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

[TMHeimer]

It would not have been difficult to write this question in such a way that we didn't have to caveat with "well, if we pretend what we think they're asking in this question, the answer is D"

Hell, one could even make it more situational to diving:

An empty lift bag with 40l capacity is at 40 meters. Approximately how much air must be pumped down from the surface to fill the bag?​

I also agree here. I'm sure all these questions can be worded so there would be nothing to post on and on about here.

 

[saxman242]

so there would be nothing to post on and on about here.

Of course, where's the fun in that?

 

[Storker]

I'm sure all these questions can be worded so there would be nothing to post on and on about here.

Um, "here" is Scubaboard. Do you want to reconsider that?

 

[Angelo Farina]

Reading the question I imagined the container as the open plastic canister I used many times for raising heavy objects.
A plastic canister is better than a ballon, as when fully filled with air it will provide a constant positive thrust, which does not change during the ascent. The excess air simply comes out from the opening...
That is the only kind of "container" which I did bring underwater, full of water, and did fill with air at depth.

 

[tursiops]

Reading the question I imagined the container as the open plastic canister I used many times for raising heavy objects.
A plastic canister is better than a ballon, as when fully filled with air it will provide a constant positive thrust, which does not change during the ascent. The excess air simply comes out from the opening...
That is the only kind of "container" which I did bring underwater, full of water, and did fill with air at depth.

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

 

[Celt]

I figured a 40 litre tank @ 4 atmosphere would contain 160 litres

P.S. should I have added the mile of air over my head?

 

[BlueTrin]

I figured a 40 litre tank @ 4 atmosphere would contain 160 litres

P.S. should I have added the mile of air over my head?

Yes you should have added it.

If you redo the question for 10m, it will become easy to see why you need to add it.

 

[Celt]

At 40m you are at 5 atmospheres.

The formula is (depth / 10) + 1.

At 0m, you have 1 atmosphere (by definition)
At 10m of depth under water you have 2
At 20 you have 3

... etc

You could say that at 40m you are at 4 ATA relative to the surface (which is at 1ATA) so you are at 5ATA total if that makes it easier to remember ...

Thank you. 4 ATA of water + surface pressure, Maybe the question should have said lifting bag. They would be sent to the bottom upside down and any air at the surface would be driven out.

 

[TMHeimer]

I figured a 40 litre tank @ 4 atmosphere would contain 160 litres

P.S. should I have added the mile of air over my head?

Or simply, It contains 40 litres at 1 ata. At 5 ata it needs 5 times as much.

 

[Celt]

Or simply, It contains 40 litres at 1 ata. At 5 ata it needs 5 times as much.

Thank you, it needs 5 times as much to fill the same space. Very handy to know, do you mind me asking what size bag would a 7ltr cylinder at 200bar fill at the same depth and can you show how you got the answer