Specific Gravity (SG) is the ratio of the density of the substance to a reference density, usually deionized water. In other words, it is the ratio of the mass of the substance to that of a reference mass for the same given volume at room temperature. Being a ratio of densities, specific gravity is a dimensionless quantity but varies with temperature and pressure. For this reason, it is crucial that densities of the sample and reference are determined at the same temperature and pressure.
Test Method A as per ASTM D792-08 is used to determine the specific gravity and density for polymers. This test method uses water as the immersion liquid and an electronic single pan balance.
Calculate the specific gravity of the plastic as follows:
sp gr 23/23°C = a/(a + w – b)
a = apparent mass of specimen, without wire or sinker, in air,
b = apparent mass of specimen (and of sinker, if used) completely immersed and of the wire partially immersed in liquid, and
w = apparent mass of totally immersed sinker (if used) and of partially immersed wire.
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