Specific Gravity (SG) is a special case of relative density defined as the ratio of the density of a given substance, to the density of water (H2O). Substances with a specific gravity greater than 1 are heavier than water, and those with a specific gravity of less than 1 are lighter than water.
The density of the denominator, or reference, is = 1000kgm-3 (at 4°C/39.2°F) in SI units.
SG is by definition dimensionless and therefore not dependent on the system of units used (e.g. slugsft-3 or kgm-3), in so far as the units are consistent. Based on the SG-value of a given substance, the density of that substance can be calculated.
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- Introduction to Fluid Mechanics Fourth Edition, Wiley, SI Version, R.W. Fox & A.T. McDonald
- Thermodynamics: An Engineering Approach Second Edition, McGraw-Hill, International Edition, Y.A. Cengel & M.A. Boles