Various mechanisms. The plan for the Hindenberg (a rigid frame airship, of course, not a blimp) was to have concentric gas cells -- spherical gas-bags inside t

Various mechanisms. The plan for the Hindenberg (a rigid frame airship, of course, not a blimp) was to have concentric gas cells -- spherical gas-bags inside the truss framework. Each outer sphere full of helium would contain a large inner sphere of hydrogen. The He would effectively insulate the H2 gas from atmosphere, reducing the fire hazard, while taking advantage of the superior lift provided by H2; to descend, they could simply vent hydrogen (via a valve out the top of the lift cell). Unfortunately the US helium export ban put paid to that plan, forcing them to use only hydrogen. The rest is history. (Another key breakthrough in the early noughties (of the 20th century) was the invention of Blaugas -- a gaseous combustible fuel that was of neutral buoyancy in air. You could store fuel in gas cells and pump it straight to the engines without having to make any trim adjustments.) Blimps -- non-rigid airships -- use concentric gas bags as well: there's an inner cell hidden within the outer lift cell that can be inflated or deflated with air to maintain internal pressure (keeping the outer bag taut but not over-pressurized) and regulate lift. But the ability to compress or liquify helium is indeed a valuable new technique that will reduce operating costs enormously for future airships.