calculations for lift bags????

[BVI Hunter]

Is there a formula I can use to calculate the size of lift bag to lift an object off the sea floor?

I assume depth is a consideration, anything else I can factor in?

Thanks!!!

 

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[ropesfish]

Check other thread for input.

 

[Zerovis]

Weight of object in Kg - displacement volume of object in L (litres) = volume of lift bag in litres.
Regardless of the depth if you have a an object that weighs 60 kg and displaces 20 litres you will need a lift bag that can hold 40 litres of air.
The deeper it is the more air you have to bring down because you need 40 L at the depth of the object so if its at 30 meters you will need the surface equivalent of 160 l or 16 bar in a 10 litre tank .

note your lift bag should be at rated for a larger capacity than you need and you should bring more air than that too

 

[Uncle Jeff]

Lift Bag Size/Volume Calculator

Bah I got tricked into posting to a 3 year old post.. who bumped this LOL

 

[Jason in Enid]

Weight of object in Kg - displacement volume of object in L (litres) = volume of lift bag in litres.
Regardless of the depth if you have a an object that weighs 60 kg and displaces 20 litres you will need a lift bag that can hold 40 litres of air.
The deeper it is the more air you have to bring down because you need 40 L at the depth of the object so if its at 30 meters you will need the surface equivalent of 160 l or 16 bar in a 10 litre tank .

note your lift bag should be at rated for a larger capacity than you need and you should bring more air than that too

Lift Bag Size/Volume Calculator

Bah I got tricked into posting to a 3 year old post.. who bumped this LOL

realizing this is an old thread... zerovis explanation is wrong. The size of your lift bag does NOT change with depth. The volume of air to fill it at depth changes because air compresses, water does not. His explanation could get someone hurt or killed. If you are using a massively over-sized bag, the air inside will expand in volume thus increasing it's lift as it moves towards the surface. Too large of a space can let the air expand faster than you can release resulting in a run-away lift. This results in a violent surface breach (think of a polaris missle) and injuries to any surface support above, followed by a bag collapse (if using the wrong type) and your object crashing back down to the bottom of the ocean with you directly in it's path.

 

[Salvor6]

Jason is right. To lift 1,000 pounds you need a 1,ooo pound lift bag. To lift 2,000 pounds you need a 2,000 pound lift bag (duh!). Modern lift bags are fully enclosed and have release valves to prevent over pressurization like the sub-salve bags.

 

[Zerovis]

realizing this is an old thread... zerovis explanation is wrong. The size of your lift bag does NOT change with depth. The volume of air to fill it at depth changes because air compresses, water does not. His explanation could get someone hurt or killed. If you are using a massively over-sized bag, the air inside will expand in volume thus increasing it's lift as it moves towards the surface. Too large of a space can let the air expand faster than you can release resulting in a run-away lift. This results in a violent surface breach (think of a polaris missle) and injuries to any surface support above, followed by a bag collapse (if using the wrong type) and your object crashing back down to the bottom of the ocean with you directly in it's path.

HI Jason
perhaps you should re read my post. I said " Regardless of depth if you have an object that weighs 60 kg and displaces 20 litres of water you will need a lift bag that can hold 40 Litres of air" ergo the bag does not change size for a given target.

Regarding your assertion that I could get someone killed , I said " your lift bag should be rated for a larger capacity than you need" There are 2 reasons for this firstly you should never use a lifting tool to its max capacity . and secondly if you have it matched exactly then you can only make it neutral buoyant at best leaving you having to hold it at the surface.


Furthermore the correct position to be lifting from is not below the target but along side the bag where you have access to the dump valve allowing you to keep it neutral during the assent.

It is not the size of the space that the air has to expand into that accounts for the rate of expansion it is your rate of assent, if you are competent to control the rate of assent you should have no difficulty in managing the air expanding in the bag your dry suit etc.