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Balloons that float midway in air

Neither sinking nor rising, balloons of neutral buoyancy:


If a party balloon full of helium is released indoors it will rise to the ceiling, or, if there's a big weight attached to the balloon, it will remain on the floor. But what if you carefully chose exactly the right small weight to attach to the balloon so it didn't rise up to the ceiling nor did it stay on the floor? If the weight was exactly the same as the floating/lifting ability of the balloon, what would happen then?

This is the kind of thing that occurs to the enquiring mind. Instead of just being amused that a helium balloon can float up in the air, or understanding that a balloon can be prevented from rising by tying it to a weight, the clever person thinks What If!

This is a scientific experiment which you SHOULD try at home. Yes, it is possible to balance a balloon so it floats midway in air. It's got to be exact, so things such as very tiny pieces of paper or Blu-Tack are required to control the weight to balance it precisely. A dose of sand in a tiny plastic coffee cream pot can be weight adjusted accurately.

The results with a neutral buoyancy balloon are quite curious. What happens is that the balloon doesn't just hang there in mid air, as might have been initially presumed. It starts to wander about. Even indoors the air is not a static medium, but has air currents in it. Convection currents, draught, and the slipstream from anyone moving about, all affect the movement of the neutral buoyancy balloon. In a house which seemed still, a balloon weighted carefully will float about on invisible currents of air, and will be at unexpected locations in its wanderings. Like a cat, you can't be sure where it is at any time.

You don't have to take my word for this. You can try it. You may be surprised when you see for yourself the way the indoor air has currents which can move a neutral buoyancy balloon about at quite a pace.

Balloons aren't perfectly airtight, and after a while the gas will slowly escape, so some maintenance is required to keep the buoyancy neutral. Also, the weather affects the balance of rise and fall, because of changes in air pressure, as seen on a barometer.

It has been noticed that balloons of different qualities behave differently regarding the leakage of helium gas. Cheap balloons (available in packs of 100 from the Pound Shop) lose gas quite quickly and will drop in a day or so. Quality latex balloons (like the ones that flew away with the DV5300 (reward available)) will stay up for three days, but the best balloons found so far for neutral buoyancy are the metallic-effect celebration balloons available from party shops. Some of these will stay up for more than two weeks, which means the helium is only leaking out very slowly, so they are easier to balance for neutral buoyancy! In one experiment, a Happy Birthday balloon was kept adjusted for buoyancy with Blu-Tack, and it wandered around the house for 41 days, performing some remarkable movements in the convection air currents.

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