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

[Celt]

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:
View attachment 583182
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:
View attachment 583183
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.

Angelo when you say a canister of proper size allows for a controlled lift do you mean it won’t speed up as it rises and the lift will stay the same?

 

[tursiops]

Angelo when you say a canister of proper size allows for a controlled lift do you mean it won’t speed up as it rises and the lift will stay the same?

He is assuming a container that expells the increased volume caused by the decreasing ambient pressure.

 

[Angelo Farina]

Angelo when you say a canister of proper size allows for a controlled lift do you mean it won’t speed up as it rises and the lift will stay the same?

Exactly. If you use a canister of the proper capacity (20 liters usually for a 20kg thrust), it will not expand during ascent.
Lift bags are instead almost invariantly of much larger maximum capacity than needed, and filled only partially. Being soft, it is not easy to dump the excess air, apart the case of professional lift bags equipped with a dump valve at the top.
Instead a rigid canister, also when the required thrust is less than their full volume, can be easily emptied of the excess air, rotating them on one side.

 

[Celt]

Exactly. If you use a canister of the proper capacity (20 liters usually for a 20kg thrust), it will not expand during ascent.
Lift bags are instead almost invariantly of much larger maximum capacity than needed, and filled only partially. Being soft, it is not easy to dump the excess air, apart the case of professional lift bags equipped with a dump valve at the top.
Instead a rigid canister, also when the required thrust is less than their full volume, can be easily emptied of the excess air, rotating them on one side.

But it would have to be able to spill as fast as the rate of expansion, but if it over spilled at the surface it would no longer have enough lift and would drop back to the bottom, that could be tricky, The problem with open lift bags is controlling speed and stopping them falling over at the surface especially with a sea running

 

[tursiops]

Lift bags are instead almost invariantly of much larger maximum capacity than needed, and filled only partially.

They come in many sizes, as needed. A dump valve on them makes it possible to make fine adjustments, which is sometimes useful, and not possible with your plastic jug. Also, a rolled-up lift bag is much easier to transport underwater.

 

[Angelo Farina]

They come in many sizes, as needed. A dump valve on them makes it possible to make fine adjustments, which is sometimes useful, and not possible with your plastic jug. Also, a rolled-up lift bag is much easier to transport underwater.

I used both bags and canisters, and in the end I prefer the canister...
Only once I was given a good bag, equipped with a properly-sized dump valve, and proper lift (not too big). In many other cases the bag was just too big, the valve absent or too small, and once the expansion begins, you cannot control it easily...

 

[tursiops]

I used both bags and canisters, and in the end I prefer the canister...
Only once I was given a good bag, equipped with a properly-sized dump valve, and proper lift (not too big). In many other cases the bag was just too big, the valve absent or too small, and once the expansion begins, you cannot control it easily...

LOL. Have you EVER been wrong?

 

[Storker]

Have you EVER been wrong?

Some folks think "diff'rent strokes for diff'rent folks".
Some folks think "my way or the highway".

So, diff'rent strokes for diff'rent folks.

 

[tursiops]

Some folks think "diff'rent strokes for diff'rent folks".
Some folks think "my way or the highway".

So, diff'rent strokes for diff'rent folks.

Yep. I'm just looking forward to Angelo taking down half a dozen different size canisters because he doesn't know what he is going to find and/or how heavy it is. Sometimes a little extra lift to get an object free of the bottom is nice too, then release the extra air in the bag. I've taught a lot of Search and Recovery classes; nobody seems to have the problems with a lift bag that Angelo is fearful of, or has experienced.

 

[Angelo Farina]

Yep. I'm just looking forward to Angelo taking down half a dozen different size canisters because he doesn't know what he is going to find and/or how heavy it is. Sometimes a little extra lift to get an object free of the bottom is nice too, then release the extra air in the bag. I've taught a lot of Search and Recovery classes; nobody seems to have the problems with a lift bag that Angelo is fearful of, or has experienced.

The problem was simply to get proper bags, which simple were not available (in a small island in the middle of the Mediterranean with no shop of technical equipment). At the LDS, there were only two 50-liters bags, and the dump valve was available only for one third, very large 500-liters bag...
And it revealed to be under-dimensioned for that bag.
Instead there were a lot of plastic canisters of any shape and size, as they mostly produce wine...
The objects to be recovered were all almost perfectly identical (you know, Romans had very precise standards). A 20 liter canister was just perfect... I cannot say more, sorry!