Load cells and weighing multihull builds

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Jan 2, 2022.

  1. gonzo
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    gonzo Senior Member

    I don't understand the question. You should calibrate them with no load and verify the output is zero. The you calibrate them with known weights and compare the output. You can then either calibrate the system (ideal), or you can make a table with known errors and correct them later.
     
  2. fallguy
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    fallguy Senior Member

    Thanks @gonzo .

    The calibration sequence is no load; add load.

    But the calibration is done to determine the bending moment of the wheatstone bridge? which is a function of the mounting of the load cell.

    Change the mount; recalibrate the cell.

    How can I calibrate these?

    Only way I can see how is to mount them on a steel plate and then move the steel plate to my stands; or to buy 3 more sets and weigh the cradles they are on as the calibration.

    I cannot just mount each load cell to each stand. That would require a lot of effort, but the load cell cannot support the stand; it will tip over.

    So, feel a bit stuck.

    perhaps mounting to a steel plate; then I can move the plate to the stands and even just c clamp the plates to the stands and lift them

    It is really not easy. Not sure how anyone would do it unless they have them in some standardized mounts.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Simply as Gonzo notes:

    That's it.
    It appears you're over thinking this.

    You just need to ensure the cells are reading correctly - via the known weights.
    Once done, you're good to go...place the cells wherever you like, on 4 points at known distances apart and relative to a datum on the boat, that's it.
     
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  4. fallguy
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    fallguy Senior Member

    Not sure I am overthinking.

    Mounting the cells is a bit of a nightmare.

    I got only one spacer. So I will need to make the same spacers and that will drive the calibration. As long as the spacer is the same; the bending will not change.

    Then I have to find a way to mount them under the stands. No picnic. It requires lifting the boat off the cradle; unbolting and flopping the stands down; drilling out the mounting screws and attaching the cell and then putting the stand back together and setting the boat back down. Complicating things firther; it is minus 9F here now.

    I was not thinking the spacers drive the calibration, but they will. Too bad three of them were lost out the box bottom. Some metalwork is in my future is all.

    Thanks for the replies.
     
  5. Heimfried
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    Heimfried Senior Member

    @fallguy
    Sometimes a new suggestion is helpful - sometimes it's not.
    So, are you interested in another approach?
     
  6. fallguy
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    fallguy Senior Member

    sure

    It is just tricky to do this unless I build mounts for the cells and then move them.
     
  7. fallguy
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    fallguy Senior Member

    A8CD6613-48A3-4543-BE4E-853D946BE4C6.jpeg
    Okay. I am starting to get somewhere in my comprehension of these things.

    Revelations today. The load cell foot is up; not down like the one in the background. Or mounted further away is perhaps better word choice.

    So, I need to build mounting plates for these.

    Then the mounting plates are moved under the stands.

    it did not help that the box was broken and I only got one of the spacers

    no worries; not going to build the stands with light
    Corecell
     
    Last edited: Jan 20, 2022
  8. Heimfried
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    Heimfried Senior Member

    I don't think so, have a look at this pic:
    Load cell.jpg

    It is mounted at the live stock stand with its foot down.
     
  9. Heimfried
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    Heimfried Senior Member

    Another one:
    Load cell 2.jpg
     
  10. fallguy
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    fallguy Senior Member

    I can operate them the other way. So, I really need to build mounts for them.

    I ordered 4 pieces of 6" c channel.

    I will go an make the spacers and mount the cells on the c channel and use the c channel as floor plates and use them in reverse. Then I can use them to weigh anything and the calibration is permanent. Not sure if I have Covid, but headachy today. The c channels will be ready tomorrow and so I might be able to weigh something inside perhaps as soon as tomorrow.

    38BE9308-9AC1-4B6E-98F3-1C4620210937.png
     
  11. fallguy
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    fallguy Senior Member

    The way I am setting these up, I can even use four of them to weigh each stand. Then move each unit to under each stand and weigh the entire boat.

    @Heimfried not saying you are wrong; just that I was wrong about how they work, so that was adding confusion

    I intend to use them with the foot up and make a solid(er) base.
     
  12. Heimfried
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    Heimfried Senior Member

    Good luck, stay healthy!
     
    Last edited: Jan 20, 2022
  13. Heimfried
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    Heimfried Senior Member

    Four cells under one stand has the upside that it is easy to hold the stand in its balance. The downside is you don't know about the actual loads at the other 3 stands as they will change every time you change the arrangement, moving your load cells from one stand to the next stand.

    Each unit under each stand is in principle the best solution but in practice it will be difficult to find the 4 spots under the stands to bear the loads and hold the stands in balance at the same time.
    As the bending moment is the same I think the cells will work this way also, not sure about it.
     
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  14. Heimfried
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    Heimfried Senior Member

    I did prepare a sketch suggesting another approach. I don't think you will be open for it now but I'll post it just in case your approach will not work as good as you thought.

    LC3.jpg
    You said in post #23 there is a 8' beaching keel 4" over ground under each demihull. I suppose the LCG is within the lenght of this keels. Take 2 steel beams may be 6' long (U profile) mount one load cell at each end of the beams. Put each beam under a keel lenghthwise and prevent lateral tilting by connecting the beams with a board or something.
    This way the balancing issues should be solved.
     
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  15. Ad Hoc
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    Ad Hoc Naval Architect

    Im confused, or misunderstanding.
    What bending...bending of what, are you referring to?
     
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