Good point.  I was considering that if the temp and pressure at the start and end of the test were identical I should be able to determine if any air was lost.  This was based on the (potentially bad) assumption that no air escapes through the balloon skin, which was made on the observation that several identical balloons filled and tied off took more than a week to show significant deflation.  A 24 hour test period resulted in virtually identical size.

However, if I extrapolate your comment, the equal size may be due to the skin relaxation making up for the reduced air mass and thus pressure within the balloon, resulting in the same dimensions but not the same pressure.  This matched relationship would not hold if the volume and total air mass were different (for instance, with the wing plumbed in) but the air escape potential (balloon skin type and surface area) was unchanged.  However, the greater air volume within the wing should sustain the attached balloon longer and result in a larger balloon size and greater wing/balloon air pressure after a certain length of time than the limited air mass within a tied-off balloon.

In any case I think we can agree that measuring balloon dimension is not a very dependable method for measuring pressure.  I'll try to test again with a good pressure gage and ensure that the start and end ambient temps and pressures are equal to simplify the math.  Otherwise I'll pump in some 100LL and look for blue stains ;-)

Cheers,
- Kyrilian


Brent Regan <brent@regandesigns.com> wrote:

In order to make an accurate assessment of wing integrity using the balloon method you must track the barometric pressure, the pressure in the wing (balloon latex will relax with time, lowering the pressure), the temperature of the air in the wing and the diameter of the balloon. You must also know the approximate volume of the wing. The rest is just math.

Regards
Brent  Regan

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