Specific gravity and density: how do they differ?

Confused about specific gravity (SG) and density?

SG is the ratio of the absolute density of the specified material at a specified temperature over the absolute density of water at a specified reference temperature (usually 15^oC). The density values should of course be in the same units. Because it's only a ratio, SG has no units. However, because the density of water at 15^oC happens to be very close to 1.00 grams per mL, the units of SG are also close to grams per mL.

Why use SG instead of density?

Firstly, it's universal - whether you're thinking in pounds and cubic inches or kilograms and millilitres, the value doesn't change. Secondly, when you're measuring and reporting density at a high level of precision, you have to consider the difference between absolute density (which is the density of the specified material in a vacuum), and the density in air, which is slightly lower and varies to a small degree with air pressure. Air exerts a buoyancy effect of roughly 0.0012 grams per millilitre at 20^oC, so the weight of a material in air is slightly lower than what would be measured in a vacuum. Using SG eliminates any ambiguities between the absolute and measured density.

Is there really much difference between SG and density?

If you're using units of g/mL, or kg/L, then unless you're working at very high precisions the difference is tiny. At densities close to 1.0 g/mL, SG exceeds "density in air" by roughly 0.002 SG units. The relative difference diminishes at higher densities. For a solution with a "density in air" (at a normal air pressure) of 1.000 g/mL, the absolute density would be 1.0011 g/mL (if you used stainless steel weights to calibrate your balance), and the specific gravity relative to water at 15^oC would be 1.00205.

Conclusion: unless you're concerned with the third decimal place of SG, you can safely assume that SG has units of grams per millilitre.

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