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I'm working on a project and I need to measure the volume of a balloon. In fact, I need to measure its radius. I want you to give me advice and ways of measuring it.

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    $\begingroup$ Measure its circumference and divide by $2\pi$. $\endgroup$ – John Rennie Jan 27 '15 at 18:59
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    $\begingroup$ @JohnRennie It will be the radius :-) $\endgroup$ – peterh - Reinstate Monica Jan 27 '15 at 19:23
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    $\begingroup$ The balloons I know aren't spherical. $\endgroup$ – BMS Jan 27 '15 at 20:00
  • $\begingroup$ Do you want the volume or the radius? "In fact, I need to measure its radius" makes that ambiguous (the title says "volume"). $\endgroup$ – Floris Aug 23 '15 at 19:00
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If you assume that the balloon is spherical you can fill a bucket completely with water, gather the volume it displaced and measure it. Then you use the formula $V=\frac43\pi r^3$ to get the radius.
To do this i am assuming that the compression of the balloon is negligible when it is underwater.

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    $\begingroup$ And you can get a sense of just how good the approximation of negligible compressibility is by using a very deep container and seeing if the displaced volume changes with different depths. You could also use oil or alcohol if you want to minimize this issue since they are less dense than water. $\endgroup$ – BMS Jan 27 '15 at 20:10
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Fill the Balloon with water instead of Air, and use the Water volume displacement method in the prior answer.

A Displacement cylinder that is either rectangular, or, a Cylinder is far easier to calculate volume change, with and without the water balloon.

A short piece of smooth walled large diameter plastic water pipe, with a homemade flat bottom would work well. Plywood disc glued into the cylinder with silicone sealer ( adhesive ), and varnished to make it waterproof inside.

A 6 inch by 12 inch concrete testing cylinder would also work, as the diameter is then known.

Mike Clark Golden, Colorado, USA

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    $\begingroup$ Once you filled the balloon with water, you can measure its volume in a much simpler way :) $\endgroup$ – gigacyan Jan 28 '15 at 12:10
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You can do sth better : 1 - first inflate the balloon and then deflate it to a spherical shape (Because usually ,balloons aren't spherical shape and this is the matter of this question.)

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