Load Cell and HX711 Questions

Other Hardware Sensors
1

Hi! i want to make a scale to weight up to 25kg, but i need at least 1 gram precision.

i have 4 load cells from a wii balance board, they are rated for 75kg each.

my question is: how the 24bits precision from HX711 correlate to final gram value precision?

using load cells with smaller "kg" limits would improve precision?

i can use 4 HX711 with those load cells, will this increase precision ? (sum each values?)

i also can get those load cells from kitchen scales (4x 10kg max), can i get better precision this way?

thanks in advance!

2

First you need to know the loadcell sensitivity, it's usually given in mv/V. From that you can determine the voltage/gram output of the load cell.

For example:
Say the sensitivity is 2mV/V
With 5V excitation voltage you will have 5V x 2mv = 10mv output at full scale.
If it is a 75kg load cell then, then the voltage/gram is 10mv/75000 = 133nV/gram

i also can get those load cells from kitchen scales (4x 10kg max), can i get better precision this way?

If the sensitivities of the 10kg and 75kg are the same then yes.

3

1 gram precision at 25 Kg. that is quite precise, it means the maximum error is 0.5 gram or less. This precision implies you need a very stable setup, free of vibrations and other noise. You need temperature control to keep the precision of the load cells within a defined range.

My HX711 library includes functions that help to reduce noise, don't know if you can get the 1 in 25000 level of precision with the load cells you have.

Use read_median(times) as it eliminates outliers, where average count them in which is not what you want with the precision needed. (times = nr of samples).

Q: Is it possible to weigh e.g. 5 x 5000 gram instead of 1 x 25000 gram?