Originally posted by Shallow Blue
As an aside to the answers you already have, I'd like to note that the exact amount varies with the air pressure in your room, and therefore with your position on or above (or in my case, below!) sea level, and on a day to day basis, with the weather.
Yea, I already found that out playing with adding small weights to a regular helium balloon. It is only marginally static. The air in the room is moving by convection a little bit and the temperature differential between the floor and ceiling changes things too.
For instance, I was able to take a balloon and get it to stay mostly in one place, but I could hit it with some energy from an IR lamp and that would heat up the balloon a bit and it would start to rise till it shed the heat and came back to room temp, then it would fall down to where it was. That was a fun experiment.
I imagined a helium balloon with phosphor coatings where it floats around and some IR lasers hit it, say from three sides, with a wide beam as wide as the balloon, and the phosphor reacts to IR to produce visible light, it could be a floating light source with no visible energy source! I know there are IR phosphors because we use them in the photonics industry to be able to visualize where an IR laser beam was, used for collimation of ion implant beamlines where focusing magnets and such have to be accurately placed around the ion beam so it focuses magnetically, if the beam is off center it gets into a shape that makes for non-uniformity doing its dopant thing on silicon wafers. The sensor is a business sized card with a phosphor coating that allows you to see where the beam is, it lights up when hit with IR.