Specific Gravity - SG - is a dimensionless unit defined for liquids as "the ratio of the density of the substance to the density of water at a specified temperature".For gases the Specific Gravity is related to air.. Specific Gravity for common fluids are indicated in the table below.Material Specific Gravity Temperature 1,1,2-Trichlorotrifluoroethane 1.558 25 °C Specific gravity is a measure of the density of a mineral or in this case optical lens material. It is derived from the density of the lens material divided by the density of water. Since water's density equals 1 gram per cubic centimeter (at specific conditions), then a mineral's specific gravity (SG) would also correspond to a mineral's ... The pipes used for gravity fed systems are resistant to a certain pressure, call Nominal Pressure (NP): if the pressure in the pipe is higher than this NP, there is a risk of rupture. The range of pipes nominal pressure generally used for the gravity fed system are given in table 1. Table 1: Pipes pressure level NP Pipe type Nominal Pressure