperature and pressure, “Relative” to each other. Hence, the term “Relative Density.” This is the most commonly used definition today. The fixed or “Specific” requirement of temperature and pressure, (0 degree C and 760 mm), had been removed over the years. The American Gas Association in 1985 officially replaced the term Specific Gravity with Relative Density. However, both terms are still used synonymously