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By now, you should have a pretty good idea of how liquid and vapor fuels react. You should be ready to step a little closer into the actual chemistry of the fuels. We'll begin by discussing the most basic and well-known property: their "Flash Point."
In the US, a fuel's flash point is the primary measurement to determine it's safe usability. This is expressed in 3 distinct classes: I) room temp and below, II) room temp to max atmospheric, and III) above max atmospheric. Any given fuel falls into the category at which it begins producing enough vapors to ignite.
...theoretically...
The flash point is a very old measurement going back to the alchemical days. Back then it was discovered that nearly anything would produce a "flash" if heated in the presence of a flame. The flash point was temperature at which any given mix of... stuff... would produce this flash.
In the more modern, scientific world, we know this flash to be the ignition of volatile vapors produced by almost all organic substances. It is most used in the waste management industry to determine how safe a pile of ground organic muck will be in a landfill.
The process of measuring the flash point of a substance varies slightly, based on the type of substance you're working on. But the basic setup is the same. An open metal cup is attached to a heating coil. A thermometer rests within the contents of the cup and keeps accurate track of the temp of the testing substance. About 8 inches away, a small open flame burns. The whole thing is kept in a stable air environment at sea level pressures and at an ambient temperature of about 70 degrees (f).
It's all very clean, but it's that cleanliness that makes it less useful as a broad indicator of general safety. Too many people simplify the definition of flash point to "the temperature at which a substance produces volatile vapors." But as you can see from the testing apparatus, the true definition is more like: "the temperature at which a substance will produce a sufficient density of vapors to ignite, from an open cup, at sea level."
As soon as you start forgetting some of the extended definition, you forget about a lot of the variables that make the real differences between Class I and Class II. The density of vapors thing is an important one: Class I fuels won't produce enough density to ignite in an open environment, and have to fill a closed environment first, then build enough density to flash back to the source.
Class II and III fuels almost always produce enough density to ignite, but change when you talk about the sea level or open cup issues. If you're at 4000 feet, the atmospheric pressure is much lower than sea level, so your flash point drops noticeably. Cheap kerosene could act like white gas and burn on your clothing, cheap lamp oil or biodeisel could react like kerosene.
And finally, if there's anything in your fuel, like bits of wick that have fallen off, sand, lint, etc, the extra surface area could radically change how fast your fuel is able to produce vapors.
So, flash point is a handy tool for determining how a fuel will react in stringently sterile conditions, however, in the real world, enough variables exist to make it almost dangerous to use as the exclusive indicator of a fuel's safety.
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