Jim,

Your question highlights the previous discussion on definitions.

The true definition of Vapor Density (mass/unit volume) for air and propane changes with temperature (assuming the same mass and pressure) and can be calculated for any temperature using the Ideal Gas Law (PV=nRT).

In contrast, the specific gravity of propane does not change with temperature since both gases are calculated at the same reference temperature. The density of an ideal gas is linear and inversely proportional to temperature. Assuming the same mass and pressure, both air and propane will change in the same proportions with temperature. Thus, the specific gravity does not change with temperature since both gases are calculated at the same pressure and temperature )i.e. . If using the same reference pressure and temperature, you can calculate the specific gravity of a gas by using a ratio of molecular weights (MW) or the inverse ratio of specific gas constants.

If you are asking for the specific gravity of propane in ratio to air with a different temperature and pressure than the reference gas (i.e. air), then the Ideal Gas Law is the appropriate equation for your analysis.

I hope this is helpful to you.

Sincerely,

Doug Carpenter

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
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