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I have a three different huge kitchen (?) scales with accuracy / being able to measure stuff to up to 200 kg / 440 lb.

When I compare the results, for first two scales they're:

  • comparable or identical (+/- 0 to 0.5 kg) for 0-100 kg range,
  • comparable (+/- 0.5 to 1.5 kg) for 100-200 kg range.

When I compare these two scales to the third one then the results are:

  • comparable or identical (+/- 0 to 0.5 kg) for 0-100 kg range,
  • way not comparable (+/- 6 to 9 kg) for 100-200 kg range.

How can that be? Has anyone ever heard about such situation? I can't say that my third scale is broken or inaccurate, because for 0-100 kg range it measures identical results as two other scales. Only going above 100 kg causes it to go wako.

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    Yes you can say it's inaccurate for higher weights, because that's what it is. It's unclear what your question actually is, so long as didn't make a mistake while testing, one of your scales clearly loses accuracy as weight increases. Depending on the technologies used for each scale, it can happen for different reasons.
    – eps
    Commented Aug 13, 2022 at 17:58
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    For example if it uses springs, Hooke's law only applies within certain ranges, and beyond that F ~ x^2 no longer applies
    – eps
    Commented Aug 13, 2022 at 18:01
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    Springs can degrade or deform permanently, electronic components can drift over time, which correlates to error in measurements.
    – eps
    Commented Aug 13, 2022 at 18:10
  • @eps Thanks for trying to answer. I forgot to mention that this is a brand new electronic scale. So neither springs (affected by Hooke's law or by springs degradation) nor electronic components degradation. If that matters.
    – trejder
    Commented Aug 13, 2022 at 18:50
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    Despite this being a kitchen scale, it's not a good question for SA. You really need one of the electronics or DIY SE; answerers who know the internals of scales just don't hang out here. When posting it in another group, include the brand and model of each scale. It's not technically off-topic, so not voting to close, but I really think you'll get better answers elsewhere.
    – FuzzyChef
    Commented Aug 13, 2022 at 19:19

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What you're observing matches my experience for inaccurate electronic scales. Inaccuracy is more severe at higher weights. Usually this is because the inaccuracy is progressive; the scale is, for example, 2% inaccurate. At low weights that's hard to observe because it's smaller that the scale's own precision limitations, but at large weights it's quite apparent. This is why calibration of scales usually uses weights that are close to the top capacity for the scale (for example, my 0.01-200g scales use a 100g weight for calibration).

Speaking of calibration, you really need calibration weights if you care about scale accuracy. It's more likely that the one scale is off -- but it's certainly possible that that scale is accurate and it's the other two that are off. Further, most scales come with calibration procedures, but that calibration requires you to have a scientifically precise calibration weight to use.

(My experience is based on teaching ceramics classes using cheap electronic scales, which has involved buying dozens of them and calibrating them myself.)

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