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hx711 weight scale

Why does it take time for electronic scale to measure?

Ask QuestionAsked yesterdayActive todayViewed 2k times112

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)?sensormeasurementShareCiteEditFollowFlagedited yesterdaySamGibson15.8k44 gold badges2828 silver badges5454 bronze badgesasked yesterdaySharon G12733 bronze badges New contributor

  • 2The 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. – swer yesterday
  • 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. – Tony Stewart EE75 yesterday
  • 3Weight is measured by extension of a spring. A mass on a spring has a time cona – Andrew Morton yesterday
  • 2my 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) – Aaron F yesterday 
  • The spring has a time constant, and there then has to be an analogue to digital conversion. – user207421 yesterday

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4 Answers

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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.


ShareCiteEditDeleteFlagedited 20 hours agoanswered yesterdaytlfong012,31111 gold badge77 silver badges1515 bronze badges

  • 4More correctly, the hx711 has 24 bit resolution. Some of those bits will be noise. – Kartman yesterday
  • I very much agree. The body weight balance does not need that many bits accuracy. I guess 16 bits is more than enough, the rest should be truncated. But one thing I found HX711 very good is that it has a mains 50/60Hz low frequency filter. – tlfong01 yesterday    
  • 5Yes, 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. – TonyM yesterday
  • 1Ah, I forgot what is the difference between precision and accuracy (I know what is resolution though.). So I need to google to understand you comment. 🙂 (1) Accuracy vs. Precision: What’s the Difference? – Minitab Blog, 2012jul09 blog.minitab.com/en/real-world-quality-improvement/…. – tlfong01 yesterday   
  • 2Glad it helped. My comment is missing the statement that accuracy is how close a measured value is to the true value. – TonyM yesterday 
  • (1) Yes, your first comment does not seem to define the term “accuracy”, so I was confused. (2) My first suggested reference is a bit commercial and does not clarify my confusion. (3) This wiki article is a bit better. Accuracy and precision – Wikipedia en.wikipedia.org/wiki/Accuracy_and_precision. (4) The wik article is not clear. I am still confused. I need to google harder. – tlfong01 yesterday    
  • 6I 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. – TonyM 19 hours ago 
  • @TonyM, I agree that the answer has diverted too far, and too long winded. Perhaps I would try to give a short answer, with concise explanation, and another long answer with more details, for those readers who are interested. – tlfong01 15 hours ago   
  • 2It’s too much, it’s a dissertation rather than an answer. Stick to short and simple. Look at other successful answers on this site. Less really is more. It’s only one answer to one question. If you’ve got spare answering energy, give short concise and clear answers to other questions, not dwell so much on this one giant answer. It’s hard persuading you 🙂 – TonyM 13 hours ago 
  • 3Are 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? – ilkkachu 6 hours ago
  • 3How 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. – ilkkachu 6 hours ago
  • 3Then, you say “Wiki says the following”. But “wiki” is just a general name for a particular sort of software. Which wiki is that? Here’s the part where you actually should have a real reference (or, more often, just a link) for the part you quote, and where you should actually mark the quote as a quote. I mean, seriously, I’m not sure if you trying to write a concise on-point answer here, or something that looks like a homework paper for a university course, what with emulating the style of an actual thesis or such, but which ever one it is, it doesn’t really pass muster. – ilkkachu 6 hours ago

Add a comment13

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.ShareCiteEditFollowFlaganswered yesterdayMarcus Müller68.6k44 gold badges102102 silver badges184184 bronze badgesAdd a comment7

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.ShareCiteEditFollowFlaganswered yesterdayIgor G29622 silver badges55 bronze badgesAdd a comment3

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.ShareCiteEditFollowFlagedited yesterdayanswered yesterdayAndrew Morton1,75711 gold badge1212 silver badges2323 bronze badges

  • 2While this is true I suspect the human-on-scale is overdamped, and the settling time would be a fraction of a second. – tomnexus 20 hours ago
  • 2@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. – Andrew Morton 7 hours ago 

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