REFLECTOR: Leak testing

["Al Gietzen" <alventures@email.msn.com>]

A word about leak testing the strake tanks:

If you're going to leak test your fuel tank by looking for changes in
pressure and volume of air; as in the balloon method, be aware that a given
pressure and volume are only valid at one temperature.  In other words,
seeing changes in the size of an inflated balloon on the fuel line may only
mean that the temperature has changed.

 A decrease of about 15 degrees F in your work area, say from afternoon to
the next morning,  can completely deflate a 8" diameter balloon, even if
your tank is leak tight.  A typical balloon, by the way, puts on about ½ psi
or less, which is about the same as the pressure head at the bottom of a
tank of fuel.

A very sensitive way to leak test is to use that water level you may have
made when you started building (I made one, then never used it because I
wasn't satisfied with its accuracy).  Add some pressure to, or extract some
air from, the tank and connect on your water level tube (now a manometer).
One foot difference on the water levels at the two ends is about ½ psi.
While monitoring for constant temperature in your work area, even the
tiniest leak would show up fairly quickly as a change in water levels.

I did this on my one completed tank this afternoon.  Extracted a few pumps
of air with the "top-side" oil changer I once bought for my boat, to give me
about a foot difference in the water levels.  Maintaining temp constant for
three hours the level did not change.  Feels good to know that my tank is
absolutely air tight.  This, of course was after I sealed over the filler
cap which leaked like a sieve, and finding a tiny leak at the flange of the
fuel lever sender unit.

I also found what a sensative thermometer this thing made.  I noted that the
level changed about 1/4" per degree in my 5/16" o.d. tubing.

Or you can build your plane as best you can, put fuel in, and see if you
notice any leakage.  Could be that testing will show you an air leak where
fuel won't go.

Happy building, and very best wishes for good progress in the New Year!

Al Gietzen