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Lift Bag Calculation - Volume Question

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  • Flashman
    TDF Member
    • Aug 2016
    • 61

    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
  • nigel hewitt
    Established TDF Member
    • Sep 2013
    • 3199

    #2
    Steel/Iron is 8Kgm/Litre so 20Kg would be 2.5L
    Helium, because I'm worth it.
    Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.

    Comment

    • Tim Digger
      Prior Member
      • Feb 2013
      • 5534

      #3
      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.
      Evolution is great at solving problems. It's the methods that concern me.
      Tim Digger

      Comment

      • Flashman
        TDF Member
        • Aug 2016
        • 61

        #4
        Originally posted by nigel hewitt
        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.

        Comment

        • Flashman
          TDF Member
          • Aug 2016
          • 61

          #5
          Originally posted by Tim Digger
          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.

          Comment

          • Nickpicks
            Established TDF Member
            • Dec 2012
            • 4206

            #6
            Originally posted by Flashman
            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?
            Proud to be a boring health and softy crap following sissie!

            Comment

            • Flashman
              TDF Member
              • Aug 2016
              • 61

              #7
              Originally posted by Nickpicks
              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.

              Comment

              • The doctor
                TDF Member
                • Feb 2013
                • 37

                #8
                Just use a
                Last edited by The doctor; 11-12-2022, 10:00 PM.

                Comment

                • Flashman
                  TDF Member
                  • Aug 2016
                  • 61

                  #9
                  Thanks Doc, much appreciated 😃

                  Comment

                  • jb2cool
                    Sorry for being a dick
                    • Dec 2012
                    • 1641

                    #10
                    Why an open ended bag? With these there is a risk of spillage on the surface if the bag is knocked over.

                    Comment

                    • BcDiver
                      TDF Member
                      • Jan 2013
                      • 227

                      #11
                      Originally posted by jb2cool
                      Why an open ended bag? With these there is a risk of spillage on the surface if the bag is knocked over.
                      Not a fan of the ap sealed bags as have had issues with one in the past where upon using it the baffle was stuck together and I was unable to get any air into it. Had I not lost it on its last dive I was going to cut the self sealing part of it out to turn it into an open ended one. It's odd as I've never had the same issue with the ap self sealing dsmbs which, I believe, work on the same principle.

                      Comment

                      • MikeF
                        Established TDF Member
                        • Dec 2012
                        • 4077

                        #12
                        if lifting something @ 20Kg use a 25kg bag so that it lifts the weight in a controlled manner by just having enough lift to overcome the weight but any expanding air during ascent spills from the bag as the bag ascends. Using a 65kg bag just means you will have 20kg of lift at the seabed and an uncontrolled accelerating bag with 65kg of lift by the time you hit the surface and it will may well shoot out like a breaching whale, spill the contents and sink again with the anchor heading back down your way. I've seen it happen many times (including having a 10Te boat heading back down)

                        whilst a line can be run from the eye at the top of the bag to the lower lifting point to aid handling at the surface, the primary purpose of the top attachment point is to attach an inverter line to something else on the seabed that's heavier than what you are lifting so that if you cock up and overfill the bag whilst using a bag to move things about subsea, it will invert and spill the air lowering your load back to the seabed rather than shooting it to the surface in an uncontrolled manner.

                        Comment

                        • cathal
                          Established TDF Member
                          • Jan 2013
                          • 638

                          #13
                          Originally posted by Flashman
                          Hi Nickpicks,

                          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.
                          Purely empirical, but I have used a 3L Pony to lift a 20kg item from about 64M. I also use it for my suit and at the end of this dive I still had about 150 bar left after starting with 240bar. So a pony should do the job for you.

                          Cathal

                          Comment

                          • Spirit of Guernsey
                            Established WTF Member
                            • Dec 2012
                            • 7370

                            #14
                            Why not get a 25kg lift bag with a crack bottle? It's what I use for sending up the shot, makes it so easy.
                            There are four varieties in society: the lovers, the ambitious, observers and fools. The fools are the happiest.
                            Hippolyte Taine – French critic and historian (1828-93)

                            Comment

                            • EmpireMica
                              TDF Member
                              • Nov 2018
                              • 39

                              #15
                              No, no, no!

                              The first thing to ask is, "do you have a licence to lift anything from the seabed using equipment other than your hand? That could be "Britain's Maritime Heritage" (tm) you're destroying!"

                              Comment

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