Quiz - Physics - Fill a Container at 40 metres/132 feet

[saxman242]

I’d be hoping to have enough lift by half filling both bags which would be 1 ton so 2 15ltr tanks would be plenty, now all I have to figure is what the tank weights? Thanks very much for your reply

That's probably a discussion that would be better served by its own thread.

 

[Mike1967]

I’d be hoping to have enough lift by half filling both bags which would be 1 ton

You found some treasure?

 

[Pedro Burrito]

Spoiler: Answer

d. 200 litres

The forty litres of the container times the atmospheres at 40 metres (5).

 

[Blackcrusader]

You have to make an assumption about the container, I'll assume this is like a balloon, that was empty at the surface

I can count my SMB as a container.

 

[ToneNQ]

But an incompressible container doesn't have to be filled with anything at whatever depth. It is what it is. It could be a 20 L cylinder with 40 L of air at the surface (1 ATM gauge pressure) and still be a 20 L cylinder with 40 L of air at 40 meters (1 ATM gauge pressure) without adding any air from the surface at all. That's the whole principle behind the S and the C in open circuit scuba. The question must assume a compressible container with a volume of 40 L at the surface for 200 L to be an answer.

The compressible nature of the container itself is not relevant, providing that the ambient pressure can act on its contents. An obvious scenario is an chamber or bell on the bottom, ie a decompression habitat. You'd largely flood it to sink it, only enough air to manage its negative buoyancy as the pocket compressed and the displacement reduced. At 40m in depth you anchor it down and then fill it with air.

 

[formernuke]

Volume is volume, the pressure will change but the volume will not.

 

[saxman242]

Volume is volume, the pressure will change but the volume will not.

So does the temperature change or do more moles of gas show up?

 

[Angelo Farina]

Yes, you are describing a lift bag. The "balloon" is a physics metaphor, like a spherical cow.

Not exactly.
I used lift bags, and they tend to expand while you ascend. A rigid canister cannot expand, the excess air simply comes out...
This i what I was using at the time:

Of course without cap, you attach a piece of rope to the handle, so that the canister stays upside-down when filled with air...
A lift bag is something as this one:

This does not have a prefixed volume, just a maximum one, which often is excessive, causing the object to sky-rocket to the surface...
A canister (of proper size, of course) allows for a controlled ascent.

 

[Khrissi]

Define “fill”. Is the container compressible? What was the volume of the container on the surface? Are we filling it so that it occupies the same volume as in the surface?

....there's always One K

 

[tursiops]

I used lift bags, and they tend to expand while you ascend.

No. A real lift bag does not expand except insignificantly. A cheap one probably does. People have used plastic garbage bags inside mesh bags.....
There is a reason professional salvors use lift bags.