[dotancohen]

What ambient stuff is available at the altitude where a zeppelin typically flies?

[Retr0id]

there is generally plenty of ambient air in the earth's atmosphere

[dotancohen]

And how can that ambient air be stuffed into bags to control buoyancy?

[Retr0id]

https://en.wikipedia.org/wiki/Air_pump

[dotancohen]

An air pump will not compress the air such that it becomes dense enough to overcome the buoyancy of the helium. The idea is ridiculous.

[Retr0id]

Tell that to the airships I guess

[numpad0]

so balloons appear to have negative mass, it's actually just the result of having lower density than the air. the upward force balances out with the gravity where the lbs/in^3 figure of its entirety matches that of ambient air. it's exactly the same as how an empty tank underwater floats, and a water filled tank underwater sinks.

or I guess one could say it's the bottom side getting more compressive load from air than the topside, given the observable effect, whatever floats our zep...

[cenamus]

The stuff the Zeppelin/Blimp floats in

[dotancohen]

And how do you propose that stuff be stuffed into the bags to control buoyancy?