Ron and Jim,

Some comments:

Jim's definition of specific gravity is correct, but could be slightly misleading in that the Specific Gravity can be calculated for solids, liquids, and gases and by definition is not solely associated with liquids. The difference being the standard refernce used to calculate the Specific Gravity, which is what Jim has correctly stated. For liquids and solids, the standard reference density (at least in engineering) is the density of pure water. For gases (also in engineering), it is the density of air at the same temperature and pressure as the gas.

I did not typically find the term Vapor Pressure in text books associated with topics in mechanical and fire protection engineering. It seems to show up in other disciplines such as meteorology (Chris Bloom may be able to provide more insight). It also seems to show up in trianing materials for the fire service, which may be the reason that NFPA 921 has a definition of Vapor Pressure in stead of the more general definition of Specific Gravity. The use of the term Vapor Pressure is the same as the definition of Specific Gravity of a gas, although in some contexts such as calculating the Relative Humidity using the Saturated Vapor Density, units of mass/unit volume are used while the Specific Gravity of a meterial is non-dimensional.

I hope this is helpful to your cause.

Sincerely,

Doug

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
(410) 884-3267 (fax)
www.csefire.com