Bill wrote:
What about a pressurized tank of helium? Heavier full of helium than empty?

What if you released the gas from the helium into a large balloon which was
attached to the helium tank. Would the balloon at some point lift the helium
tank once it was filled with helium from the tank?


Empty is a relative term. A tankful of helium at
1 atm is lighter than a tankful of air at 1 atm.

Of course if you release helium into a large
balloon at some point it will lift the tank of
compressed helium.

It's all about floating; floating in air is the
same principle as floating in water. Basically if
an object weighs less than surrounding gas or
liquid weighs, it will float. For example as
soon as the balloon inflates, it gets larger and
displaces more air. As soon as the volume is
large enough that the weight of air it displaces
slightly exceeds the weight of the balloon, the
internal helium gas, and everything attached to
it, it will start to float.