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If I am standing on a digital scale or measuring the weight of my luggage, the display doesn't show the weight immediately - it takes a few fractions of a second.

Why is that and can you overcome it to always, immediately show the current weight (e.g. weighing the exact weight of a continuously filling cup)?

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    \$\begingroup\$ The filter in the device should be settle down to show the results correctly. Otherwise, it will vibrate up and down until the filter is settle down, which is undesirable by users. \$\endgroup\$
    – swer
    May 20, 2021 at 11:08
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    \$\begingroup\$ Each design has a tolerance to settling time due to required accuracy an threshold for consecutive readings within that tolerance dependant on the designers choice. \$\endgroup\$ May 20, 2021 at 11:44
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    \$\begingroup\$ my bathroom scales show the weight immediately, and show it fluctuating... then, after it settles down the number freezes and the display flashes three times (my previous bathroom scales acted as you describe) \$\endgroup\$
    – Aaron F
    May 20, 2021 at 23:28
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    \$\begingroup\$ The spring has a time constant, and there then has to be an analogue to digital conversion. \$\endgroup\$
    – user207421
    May 21, 2021 at 2:30
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    \$\begingroup\$ To some extent I suspect it follows from the (really annoying) standard behaviour of freezing the display. If you're going to freeze it, you have to let it settle down first. I rather have (the option to have) a live display so I can get on, pick something up, and watch it update - but like many here I'm used to reading meters, and not the target market for bathroom scales. Digital kitchen scales do this with something like 1s settling times \$\endgroup\$
    – Chris H
    May 21, 2021 at 12:33

4 Answers 4

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Answer

Well, let me take the digital body weight to explain why it takes some time, usually less than two seconds to get the results.

  1. The balance usually has four load cells at four corners, each of which connects to a HX711 weight sensor with a sort of Wheatstone bridge measuring voltage.

  2. When you step on the balance (or placing your luggage on the balance), you don't place yourself in a good balance, spreading evenly over the four corners. (For luggage balance, there might be many more load cells.)

  3. The host controller would wait for the slow human's four readings to settle down, and use some simple algorithms (spatial/time moving average etc) to try to "balance" the unbalanced/unevenly spread readings.

  4. In short, it is the unsteady human body, or the "unsteadyily placed" luggage that take time to settle down. The HX711 sensor itself is 24 bit accurate, and only takes 0.1 second to do the analog to digital conversion.


References

(1) Hacking a body weight scale - rpi.org.forum 2018dec16

(2) HX711 weight sensor - Rpi.org.forum 2018nov17

(3) HX711 24-Bit Analog-to-Digital Converter (ADC) for Weigh Scales Datasheet - Avia Semiconductor

(4) HX711 24-Bit ADC (with load cell mV output pre amp, no programming needed) Tutorial - Components 101, 2018sep28

(5) Mini Pocket Digital Scale for Gold Silver Jewelry 100g-0.01/200g-0.01/500g-0.01 - AliExpress US$5


Appendices

Appendix A - HX711 weight sensor and load cell

hx711 weight sensor and load cell


Appendix B - Hacking a body weight scale

hack1


hack 2


hack 3


Appendix C - HX711 Evaluation Notes

hx711 block diagram


Appendix D - Accuracy and Precision Measurement Results

accuracy measurement results

Wiki says the following:

Accuracy in a set of measurements, is closeness of the measurements to a specific value

Using 50g standard weight as the bench mark, I found the closeness is max 50.00g and min 49.96g. So the closeness is

(50.00g - 49.96g) / 50.00g

= 0.06 / 50.00 * 100%

= 0.12%


For 10g standard, accuracy

= (10.01 - 10.00) / 10.00

= (0.01 / 10.00) * 100%

= 0.1%


Wiki also says the following:

Precision is the closeness of the measurements to each other.

With a very limited sample size of only two 100g/0.01g weight scales, I also found the closeness is about 0.1g, so the precision is also 0.1%.

Of course the cheapy US$5 weight scale's 0.1% accuracy and precision is far from HX711's 24bit accuracy/precision. However for everyday casual use with 6 decimal digits LCD display, 0.1% is good enough.

For higher accuracy and precision, we can use HX711 to connect to the digital scale's torn down load cell, and get higher accuracy and precision.


