Calculating balloon neutral boyancy

[ihavegoats]

Hi,

I'm having quite a time figuring out this problem. I have a weather balloon (well, a choice of several) and a payload of about 2kg. Most people release balloons like this such that they keep climbing until they pop, however I'd like to calculate how much to fill the balloon such that it is neutrally buoyant at some height, say 60,000 ft.

Calculating this near sea level is easy: when the overall density of the balloon and payload (ie, mass of payload and balloon and helium over volume of balloon) equals that of the air, it floats. The problem gets more complicated however with altitude: as you go up, the air density decreases, and the balloon expands. The air gets colder also.

Is there a better way to go about this than plotting the air density curve, and testing different initial fill values by plugging them into excel and running the numbers every 1000ft?

Thanks,

-Jack

 

[rock.freak667]

I think you would need to do it by trial and error as I am not sure there is a formula for finding the density at various heights.

Also at 60,000 ft. you'd need to re-calculate the value of 'g' to use upthrust = ρVg.