How come helium balloons stop floating after a while?

Question

Do they lose helium? How come the helium that is left doesn't keep floating?

Answer 1

After they lose a certain amount of helium, it isn't light enough to float. It's mass is greater than air because of balloon itself.

Answer 2

They tend to lose helium. When you lose enough, there is not enough buoyancy to support the weight of the balloon.

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Answer 3

Yes, they lose helium

Helium is a very small molecule and it can slowly diffuse through the skin of the balloon. Mylar balloons last longer because the mylar is less permeable to helium, but even they lose their helium after a few days.

Eventually the balloon loses enough helium that the weight of the balloon and the helium inside it exceeds the weight of the air that the volume of the balloon displaces. Once that happens, the balloon will sink.

Answer 4

Helium diffuses out of the balloon, so yes, it does lose helium. When too much helium is lost, there isn't enough "lift" to keep the balloon afloat and it sinks.

Answer 5

The weight of the air each one is displacing is equal to the weight of the balloon and included gas.   When that happens, the balloon will eventually settle down at some altitude and stay there (discounting air currents etc.)..

Let f = (m + M)a = W - B; where the net force (f) on the balloon (M) and included gas (m) is equal to the balloon and gas weight W = (m + M)g minus the buoyancy B = Rho V g of the displaced air.  V is the volume of the balloon and consequently the displaced air.  Rho is air density; so the mass of the displaced air  = Rho V and the weight of that air = Rho V g = B, which is the buoyancy force.

When W = B, so that f = (m + M)a = 0, the balloon no longer accelerates because a = 0.  But the balloon continues to rise at a fixed velocity because of momentum.  

Eventually, the air gets so thin and cold that B = Rho V g becomes b = rho v g; where rho < Rho because the air is thinner at higher altitudes and v < V because the cold reduces the pressure and consequent volume of the balloon.  When W > b, a < 0 so that the balloon begins to slow down and, eventually, accelerate back downward.  

As the balloon descends, the air density and temperature increase again and soon we have W < B; so the balloon reverses course as a > 0 and starts to climb back up to where W = B.  The balloon goes through cycles of oscillating around the neutral point where W = B and finally dampens out and settles there.  So the balloon is happily floating around.  But, and this is a big BUT...

B = Rho V g  and if the volume of the balloon gets smaller (V ---> v), it will lose buoyancy; so that W > b and the balloon will start to descend because a > 0.  And there's your answer, balloons eventually stop floating because they lose their displacement volume V.  That is V ---> v where v < V so b = rho v g < rho V g = B and b < B.

Now why would V ---> v over time?  As you said, it loses He and the pressure the gas puts on the balloon's surface.  As the pressure inside decreases, the volume of the balloon and consequent displaced air decreases.  That lowers buoyancy and keeps lowering it until the balloon finally touches ground.

The balloon still has some He inside, but there is so little that the volume of the balloon no longer displaces enough air to give the balloon buoyancy B >= W.  In which case, there is insufficient net force to keep the balloon in the air even though some He remains inside.

Although the balloon's material is impermeable to He, some of the gas still leaks through  There is no such thing as 100% sealed because molecules of the gas slip in between the molecules of the balloon's surface.  So eventually the volume V ---> v and the B ---> b; so that the balloon sinks until it hits the ground.

There is one other reason a balloon will stop floating...it explodes.  Here the balloon's surface is not up to holding in the net force exerted by the gas on its inner surface.  The net force is the force of the gas minus the force on the surface by the surrounding air.  While the force of the gas remains high during ascent, the force of the ambient air diminishes because the column of air pushing on the balloon is less at altitude than on the ground.  If the surface of the balloon gets really cold, it might become less flexible.  In which case the net increase in outward force would suffice to burst the balloon.  And down it comes.