Specific gravity is also called relative density. It’s used to define the weight or density of a substance as compared to the density of an equal volume of a reference substance at a specified temperature.
Specific gravity for liquids is almost always measured with reference point to water with the temperature at 4 °C or 39.2 °F.
Specific gravity for gases is almost always measured in the reference to air at room temperature of 20 °C or 68 °F.
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Specific Gravity Is A Measurement
Specific gravity is the ratio of the density of a substance to the density of a reference substance. (source 🗗)
In most of the cases the reference substance is water for liquids or air for gases. Specific gravity represents a ratio and is therefore dimensionless. (source 🗗)
For example the density of sampled liquid / density of water = specific gravity. (source 🗗)