[Andy P]

HI
I am an assistant instructor and have been asked to arrange an open water lesson using lifting bag(s). I can demonstrate the use of these no problem but just in case im asked the following question I would like to know the answer.
At 20m what size of lifting bag would I need to lift and move a 3kg object?
If someone could provide maths to explain this it would be a great help. ie how much air ar 20m etc.
Many thanks.

 

[bubbleless]

Quick guess.......

3kg lift bag or possibly a dsmb....... or any size of lift bag with 3kg of lift added.

If your an assistant instructor shouldn't you know this sort of thing??

whats an open water lesson?? padi?? Bsac?? is this part of the open water/ocean diver??

Tips weight / mass of water is 1 gram per cubic centimetre, pure water not salty,

or 61p will buy this.....

https://play.google.com/store/apps/details?id=com.phonegao.liftbag

 

[Andy P]

Assistant instructor (DL + IFC) is a learning grade...??? and can only teach under the direct supervision of an OWI. The OWI has asked me to present a lesson to him before being unleashed on trainees. I had an idea that a Buddy DSMB would move a 3kg 'shot' but I have a 25kg bag that i will use as this lesson is aimed at trainee DL's.

 

[Aqua Maria]

Take a look at the Instructor Resources CD that you were given on your IFC. The notes for the Search & Recovery course are on there, with some good information on how to work out lifting bag required.
A 25kg bag to lift a 3kg object is excessive and could result in an uncontrolled lift.

 

[bubbleless]

There are several levels of assistant instructors across the different agency's, so just curious to which agency/ level of diver you were presenting this to.

As Aqua Maria said a 25kg lift bag is a little on the large side.

 

[Woz]

Depends on the volume of the shot. See Scuba this month for how to calculate buoyancy required.

 

[Woz]

3kg I'd pick it up!

 

[bubbleless]

So would I, but then all the explanation about lift bags goes out the window,,

 

[Woz]

You have to take into account the buoyancy of the shot so you need to know the density of it. 3kg steel or 3kg lead? More volume = greater buoyancy = smaller lifting bag.

 

[Ralf]

Haha,this is fun A 3 kg shot weight that would have 3½ litres of volume, would float

Let me guess that it is a lead shot. A 3kg lead shot might take up ½ a litre of volume and would thus weigh 2½ kg.
A 2½ litre "lift bag" would then be needed. A 2½ litre lift bag can only contain 2½ litres of air at any depth. But that air is of course compressed air.

Assistant instructors should be learning dive instruction, not basic dive physics. Read a good book. The Naui master scuba diver manual is a good start.

 

[sheesh]

My understanding...
If you need to lift 3kg you need to displace 3 litres of water - less if it is salty.
So... 3l - volume of the object to be lifted = size of lift bag (in very simplistic terms).
IF you work on it being the densest object known to man and it has of volume of e.g. 50cc then you need a 3 litre lift bag (assuming the lift bag itself is neutral). At a depth of 20m this will require approx 9 litres of air. Obviously as it ascends the excess air will expand and escape from the lift bag, so at the surface it will contain 3l, having vented the excess 6. The bigger the bag, the faster the lift.
Obviously as the volume of the object increases the minimum size of lift bag required decreases proportionately, so if it had a volume of e.g. 2.5l you would need a lift bag of just over 500ml.

Note I am not an instructor, just referring back to my 'O' level physics from 30 years ago. It all depends on how much you trust my memory.

 

[Ralf]

[deleted]

 

[Ralf]

At 20m it still needs ONLY 3 litres of air.

That 3 litres contains the same amount of matter that 9 litres do on the surface,
bacause air is compressed to one third of its original volume. It is three times as dense.

A litre is a litre, but the amount of matter in that volume depends on pressure.

The correct term is "litres of surface air" or "litres on the surface".

 

[sheesh]

At 20m it still needs ONLY 3 litres of air.

That 3 litres contains the same amount of matter that 9 litres do on the surface,
bacause air is compressed to one third of its original volume. It is three times as dense.

A litre is a litre, but the amount of matter in that volume depends on pressure.

The correct term is "litres of surface air" or "litres on the surface".

I prefer to think of it as 9 litres of air from my cylinder, but you use what ever semantics you want.

 

[bubbleless]

and if its stuck in the mud all this goes out the window... lol