Join Date: Feb 2004
Thanked 0 Times in 0 Posts
Rep Power: 0
Throughout this interesting thread conflicting statements have been made with regard to the behavior of gases and vapors. Unfortunately, none of the posts seems to provide a fairly comprehensive picture. Of the top of my head; these are the fundamental mechanisms underlying the observed phenomena:
At low concentrations only the relative molecular weights of molecules in the gaseous state, whether representing permanent gases, vaporized liquids or sublimated solids, determine which gas/vapor is heavier or lighter. At high concentrations, however, nonideal behavior can occur if the gas/vapor molecules are "sticky" (e.g. because of polar groups) and start clustering to form "dimers", "trimers", etc. thereby multiplying their effective molecular weights by a factor 2, 3 or more.
The molecular weight of the two major air components nitrogen (~80%; MW=28) and oxygen (~20 %; MW=32) puts dry air in an average MW range somewhere between 29 and 30. Low concentrations of water vapor (MW=18) will lower that number whereas higher water concentrations may increase the effective average MW of air through the formation of dimers and larger molecular clusters. Of course, below the dewpoint temperature water vapor will tend to condense out again, & s.o. & s.f..
Normal gasolines contain only trace amounts of gases/vapors lighter than air; e.g. methane (mw=16) or ethane (MW=28) and are primarily composed of hydrocarbon molecules with anywhere between 4 and 10 carbon atoms plus small amounts of non-hydrocarbon additives and degradation products; probably putting the average molecular weights of gasoline vapors somewhere in the MW 80 - 120 range.
So in completely still air gasoline vapors will tend to sink to the bottom of any airfilled containers, lockers, cockpits or cabins. In practice, however, most open spaces aboard an operating vessel will have significant air movements because of mechanical disturbances (people moving, doors opening, fans running) or convective air movements induced by temperature (and thus density) differences. This is the reason we can often smell gasoline and other relatively heavy chemical vapors , even though we are not direcly sniffing along the ground.
For the same reason convective air flows and other types of air currents can easily bring so-called lighter-than-air gases such as methane or even hydrogen down into open cabins or near open galley flames, even if originally released well above such danger points.
Last edited by HenkMeuzelaar; 04-29-2006 at 01:55 PM.