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  1. #1
    TDF Member Flashman's Avatar
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    Lift Bag Calculation - Volume Question

    I'd be most grateful for any help regarding a lift bag calculation.

    I wish to raise a 20kg grapnel anchor from the sea bed at 20 metres depth in salt water. I am happy with the calculation apart from working out the actual volume of the anchor. I have guesstimated the volume but I would be grateful if anyone has experience of calculating the volume of an anchor or has actual experience of lifting an anchor which they would be happy to share? I intend to use a 20kg lift bag. My guesstimate of the volume is 5 - 10 litres but I may be miles out hence this post.

    If the object in Salt water 20 m, weighs 20 kg and has a volume of 5 L.
    you will need a lift capacity of 15.6 kg (or 15.21 L) and the lift bag will use 257.15 L of air.

    If the object in Salt water 20 m, weighs 20 kg and has a volume of 10 L.
    you will need a lift capacity of 11.25 kg (or 10.97 L) and the lift bag will use 252.91 L of air.


    Thanks

  2. #2
    Established TDF Member nigel hewitt's Avatar
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    Steel/Iron is 8Kgm/Litre so 20Kg would be 2.5L
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    Prior Member Tim Digger's Avatar
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    I'm probably wrong about this as I never do such things underwater but. If you need 15kg of lift or 17.5kg as Nigel's density gives, then surely you need 3* 17.5 litres of air at STP in bag at 20meters depth (leaving aside density of sea water difference from 1. Equals 52.5litre of air not 257.15. I guess in practice it may be a little more as bag and line attachments will all be negatively buoyant.
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    TDF Member Flashman's Avatar
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    Quote Originally Posted by nigel hewitt View Post
    Steel/Iron is 8Kgm/Litre so 20Kg would be 2.5L
    Thanks very much Nigel. That was exactly what I was after. If the object in Salt water 20 m, weighs 20 kg and has a volume of 2.5 L.
    you will need a lift capacity of 17.78 kg (or 17.33 L) and the lift bag will use 259.28 L of air.

  5. #5
    TDF Member Flashman's Avatar
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    Quote Originally Posted by Tim Digger View Post
    I'm probably wrong about this as I never do such things underwater but. If you need 15kg of lift or 17.5kg as Nigel's density gives, then surely you need 3* 17.5 litres of air at STP in bag at 20meters depth (leaving aside density of sea water difference from 1. Equals 52.5litre of air not 257.15. I guess in practice it may be a little more as bag and line attachments will all be negatively buoyant.
    Pleased to be corrected if I am in error on something fundamental here.
    Hi Tim,

    Thanks for your reply. Here's some useful information regarding Lift Bags.

  6. #6
    Established TDF Member Nickpicks's Avatar
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    Quote Originally Posted by Flashman View Post
    Thanks very much Nigel. That was exactly what I was after. If the object in Salt water 20 m, weighs 20 kg and has a volume of 2.5 L.
    you will need a lift capacity of 17.78 kg (or 17.33 L) and the lift bag will use 259.28 L of air.
    How does the 259.28 litres of air get calculated? At 20m, pressure will be 3 bar (approx), so a 17.33 litre bag will need 52 litres of atmospheric air. Am I missing something?
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    TDF Member Flashman's Avatar
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    Quote Originally Posted by Nickpicks View Post
    How does the 259.28 litres of air get calculated? At 20m, pressure will be 3 bar (approx), so a 17.33 litre bag will need 52 litres of atmospheric air. Am I missing something?
    Hi Nickpicks,

    No, you are not missing anything and thanks for your input. I used the online calculator here which I presumed gave the resultant figure as the maximum amount of air required at that depth including an initial lifting force. I do not know what math calculation figures were used. I have sent an email request asking how the computations were arrived at and will post back here when I receive a reply. I also referenced another calculator here which gives +/- 50 litres of air required for my particular query but does not include a lifting force. I know the lifting force should be a maximum of 20%.

    In the meantime, I know that a 3 litre pony filled to 200 BAR should provide more than enough air to lift the anchor to the surface using a self sealing AP Valves Lift Bag.

  8. #8
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    Just use a 65k lift bag make sure it an open ended bag use 6m poly prop blue rope
    Tie the rope around the anchor then tie the rope on to the lift bag then fill with air
    Make sure you run a rope from the top of the lift bag to the bottom of the lift bag
    So when it hits the surface you can grab hold of the rope and pull it in

  9. #9
    TDF Member Flashman's Avatar
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    Thanks Doc, much appreciated 😃

  10. #10
    Sorry for being a dick jb2cool's Avatar
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    Why an open ended bag? With these there is a risk of spillage on the surface if the bag is knocked over.


 
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