Bryan wrote:

I am in the middle of testing my fuel tanks for leaks with a vacuum
pump.  How long is long enough to hold vacuum to prove it wont leak?


Be very careful with the vacuum pump. More than one Lancair builder has
"imploded" his wing tanks with this not-so tried and true method. There have
also been several mishaps from people trying to use compressed air to test
for leaks. This method is almost certain to provide comic relief to
others...don't even consider it. You might, however, consider an alternative
method that worked well for me:

Carefully seal all openings/vents, etc except one--the fuel pickup port, or
the return line work well because you can provide it with a very air-tight
fitting to your "test rig." Make a test rig from a good-quality latex
balloon--the kind used for filling with helium are generally OK--and a short
piece of aluminum pipe with a fitting that will attach to your fuel tank
opening. CAREFULLY seal the balloon to the pipe. RTV on the pipe, and a
rubber band wrapped over the neck of the balloon to hold it tightly to the
pipe works well. Blow up the balloon and twist the neck to keep the air
inside while you attach the fitting end to your tank. When you allow the
balloon to un-twist, most of the air will rush into the tank until the (very
small) pressure is equalized, but the balloon needs to remain partially
inflated. You may need a couple of tries to get it right... You have a
fuel-tight tank when the size of the balloon changes with the barometric
pressure over 2-3 days, but does not go flat during that time.

You can make a fancy test rig with a Tee and valve, etc, to aid in filling
the balloon, but everything you add must be absolutely air tight or you're
adding complications with negative benefit. I can't over-emphasize the need
for careful sealing of all openings/fittings. For example, I used clear
packaging (release) tape to seal the filler cap opening on my tanks. Since
the cap was larger than the width of my tape, I used two strips, carefully
overlapped and pressed down. My balloon went flat in about two days. Double
checked every joint/connection and opening with soapy water and concluded I
had a leak in the tank, but before proceeding with "dunking the wing" to
find it, I tested the second wing tank. Same results...a very slow leak that
deflated the balloon in a couple of days. I then went back with the soapy
water around the openings and discovered air leaking along the inside edge
of the clear packaging tape where I'd overlapped them across the top of the
filler cap. One very small bubble every half minute or so at each end of the
tape where one strip laid across the other! YGBSM! Anyway, it was enough of
a leak to deflate the balloon--in time. Re-applied the tape strips with a
very small wipe of RTV at the edge of the first tape and both tanks tested
well. The balloon stayed inflated for a week or so for each tank.

Bottom line: This method works well, and is safe to use. The tanks can stand
VERY LITTLE positive or negative pressure, so no matter what method you use,
be very careful, and don't even think about approaching your tanks with a
compressed air hose...or even a vacuum cleaner hose for that matter!

Bob Pastusek