Quote:
Originally Posted by RogerBW
This gives you a lift (at standard sea level temperature and pressure) of 1.3 kg/m³. That's as distinct from the 1.2 kg/m³ that you get from hydrogen.
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You could heat the gas, which will get you closer to the vacuum ideal with much less mass dedicated to containment. Hotter gas produces more pressure, so it takes less gas to fill a bag to equilibrium.
It should be obvious why a hot hydrogen balloon is a really bad idea in an oxygen rich atmosphere, but it has been proposed for Jupiter atmospheric probes (though, there a non-heated hydrogen balloon would sink).
One thing that is interesting is that, in Venus' dense CO2 atmosphere, oxygen and nitrogen are lifting gases. A floating city above the clouds at 1 atm is awesomely steampunk but plausible, with outside work done in shirtsleeves and a face mask (and perhaps an acid resistant umbrella).