For a long time I've been trying to make a DIY milligram-accurate scale, and milligram-accurate strain load cells are expensive. Does anyone know if the resolution of this is high enough?
My dad built one with an electromagnetic coil many years ago, can't find the design, capacity was limited but had 100? microgram resolution, maybe similar to this applied science video https://www.youtube.com/watch?v=ta7nlkI5K5g but I think simpler.
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.
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.)
My uninformed opinion based on no experience is that this will creep so you should use a Kibble balance instead. (See sibling comment by s0rce.) Or maybe use a lever arm to amplify milligrams into tens of milligrams of force. Or just a regular pan balance. How big a weight do you want milligram precision on?
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?
Probably not, PCBs are also terrible as load cells because fibers break.
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.
s0rce|9 months ago
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.
kragen|9 months ago
colechristensen|9 months ago
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|9 months ago
(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.)
kragen|9 months ago
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?
dogma1138|9 months ago
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.
flowerthoughts|9 months ago
[1] https://www.mccsemi.com/pdf/ComponentWeightInformation.pdf
stefan_|9 months ago
They don't tend to use strain gauges I think.
sadhorse|9 months ago
klysm|9 months ago