[colechristensen]

You can just buy strain gauges which are specially cut copper foils on a thin plastic substrate that you glue to something, like a metal strip. The resistance changes very slightly as the metal bends.

You measure the change in resistance with a wheatstone bridge tuned correctly.

You basically just need a strain gague (a few dollars), 4 resistors, an op-amp, and a microcontroller with an ADC.

Calibration is important and you'll run into things like the metal bar creeping, permanently bending as a result of weight being put on and off.

But also, milligram accurate scales are $20 on Amazon https://www.amazon.com/GRAM-PRES-Precision-Milligram-Reloadi...

[kragen]

Not every scale with milligram repeatability has milligram accuracy, and not every scale with milligram resolution readouts has milligram repeatability. You probably know that, but not everyone reading this does.

(This one might be fine? It does claim to have a 50.000g calibration weight, which is a good sign, but it doesn't say anything about metrological traceability, which is a bad sign.)

[dogma1138]

Precision and accuracy is very expensive there is a reason why high end measurement equipment costs as much as it does.

[kragen]

At some level it becomes expensive, but it's far from clear that five significant figures of mass is that level.

A US$2 quartz watch measures time to 5½ significant figures, US$10 multimeters routinely measure voltage to 4½, and US$5 GPS receivers can provide you with time measurements accurate to 40ns that inherit the drift of world metrology standards, a precision of 16 significant figures if you are measuring a long enough time interval (over 10 years).

[thirdhaf]

As a quick and dirty rule of thumb measuring parts per million in anything except time or frequency will get expensive. Temperature drifts will cause expansions and contractions on that order if you’re measuring lengths.

[kragen]

Yes, but in this case we're measuring weights, not lengths, and we're looking for 5 sig figs, not 6.

[userbinator]

Where are you finding $10 4.5 digit multimeters?

[kragen]

Hardware store, usually. 3.5 digits is common, 4.5 digits less so.

[userbinator]

3.5 digit ones sell for far less than that (especially the infamous 830-series based on the ICL7106 and its clones). I haven't found any 4.5 digit ones at that price.

[colechristensen]

>metrological traceability

It's a $20 scale, if you had need of metrology you wouldn't be buying a $20 scale. Most of us do not need metrology. I want to make small amounts of pickles with perhaps unreasonably precise measurements of salt at scales where half a gram or maybe a tenth of a gram is significant.

[kragen]

You said, "Milligram accurate scales are $20 on Amazon," and linked to that scale. I'm questioning whether it's really accurate to a milligram. It sounds like now you're questioning that, too. That's fine. But please don't pull this sour grapes bait and switch bullshit.