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Specific
Gravity
Specific
Gravity is a strong element
in the price factor and thus has great importance. Beyond
the price/volume relationship, however, specific gravity
is used in production control, both in raw material production
and molding and extrusion. Polyethylenes, for example, may
have density variation, depending upon the degree of "packing"
during molding, or the rate of quench during extrusion.
Although specific gravity and density are frequently used
interchangeably, there is a very slight difference in their
meaning. Specific gravity is the ratio of the weight of
a given volume of material at 73.4 °F (23 °C) to
that of an equal volume of water at the same temperature.
It is properly expressed as Specific Gravity, 23/23 °C.
Density is the weight per unit volume of material at 23
°C and is expressed as D23C, g/cm3. The discrepancy
enters from the fact that water at 23 °C has a density
slightly less than one. ASTM D-792.
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