Ron:

Hope this helps.

Relative density (also known as specific gravity) is a measure of the density of a material. It is dimensionless, equal to the density of the material divided by some reference density (most often the density of water, but sometimes the air when comparing to gases).

Specific gravity is the density of a material measured against the density of water. Water weight is approximately 1 gram per cubic centimetre (g/cc). Other materials weight is compared against this weight to where if the material has a weight of less than 1.0 it will float on the water, where any material with a weight greater than the 1.0 will sink.

Vapor Density – Is the ratio of the weight of a given volume of one gas to the weight of an equal volume of another gas, typically hydrogen, at the same temperature and pressure. It is the mass per unit volume of a pure gas or vapor.

We find it listed on documents like an MSDS. On the MSDS they compare the vapor density give it as a percentage to the density of the gas or vapor to the density of air. To show how it relates to the weight of air they assign the density of air with the value of 1. Anything below this weight is lighter than air any measurement above indicates the gas is heavier than air.

Jim