Technical Tips

Specific Gravity

Specific Gravity is an Important Property

Specific gravity (sg) is properly defined as the ratio of the density of a substance (typically a liquid or a solid) compared to the density of water.  Water has a density of 1.0 g/cc at normal pressure and temperature, so the specific gravity of a substance that is 1.6 times heavier than water would be equal to 1.6.  Note that specific gravity has no units because it is the ratio of two density numbers.  It is important that the temperature and pressure of each substance is the same so that the density values represent the properties under the same conditions.

An important feature of specific gravity is that it can be calculated based on the sg’s of the components in a mixture.  The calculation is valid only if it is assumed there is no air trapped in the material.

Example:   ‘Mixture A’ has 40 weight % of ‘liquid X’ with sg = 1.2 and 60 weight % of ‘solid Z’ with sg of 2.7.  The calculated sg of ‘mixture A’ is = 1 / [(0.4 / 1.2) + (0.6 / 2.7)] = 1.8

Being able to calculate the sg of a mixture is important because the sg of a material can be used to verify problems with the mixture.

If the ‘Mixture A’ sg = 1.5 (not the 1.8 that would be expected) this may indicate that:

  1. there is air in the mixture.  Roughly 16% air will lower the sg from 1.8 to  1.5
  2. the amount of ‘solid Z’ is less than 60 weight percent.  If the material has been deaired, then ‘Mixture A’ has only 36 weight % of ‘solid Z’ not the targeted 60 weight % value. 
  3. The low sg could indicate that the wrong solid is in the mix.  If the right weights were used, then the sg of 1.5 indicates that the solid sg is 1.8, not the 2.7 that would be the case if ‘solid Z’ was used. 

The sg of a mixture of materials is a very important property that can indicate whether the mixture of materials was made correctly.