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)?
Best Answer
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.
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.
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.)
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.
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
Appendix B - Hacking a body weight scale
Appendix C - HX711 Evaluation Notes
Appendix D - Accuracy and Precision 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
Appendix F - HX711 and 100g/0.1g 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.