I typed Vapor Pressure (a much more familar term) instead of Vapor Density. Sorry for the confusion. The paragraph should read:

I did not typically find the term Vapor Density 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 training materials for the fire service, which may be the reason that NFPA 921 has a definition of Vapor Density instead of the more general definition of Specific Gravity. The use of the term Vapor Density 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.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
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Columbia, MD 21045
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