The ratio of the density of a substance to the density of a reference substance, both at specified physical conditions. As applied to gas, air is the reference substance. Two specific gravity definitions are recognized in gas measurement: 1. Real Specific Gravity. The density ratio between a gas and air determined by measurement at the same temperature and pressure. 2. Ideal Specific Gravity. The ratio of the molecular weight of a gas to the molecular weight of air. (Mol. wt. of air = 28.9644).