Electronics > Projects, Designs, and Technical Stuff

Load cell zero offset - how much is too much?

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RoGeorge:
Seems I was wrong, thank you all for clarifying.

Don't know where from, or why did I believe the sensor was a piezo.  Even the part with repeating the last number instead of the actual weight seems weird.  Might be something else, because I've just double checked moments ago with various weights, and it tends to repeat the last value, just that it "prefers" certain values (in steps of 0.4kg - the scale has 0.1kg resolution), unless I wait a few minutes and try again, then it prefers other values, no idea why so.

I'll strikeout my previous post, shouldn't have written that at all, sorry.

magic:

--- Quote from: RoGeorge on June 11, 2023, 06:10:00 am ---Might be something else, because I've just double checked moments ago with various weights, and it tends to repeat the last value, just that it "prefers" certain values (in steps of 0.4kg - the scale has 0.1kg resolution), unless I wait a few minutes and try again, then it prefers other values, no idea why so.
--- End quote ---
When I played with the shitty scale which cheats reviews by rounding to full grams, I found similar "preference" for discrete steps of 3 counts. Dividing its full capacity (200g) by "step size" (3mg) the result is close to 65536, which I consider possibly not a coincidence.

It's a tinfoil hat theory because I haven't disassembled that scale to see what ADC was inside, but I suspect that it could be a 16 bit ADC. Still no sure explanation why it sometimes changes the "preference", but maybe there is digital drift compensation working under the assumption that if the reading creeps slowly and steadily over time, it's load cell drift rather than the weighed mass actually changing. Such a digital mechanism could work with finer resolution than the ADC by means of long term accumulation and averaging, explaining the occasional shifts of "preferred" numbers.

Similarly, 0.4kg · 256 is 100kg, so what's the maximum capacity of your scale? ;D

edit
There is the issue mentioned by Kleinstein of ADC bits being "lost" when full scale load cell output is less than full scale input to the ADC, particularly likely to occur in dodgy and not-so-optimized designs. So it's not as simple as checking the number of bits of the ADC - one needs to consider what is the actual output of the load cell, any amplifiers (if present), and the internal gain of the ADC.

RoGeorge:
Could be, on the glass plate it says "Max 150kg d=0.1kg   Max 330lb d=0.2lb".

Never mind, did you find any clues on yours, is the measurement uncertainty coming from the sensors, or from the ADC?

r6502:
Hello all,

There is a 2nd indicator that a load cell is defect: defect load cells tent to drift.

When you apply a load and release it, the output signal should return to the zero signal +- 1 digit of its resolution within a defined time. This value is given as  "Zero Return (creep) in 2 minutes" in "±% of Applied Load" with this value, you cold calculate the resolution, when you use the max load, and it is typically 3000d = 0,033%

If the output signal dos not return to this point it may have been overloaded in the past.

Example: when you have a load cell for max 3000g, it should return to 0 + - 1g after a time of 2min, and the zero point should be stable within + - 1g if not it is damaged.

BR Guido

magic:

--- Quote from: RoGeorge on June 12, 2023, 04:01:20 pm ---Never mind, did you find any clues on yours, is the measurement uncertainty coming from the sensors, or from the ADC?

--- End quote ---
What uncertainty and which scale?

Either way, the answer is probably no. The cheating scale I have returned to the seller after a few days without looking inside because I wasn't sure how to take it apart without damage. All I know about its behavior is documented in the old thread. The broken one isn't mine so I'm not familiar with it at all, and in its current state it drifts even with shorted ADC input and despite there being no detectable voltage across the short circuit (less than a few μV, certainly not a few % of full scale load cell output which was found to be several mV).

I'm not sure what's going on, there is a dumb off-the-shelf ADC chip and an unknown other IC, the ADC should simply be reporting constant readings and the controller ought to interpret them as "nothing changes". I suppose I could verify the ADC by scoping serial communications and maybe attach a load cell simulator made of resistors, recalibrate and see if the drift persists. But given that the load cell is dubious too I'm not sure if I want to bother debugging those electronics.