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Strain gauge made out of PCB

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Re: Strain gauge made out of PCB

#31

Post by colechristensen »

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.)

>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.

Re: Strain gauge made out of PCB

#32

Post by userbinator »

kragen wrote: post #21
userbinator wrote: post #19

Earlier quoted context omitted.

Where are you finding $10 4.5 digit multimeters?

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

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.

Re: Strain gauge made out of PCB

#33

Post by klysm »

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?

Milligram accuracy isn’t a really a direct property of the load cell. A lot of it comes down to creep and hysteresis behavior.

Re: Strain gauge made out of PCB

#34

Post by kragen »

userbinator wrote: post #32
kragen wrote: post #21

Earlier quoted context omitted.

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

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.

I might be misremembering. How much have you found 4½ digit multimeters to cost?

Re: Strain gauge made out of PCB

#35

Post by kragen »

colechristensen wrote: post #31
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.)

>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.

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.

Re: Strain gauge made out of PCB

#36

Post by 4gotunameagain »

kragen wrote: post #13
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…

For temperature dependent effects there are things like self-compensated strain gauges (where their temperature related deformation counteracts that of a target material), or dual side measurement (sandwich a bar between two strain gauges, strain cause compression on one, elongation on the other, so +-/-+, while temperature compresses or elongates both (++/--), cancels out.)

Re: Strain gauge made out of PCB

#37

Post by kragen »

4gotunameagain wrote: post #36
kragen wrote: post #13

Earlier quoted context omitted.

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…

For temperature dependent effects there are things like self-compensated strain gauges (where their temperature related deformation counteracts that of a target material), or dual side measurement (sandwich a bar between two strain gauges, strain cause compression on one, elongation on the other, so +-/-+, while temperature compresses or elongates both (++/--), cancels out.)

You might be able to do that kind of thing with traces on both sides of the FR4?

Re: Strain gauge made out of PCB

#38

Post by sadhorse »

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?

Use vibration to measure small masses. Measuring natural frequency can be very accurate and sensitive.

Re: Strain gauge made out of PCB

#39

Post by userbinator »

kragen wrote: post #34
userbinator wrote: post #32

Earlier quoted context omitted.

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.

I might be misremembering. How much have you found 4½ digit multimeters to cost?

Around $30 at the very, very least.
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