For this, while I'm sure it works, if the humidity and/or temperature changes, the same deflection will result in different readings.
If you can calibrate it immediately before each use, or you don't care about absolute values, this is a completely valid option.
"Real" strain gauges generally use a constantan resistive element to deal with the temperature variability, deposited on a plastic carrier film (typically polyimide). The film elements then get glued to the stress sensing member. They're fairly inexpensive too.
The temperature coefficient of resistance of the strain element seems like a concern, though, and so do thermal EMFs. My kitchen scale zeroes when I turn it on, a procedure that should be able to cancel one of these two but not both. Maybe you could have a diode thermal sensor, as an MCXO does, to measure the temperature so you can cancel both?
This project seems to do the first-order temperature correction thing:
> The included sample firmware will wait until a serial console is opened, perform a 5 second offset calibration, then sample continuously at the lowest gain setting. The graph.py script can be used to display the output.
> For high sensitivity measurements, it's important to let the board reach a stable operating temperature for at least 5 minutes before calibrating.
The thing I intuitively worry about here is creep. Does FR4 creep enough to worry about? Normally you make strain gauge bodies out of steel because it doesn't.
Where I mostly seen this approach is with 3D printers where we just want to know if the nozzle is touching the print base.
But if we can quantify the general worse case variation between most PCBs then maybe we can create a recommended strain sensor element with a semi-quantified level of accuracy so it's not just an on/off sense.
maybe something like this: https://www.erowid.org/archive/rhodium/chemistry/equipment/s... https://www.erowid.org/archive/rhodium/chemistry/equipment/s...
I found some old emails about the scale from 2005/6 but can't find the link and it would probably be gone anyways.
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...
(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.)
If you have some way to cut a precise shape out of some kind of metal sheet of well-controlled thickness, could you cut out a milligram precision bismar balance or steelyard?
In general if you want a precise and accurate strain gauge you’ll be paying a lot for it, especially for one that doesn’t need to be recalibrated before every use and after nearly every measurement.
[1] https://www.mccsemi.com/pdf/ComponentWeightInformation.pdf
They don't tend to use strain gauges I think.
https://www.hbkworld.com/en/knowledge/resource-center/articl...
Piezo vs. strain gauge https://www.kistler.com/US/en/piezo-vs.-strain-gauge/C000001...
For the same weight? I would expect the opposite.
The device effectively measures mechanical strain at the surface of the PCB. The surfaces of a thicker board will experience more strain from bending because the radius of the curve (at the surface) is greater.