Reliability, repeatability, and availability casually means the same criteria. In this quick and dirty 100g weight scale tests, all three are about 0.1%


Appendix E - 100g/0.1g / 500g/0.1g Load cell interface with HX711

hx711 interface


Appendix F - HX711 and 100g/0.1g Load Cell Interface

hx711 load cell interface


Appendix G - Weight scale's response time

For body weight scales, the measurement is is of the order of one or more seconds, because the human body standing on the scale is not steady, so the controller might need to wait for the weight to become steady or the two feet to settle down, more evenly placed, taking time moving averages, spatial redistribution of the 4 load cells etc.

For the mini 100g scale, I try to use my hand to disturb the sample weight and found the response time is only less than half a second. This explains why body weight scale needs more waiting time.


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    \$\begingroup\$ More correctly, the hx711 has 24 bit resolution. Some of those bits will be noise. \$\endgroup\$
    – Kartman
    May 20, 2021 at 12:48
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    \$\begingroup\$ Yes, it's the difference between precision and accuracy. The HX711 has 24-bit precision. That's how detailed its value is. But its accuracy is much lower. I wouldn't start guessing its accuracy as 16 bits or anything else. That comes from examination of the device, the application system and the operation environmental conditions. \$\endgroup\$
    – TonyM
    May 20, 2021 at 13:59
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    \$\begingroup\$ I would stop editing and adding to this answer, it's so sprawling and hard to read now. One reference does a perfectly good and clear job but ten make it hard to piece together your point. I know what it's supposed to mean and I find it really hard work :-) After 14 edits, it still says 'unsteadyily' and wrongly says '24-bit accurate' instead of '24-bit precision' or 'resolution'. I'd trim it right, right down and shrink the pictures (see Help, easy) if you're going to do anything. Conciseness and clarity are king, not excessive detail. Thanks. \$\endgroup\$
    – TonyM
    May 21, 2021 at 8:38
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    \$\begingroup\$ Are all those references relevant to your answer text? How? Because, the text doesn't seem to actually reference them, so it's hard to tell what claims they're supposed to support. (I mean, isn't that what references are for, to support the claims the author makes?) Do you have some references for that "[each corner connects] to a HX711 [sensor]" part? Are you sure it's always an HX711, in each and every scale? \$\endgroup\$
    – ilkkachu
    May 21, 2021 at 21:20
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    \$\begingroup\$ How exactly are those appendices relevant, to anything? You're also not referencing them in the main text, so we can't know. Are you sure they're needed here, or would they be something the reader could go look up themselves if they care to? It's hard to tell now; one would expect you've added them for a purpose, but that purpose isn't apparent. If I didn't give you the benefit of doubt, I might almost say they look like meaningless filler. \$\endgroup\$
    – ilkkachu
    May 21, 2021 at 21:25
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Not an electronic thing, probably, but simply the fact that anything being moved and stood up on a surface has a mass and an elastic volume, so that the force it excerts downwards isn't constant until oscillations have stopped. Mechanical scales suffer the same.

Other than that, if you want a low-variance measurement, you need to filter your observed electrical quantity (a resistance, in this case) with a low bandwidth. But for the weights and accuracies we're talking about here, this is irrelevant.

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I would say that the delay mostly comes from the math algorithm used by the microcontroller rather than from the force sensor or force oscillation. Judging by how smoothly the indicated weight changes, I would assume the scale displays some sort of running average spanning a few actual instant measurements, with measurements taken 2-10 times per sec.

May I suggest a simple experiment to rule out the "unsteady body" hypothesis? Try pressing the panel of your scale with your hand (or both), keep pressing it for a while and then remove the hand instantly. How long does your scale take to realize the weight is zero? It took 4-5 indicator update cycles (a bit more than a second) for my household scale to drop back from 50 kg to 0. I'm pretty sure such long delay can't be due to neither oscillation nor elasticity of the glass panel.

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Weight is measured by the extension/compression of a spring (in this circumstance). A mass-spring system oscillates with a period dependent on the spring constant and the mass; the initial amplitude depends on the initial deflection, e.g., stepping onto the scales. Then there is an exponential decay of the amplitude of the oscillation with respect to time.

Once the oscillations have reduced in amplitude sufficiently, which takes some time, a fairly accurate value can be given.

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    \$\begingroup\$ While this is true I suspect the human-on-scale is overdamped, and the settling time would be a fraction of a second. \$\endgroup\$
    – tomnexus
    May 21, 2021 at 7:39
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    \$\begingroup\$ @tomnexus I have an electronic weighing scale, and it certainly takes a noticeable time to settle on a weight to display. Also, the weight (not the mass) of a human varies slightly over time on the order of fractions of a second with breathing, heartbeat, peristalsis, and maintaining balance. \$\endgroup\$ May 21, 2021 at 20:26

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