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I wrote a long piece in the Lancair Fast Packs on fuel tank leaks a few years
ago, but subsequently others have come up with better ideas.  Start as
suggested with the tank caps (use Saran wrap, see below), vents, etc.  Bubbles
work fine, but it turns out not all leaks make bubbles due to the shape of the
leak, flow rate (large leaks blow the water solution off),  type of soap used,
and the orientation (horizontal or vertical - vertical is much harder due to
run off). Bottom line best results are obtained if you use kiddies' bubble-
making solution.  It's a lot better than soapy water, and if you sponge on the
solution and then put on a thin layer of Saran wrap or equivalent, squeegee
out the bubbles, and look for the bulges as the leak bleeds air out you have
the best method for vertical surfaces, fuel caps, etc.  

On the Lancair IV, the chronic worst leak location was the leading edge
because of the porosity of the prepreg, tight curvature, and because the
vertical surface would not hold solution in place even after multiple passes
with a sponge.  Using the Saran wrap trick compensates for the run off of
water solution and quickly disclosed leaks not visible after hours of
searching.

The other toughie is the remote leak that runs down a rib/skin intersection
and exits at a point some inches away.  I had one of these that exited in a
closed bay, had to cut through the skin to get to it, and used bubbles to
localize looking internally with a mirror.  What a bitch.  The fix is to put
vacuum on the tank and suck in some runny micro (a syringe helps).  Release
the vacuum before you suck the micro all the way in leaving a micro-lined
passage way to leak later on.

Don't forget that pressurizing the tank causes the temperature to rise a bit,
and the tank pressure will cool as it sits over night reducing temperature.
Figure about a 1% reduction in total pressure for every 5 degrees F
temperature.  For reference, remember an atmosphere is about 30 inches of
mercury, or about 420 inches of water.  Pressurizing to plus one psi equals
about 27 inches of water so the total pressure will be about 447 inches of
water.  A 5 degree F drop would reduce the pressure 4.47 inches of water or
about a third of an inch of mercury or about one-sixth of a psi.  Not
negligible.  Once I got my tanks tight (a two week battle) the pressure gage
was good thermometer.

Hope this helps.

Fred Moreno