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I have built a simple scale using 4 so called 50kg load cells.

enter image description here

They are connected in a bridge and mounted on wooden feet of a square shaped box. All as a proof of concept which worked quite ok. The readings are all OK and deviation of +/- 2kg is totally acceptable for my project. I followed this discussion which also explains how the connection should be made properly using 4 cells.

How to get weight data from glass electronic bathroom scale sensors?

Now I want to keep the load cells on the feet but change the shape of the scale and instead of a square use a rectangle with the following sides.

enter image description here

In such a form a weight distribution will not be centered all the time. The object to be weighed can shift to the right or to the left.

  • Will this affect the measurements in a negative way or will the HX711 controller still take care of it and show the correct results?
  • Will the measurements still be correct using 4 cells? If not what would be the proper way of weighing a rectangular shaped load?
  • Would it more appropriate to measure the load with 2 cells on the right and two cells on the left and then sum it up?
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  • why would the shape make a difference? ... the length of the cables may or may not be a problem
    – jsotola
    Apr 19, 2021 at 20:46
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    you say that you built the scale ... why are you asking and not testing? ... i do not see a question about any problems
    – jsotola
    Apr 19, 2021 at 20:48
  • For the total measured weight the load center is not relevant. If - for example - the load center is 3/4 to the right, the right load cells will measure more weight, while the left are measuring less. The overall weight has to be equal to load center in the middle
    – chrisl
    Apr 19, 2021 at 21:00

1 Answer 1

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Newton's second law says that if a force acts on a mass, that mass will accelerate at a rate proportional to the force and inversely proportional to the mass.

All that means for your scale is that if you are not being accelerated up or down, the net vertical force on your body must be 0. Since your body has weight, and the net vertical force on your body is 0 (you aren't accelerating, are you? :) then the downward force must be balanced by an exactly equal upward force. That upward force is the sum of the upward forces exerted by each load cell. Their contributions may individually change depending on where you stand, but their sum must exactly equal your weight.

Note that we have said nothing about the placement of the cells! If your platform allows it, you could move one of the cells 6 (or 60 or 600) feet away from the the others, for instance. The total upward force must still be the same.

So the bottom line is:

F_lc1 + F_lc2 + F_lc3 + F_lc4 == your weight

no matter where you place the cells.

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  • Thanks, this is exactly the answer I was expecting to hear 😊. Clear, precise and involving physics! Of course I was almost sure it was like but needed a confirmation.
    – bortek
    Apr 20, 2021 at 5:01
  • I'd just like to point out that this refers to a different Newton. It isn't me. May 20, 2021 at 12:29

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