Questions on calculating material needed for lifting object

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    Lifting Material
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DarkMage64
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I am trying to make a floating sentry bot ; though, my plan was to use Helium or with a mixture of Ammonia. I want to find the minimum amount of gas in a balloon require to reach the max height the object should float. I know the buoyancy force is Density * Volmin * g : where the maximum potential energy of gravity is W*Hmax. I'm not sure if I am correct, but I would have to equate the Eg = Wgas correct?
 
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For low heights, there is no relevant max height - either the thing will rise, or it will stay on the ground. You probably need some active control to keep it on the same height.

I know the buoyancy force is Density * Volmin * g
Right, and all three values are nearly height-independent for the first ~100m. The same is true for the weight of your bot, its weight is either below or above that value.