Re: What's left in a helium balloon once it sinks?

Dear Lauren,

What is left inside a once floating ballon when it sinks to the floor depends on what was in it to start with!

A balloon initially filled with pure helium simply has helium in it, but not enough to keep it afloat. The helium leaked out between the molecules of the skin of the balloon. See Newton's Apple. A latex balloon leaks helium faster than a foil (mylar) balloon. One might consider diffusion of some nitrogen and oxygen molecules from the air outside the balloon into the interior beacuse of the concentration difference, i.e. osmosis. However, in the time that enough helium has leaked out to cause the balloon to sink, there is precious little air going the other way. Nitrogen and oxygen molecules are substantially bigger than helium atoms, plus they have to battle their way against those He guys streaming out of the tiny spaces between "balloon molecules"!

However, a popular gas for filling balloons is a mixture of helium and air. This is sometimes known as "balloon gas". Obviously it is cheaper than pure helium. In latex balloons particularly, a significantly higher- than-atmospheric pressure of gas is required to inflate the balloon against the elastic "contracting" force of the skin. Thus, the amount of helium required to give the balloon enough buoyancy must be increased to overcome the increased mass of gas inside the ballon - relative to a balloon which inflates at low pressure, namely a mylar balloon. Therefore, a too-dilute helium:air mixture might not be able to get a latex balloon off the ground. Even if it does, the balloon stays inflated but still sinks!

Aha! A balloon which sinks AND shrinks significantly in size, or even stays afloat while partially deflated, must have been filled with a high concentration of helium, if not pure helium. Whereas a balloon which appears to be still largely inflated but sinks (and probably sinks faster than the first balloon) was filled with something that was not pure helium, i.e. "balloon gas".

If you want to know how to do the buoyancy calculations, seeHow a Helium Balloon Works, and the answer to the question "How many regular sized helium filled balloons would lift someone? is here.

So how big and how high-flying might a balloon be? How about this one that would lift about 3,000 pounds more than 22 miles high and would stay aloft for 100 days, long enough to circle the globe about five times if the winds are right.

You might want to look into some fun experiments with balloons too.

Thanks for your question to MadSci Network