From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: gas fridge while driving
Date: Sun, 25 Jun 2000 23:19:30 -0400

retireminn@my-deja.com wrote:

> So my question is "Where do you draw the line at worrying  about flames
> and gasing up?"
>
> Cyril

Rather than make a decision on what emotionally "feels good", let's
look at what we know and then make a decision.

First off, we know that gas vapor hugs the ground.  Experience tells
us that.  NFPA tells us that placing a flame 18" above the floor
provides protection from ignition of flammable vapor.  That's the
reason for putting residential water heaters on 18" stands.

Since we know that, we know that flames far above the 18" boundary
are not likely to ignite gas vapors. In my case, my refrigerator's
flame is over 3 feet above the ground.  I therefore have a high
level of confidence that my refrigerator will not ignite spilled gas
vapors.

Next, let's consider how probable it is for there to be ignitable
vapors. We can discern that from looking at ignition sources and
assume that if vapors were there in the presence of one or more
ignition sources, then any vapors would have a high probability of
being ignited.  So let's look at ignition sources.

In fact, there are numerous sources of ignition in the form of
catalytic converters.  From SAE papers I have, I learn that a
catalytic converter runs with a surface temperature somewhere around
400-500 deg C.  From a gasoline MSDS, I see that the autoignition
temperature ranges widely with formulation but that a typical
temperature is around 300 deg C.  The converter will, of course run
hotter at idle and if the engine is malfunctioning.  A mild misfire
undetectable to most drivers will raise the temperature dramatically
because the misfire feeds raw fuel to the converter.  Plenty of
thermal margin to make the converter a reliable ignition source.

Since there may be dozens of cars at a large station such as Flying
J and since most cars now have cat converters, we can assume that
there are plenty of ignition sources.  Still, we almost never hear
of gas station fires.   They're infrequent enough that they still
typically make the national newswire.  The most logical conclusion
is that there are seldom if ever ignitable vapors in sufficient
quantities to reach other vehicles around the pumps.

Still, there is a non-zero chance of ignitable vapors being present
so now we must assign relative probabilities to postulated
scenarios.

One scenario is, of course, ignition from an RV flame high off the
ground.  Another is the scores of catalytic converters.  Yet another
is careless smokers.  Yet another are the sparks emitted from
various electrical devices on vehicles.  One doesn't have to look
long to see sparks come from starters or exposed flywheels when cars
lacking certain guards are started.

Here's where a little subjectivness creeps in.  I assign the
probabilities of ignition as follows:

cat converters - high
careless smokers - high
other vehicle ignition sources - medium
my RV refrigerator - low

Since we all readily accept the risks presented by cat converters,
careless smokers, sparking electricals and other common sources, it
follows that we should accept the very minimal to no risk presented
by an RV 'fridge.

"But", he says, wringing his hands in panic, "what if some guy
starts hosing down the island with the gas hose?" It'll probably
blow up, of course.  And if you happen to be there, your ticket
might just get punched.  But.  The relative risks don't change.
Sometimes shit really does happen.

Meanwhile the risk of letting wifey's food get hot greatly outweighs
the risk of setting off some guy hosing down the gas island so I'll
continue driving with the 'fridge on.

John