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Strain gauge made out of PCB

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Re: Strain gauge made out of PCB

#11

Post by kragen »

knotimpressed wrote: post #3

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 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?

Re: Strain gauge made out of PCB

#12

Post by kragen »

colechristensen wrote: post #5
knotimpressed wrote: post #3

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?

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…

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.)

Re: Strain gauge made out of PCB

#13

Post by kragen »

fake-name wrote: post #2

The challenge is that while you can make a strain gauge out of just about anything, making them repeatable over temperature, humidity (in the case of hygroscopic materials, like PCB FR4) and repeated flexing is where it gets difficult. 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 us…

I hadn't thought about the hygroscopic and expansion questions; I think FR4 is, like wood, almost immune to longitudinal variation with temperature and humidity due to its anisotropy. (But I haven't tried to measure that.) And strain gauges are low enough impedance that I'd expect the capacitive effects to matter.

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.

Re: Strain gauge made out of PCB

#14

Post by kragen »

s0rce wrote: post #6
knotimpressed wrote: post #3

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. 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 ol…

Thanks for the reference! If you do find the link maybe it will be in the WABAC machine.

Re: Strain gauge made out of PCB

#15

Post by dogma1138 »

kragen wrote: post #12
colechristensen wrote: post #5

Earlier quoted context omitted.

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…

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.)

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

Re: Strain gauge made out of PCB

#16

Post by dogma1138 »

knotimpressed wrote: post #3

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?

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.

Re: Strain gauge made out of PCB

#17

Post by kragen »

dogma1138 wrote: post #15
kragen wrote: post #12

Earlier quoted context omitted.

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.)

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

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).

Re: Strain gauge made out of PCB

#19

Post by userbinator »

kragen wrote: post #17
dogma1138 wrote: post #15

Earlier quoted context omitted.

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

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 lo…

Where are you finding $10 4.5 digit multimeters?
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