I would like to show the details of the situation before asking the specific question so: Conditions: The attached sketch shows a situation that is slightly after the start, since the balloon is shown with air already inside. The question that I will ask is about a situation where the balloon is completely empty to start with. Note: the pressure required to fill the balloon is not a factor. The chamber is circular with an accordion fold sidewall that will not deform or twist due to the water pressure against it, but still acts like an accordion and the chamber is fixed to the bottom of the tank. The top plate of the chamber has zero buoyancy and is attached to the bottom plate by the sidewall so that the chamber is airtight within the tank. QUESTION: The question is what volume of air would be needed to supply enough buoyancy in the balloon to create suction in the inlet pipe protruding from the bottom of the chamber to atmospheric pressure?