Walter wrote:
**Since a gas tank has a cap and air is not *supposed* to go in and out of it in large quantities, I wonder how much vapor would be introduced?**

George wrote;
**There is actually quite an opportunity for water to get in the tanks.
Each day the fuel warms and cools. Expands and contracts. The air above the fuel is expelled and re-inspired.**

Regarding the introduction of water into the tanks, whether it be vapor or liquid...I occasonally find a "spoonfull" or so of water during preflight.Often, I find none.  In the winter, about the time my old flushmount caps were going bad, I could find upwards from 1/2 fuel test tube of water, after a couple of pretty good rainstorms.

I live in San Jose, just south of the San Francisco Bay.  A few years ago, I flew to Eloy, Az to skydive for a couple of days.  The last thing I expected to see in the Arizona dessert in the summer was water...anywhere.  After two nights there,  I did my preflight, checked both tanks and gascolators for water.  I saw no water in the bottom of the test tube.  The first two tube fulls were tossed to the wind...the 3rd was dumped on the asphalt.  it looked wrong.  Puddled up instead of soaking in like gas (surface tension, I guess) It was ALL water!  There was every bit of a quart of water in the system.  I did have about 60 gallons of air in my 80 gallon tanks.  I'm sure that made a huge difference.
I know they used a lot of alcohol out there..but it wasn't isopropyl...that will make you go blind!
;p
Michael

att michael meadows
could it be warter put in your tanks intentionaly by unscrupulous peoples.
as you may know it already happened to the cessna caravan owned by the late roger nelson  iof skydive chicago .10 skydivers were killed after they lost the engine on take off due to water added by someone on purpose

Ed, what you say is true, but I believe it's only true for Mogas.  Avgas specifications are the same everywhere.