"C J Campbell" wrote in message
...
Water can get into the system at any point, as you suggest.

As for waiting 10 to 15 minutes before sampling the fuel, why? Try this
experiment: put some water and some fuel in a jar. Shake it up, and time
how
long it takes for the water to settle out.

10 to 15 minutes might be overly generous, but your experiment doesn't
really duplicate the conditions within an aviation gas tank.

Some are lined with rubber bladders, that can get small wrinkles (or
sometimes large wrinkles, which is really bad) that hinder the flow of water
toward the drains. Others have part of the wing structure inside the tank,
with holes drilled at the bottom, that impede that flow. In ALL
wing-mounted tanks, water can be quite distant laterally from the drain, and
it will take the water a lot longer to flow toward the drain along the
bottom than it would for water to be pulled by gravity straight down in a
jar.

In a jar, water will settle in a matter of tens of seconds, even if shaken
vigorously (much quicker if it's not). But in a fuel tank, it can easily
take several minutes.

So: always test your fuel (besides, you're not just looking for water).
Always give the fuel some time to rest before checking (at least a few
minutes). If you find ANY water, gently rock the wings to encourage water
to get past any obstacles and get it into motion toward the drain, and wait
a longer period of time just to make sure, and then check again. Repeat
until you have no water.

Pete