[Kirstie]

So for an example, an object weighing 1000kg with a volume of 700L is sunk in fresh water at 30m.

What size lifting bag would be required to make it neutral?
What volume of gas would be required to fill the bag at that depth?
If you have a 12L cylinder with you for lifting the object, what pressure would it need to have to contain enough gas to do the lift?

Sorry if all that is useless but it was interesting for me

:frown:

Can't I just get a reeeaaaallllyy big liftbag, fill it from a 15l pumped to 250 bar and hope for the best?


Or get lots of divers to each grab hold and fin really hard?

 

[Conor]

That would be the BSAC solution



(sorry)

 

[Finless]

:frown:

Can't I just get a reeeaaaallllyy big liftbag, fill it from a 15l pumped to 250 bar and hope for the best?


Or get lots of divers to each grab hold and fin really hard?

Or get a big straw and suck really hard! "No such thing as a sucking force" HA!

 

[Kirstie]

Or get a big straw and suck really hard! "No such thing as a sucking force" HA!

Or fill the object full of air, tie a piece of line to it and float it up like a balloon!

Hang on, that's almost a liftbag.

 

[David G]

Have a look at pages 16 to 18 of this document:

http://www.bsac.org/uploads/documents/Resources/SDC_Notes/srins.pdf

and use the first 10 or so slides in this presentation:

http://www.bsac.org/uploads/documents/Resources/SDC_Notes/VAs/SR/recovery.zip


Hope this helps.

David.

 

[Janos]

Kirstie,

Think of it this way:

There are forces acting on the Thingy that make it go up.
There are forces acting on the Thingy that make it go down.

If the total force acting up is exactly equal to the total force going down then it is neutrally bouyant.

So. Forces [1] going up = upthrust = volume of object * density of seawater
So a 2 litre Thingy will have an upthrust of 2.06kg in seawater.

Forces going down. Well if our thingy is made of lead it will have a density of about 11 so a total mass of 22 kg (2 litres x 11kg per litre).

So the forces going down are way bigger than the forces going up so it will sink like a stone. To make it neutral you need a lift bag of 22kg-2kg capacity. Ie a 20kg liftbag, which has a volume of around 20 litres.

Janos

[1] = Strictly speaking, Force isn't measured in kg, but in Newtons (N). To convert to Newtons you need to multiply by 10. If you don't know what a Newton is ignore this.

 

[Nick Kay]

Ahem

Surely, in the real world, if you have to lift an object from the seabed, you take down the biggest liftbag you have, fill it completely full of air and let it go? Do people really sit about calculating how much air they need to the nearest bar? Do they buggery!

Yes, they'd do calculations because they'd want to control the lift

Don't forget that you'd need more lift dependent on the "stickiness" of the bottom (Finless is bound to respond to that statement) - or the degree that the object has settled / become embedded

 

[Kirstie]

To make it neutral you need a lift bag of 22kg-2kg capacity. Ie a 20kg liftbag, which has a volume of around 20 litres.

Janos

That's very clear, but why 20kg minus 2kg?

 

[Kirstie]

Yes, they'd do calculations because they'd want to control the lift

Don't forget that you'd need more lift dependent on the "stickiness" of the bottom (Finless is bound to respond to that statement) - or the degree that the object has settled / become embedded

Ah yes, I remember some mention of bottom stickiness. But how do you determine that, or isn't it that relevant?

 

[Buoyant Babe]

Ah yes, I remember some mention of bottom stickiness. But how do you determine that, or isn't it that relevant?

I thought you knew all about bottom stickiness? It's what brought you and Turbs together in the first place, isn't it?

 

[Kirstie]

I thought you knew all about bottom stickiness? It's what brought you and Turbs together in the first place, isn't it?

Not so much sticky as runny, from memory. :embarassed:

 

[David G]

That's very clear, but why 20kg minus 2kg?

Its 22kg minus 2kg because the 2 litres of water that it displaces gives it an upthrust of 2Kg so it has an effective weight underwater of 20Kg not 22Kg.

David.

 

[Alun]

That's very clear, but why 20kg minus 2kg?

The 2kg is the amount of water its already displacing by its volume.

If (theoretically speaking) the object had no volume, and displaced no water at all, then you'd need a 22kg bag. As it displaces 2kg (2 litres of water) then you can deduct this from the weight.

 

[Kirstie]

Ah. I see.


You're all very patient.

 

[Conor]

We're not really

 

[Steve Summers]

That would be the BSAC solution



(sorry)

not so, we would use 2 really big lift bags

Safe diving,
Steve

 

[Woz]

[1] = Strictly speaking, Force isn't measured in kg, but in Newtons (N). To convert to Newtons you need to multiply by 10. If you don't know what a Newton is ignore this.

Very stricltly speaking it's 9.81 at STP but this varies with the distance you are away from the Earth's surface.

 

[Kirstie]

Very stricltly speaking it's 9.81 at STP but this varies with the distance you are away from the Earth's surface.

Can't help thinking I don't need to know this for the PADI exam

 

[Buoyant Babe]

Ah. I see.


You're all very patient.

We're not really

No Conor, that's just you, you stroppy oirish git

 

[dublinbay]

Right. It's dead simple. Immerse somethign in a liquid and you get an upthrust equal to the weight of liquid displaced.

So if you have a 10x10x10 cm cube of something and stick it in say water that has a density of 1 kg/litre then you'll get an upthrust of 1kg. Now- if your 10x10x10 cube weighs less than 1kg (say it's polystyrene which weighs sod all) then it will float as the 1kg upthrust is much bigger than its weight. If the 10x10x10 cube is heavier, say lead, then it will sink as 10x10x10 of lead weighs about 11kg. However it will still have the same 1kg of upthrust so it will actually "weigh" on the bottom of your sink 10kg.

For denser liquids the upthrust is greater- see Digger's post on filling the SETT with mercury.

So to achieve neutral buoyancy with Woz`s 11kg of lead you need a 10 litre lift bag (10 ltr = 10kg of lift + 1 kg of upthrust =11kg )