Specific Weight is defined as weight per unit volume. Weight is a force. The Specific Gravity - SG - is a dimensionless unit defined as the ratio of density of the material to the density of water at a specified temperature. The specific gravity of some common gases can be found in the table below:
- As a reference, normal air has a specific gravity of 1.
- If the specific gravity is greater than 1 the gas is heavier than air and the vapor will be found low; the larger the number the heavier the gas is and the lower vapors will be found.
- If the specific gravity is less than 1 the gas is lighter than air and the vapor will be found high; the smaller the number the lighter the gas is and the higher vapors will be found.
- Various vapors can be "layered" when multiple gasses are present.
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