In my opinion, vapor density is one of those poorly conceived “terms of art” used by the fire investigation community. Of course it has the nonsensical NFPA 921 definition of “the ratio of the average molecular weight of a given volume of gas or vapor to the average molecular weight of an equal volume of air at the same temperature and pressure.” Nonsense in that definition includes the idea that the average molecular weight of a gas or air changes with its volume, temperature or pressure (of course it does not, molecular weight is a function only of the molecules present and the atomic weights of the constituent atoms in the molecules). It is also nonsensical because the scientific definition of anything that is a density, has specific units, namely units of mass per unit of volume. In contrast, in most instances where I have seen vapor density quoted, it is unitless (it is a ratio of two measurements with identical units). It is also nonsensical as it encourages individuals to speak of such things as the vapor density of natural gas (which is almost completely methane, a substance with a critical temperature of -83C or -117F, and thus a substance few individuals will ever experience in a “vapor” form (a vapor is defined as a gas at a temperature below its critical temperature, the temperature at which it can be liquefied simply by increased pressure)).

A check of the scientific references provides a variety of disparate definitions for vapor density:

vapor density - weight of a vapour per unit volume at a given temperature and pressure (Hawley’s Condensed Chemical Dictionary)

vapor density – see absolute humidity – the ratio of the mass of water vapor present in a mixture to the volume it occupies (Academic Press Dictionary of Science and Technology)

vapor density is the weight per unit volume of a pure gas or vapor (Fire Protection Handbook)

(all of which meet the scientific requirements of being a density, that is having units of mass per unit of volume)

or no definition at all (McGraw-Hill Dictionary of Scientific and Technical Terms, McGraw-Hill Concise Encyclopedia of Science and Technology, Chambers Dictionary of Science and Technology).

I note that the Fire Protection Handbook goes on to define the “vapor density ratio” as the ratio of the molecular weight of the material to the average molecular weight of air.

I also note that Kirk’s 5th edition defines vapor density of a gas or vapor as the unitless ratio of the molecular weights of the gas or vapor to the molecular weight of air (but then adds in Table 4.4 “Vapor Density at standard temperature and pressure” again as if changes in temperature or pressure affects molecular weight).

In summary, the terms specific gravity, relative density and vapor density in practice by many fire investigators are all used refer to the same thing, generally a ratio of the density (mass per unit volume) of one substance (under specified conditions) to the density of another substance (under specified and not necessarily the same conditions).

Because of that, I would like to see the term “vapor density” banished from use by fire investigators and the alternative and more proper terms specific gravity or relative density used (with the appropriate reference substances and conditions cited).

In the special circumstances, where
1. the substance is a gas or vapor, and
2. the reference substance is air, and
3. the reference temperature and pressure conditions of both the gas or vapor and the air are the same, and
4. under the assumption that the gas or vapor and air both act like ideal gases,

then the specific gravity/relative density/vapor density/vapor density ratio of the gas or vapor is the ratio of the molecular weight of the gas or vapor to the molecular weight of air.

Of course that combination of special circumstances is the commonly encountered situation where a flammable gas or vapor is present in air (ignoring any temperature differences in the gas or vapor from that of air that may have been created by the pressure release auto-refrigeration process for a high pressure gas leak, or the cooling that may have been created by the evaporation of a liquid to create the vapor).

Michael A. Learmonth, B.A.Sc., M.B.A., M.Eng., J.D., LL.B., C.F.E.I., P.Eng.
Senior Associate, Giffin Koerth Forensic Engineering and Science
40 University Avenue, Suite 800
Toronto, Ontario, Canada M5J 1T1
Phone: 416-368-1700 ext. 225
Fax: 416-368-5576