# Some questions about strain gauges

• posted

I am trying to figure out a couple questions involving strain gauges.

First of all, how is the gauge factor physically determined. Anyone know the process by which a company gets that. I am particularly using a Vishay strain gauge with a GF of 2.035, and indeed in practice most standard strain gauges have a GF of around 2. What makes it this value, and how is it determined? Wouldnt it make more sense to be unity?

The resistance of a grid of wire is fundamentally given by R=K*L/A, where K is the resistivity constant of the wire, L is the length of the grid wire, and A is the cross sectional area. What contributes more to the change in resistance of a strain gauge, the change in length of the wire of the grid as it is stretched or compressed, or the cross sectional area (and what exactly is the cross sectional area of a wire grid).

Also, in regards to the previous equation, how constant is the resistivity constant of the wire of a strain gauge, is it likely to change with strain or with any other factor?

I appreciate any help or answers anyone can give me. Thanks!

Brent Ellis

• posted

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1) How to measure GF? Either sample, or token stress each one, possibly. 2) What puts the GF around 2 usually? [but not always, it can be 3 or more...]

Here's a plausibility description.

The Gauge Factor is the factor of proportionality between a fractional change in resistance and a fractional change in length. In letters: (delta R / R) / (delta L / L)

If you imagine a strain gage stuck to a long metal strip which is stretched, you might expect the resistance to double if the metal strip doubled in length, because the strain gage legs are now twice as long, are they not?

But as Mr Poisson noticed, a stretch in one direction causes a shrinking in another direction. So the strain gage legs can grow thinner in an axis across from the tension, with the effect of increasing the resistance change to double the value you might have expected.

For constantan strain gages, the Poisson ratio is about 0.33 so doubling the length, 2 X L and squeezing the width to 0.67 its rest value keeping the thickness constant. The new resistance is 2 / 0.67 = 3 and deltaR/R = 2/1 = 2

[An aside: why didn't I change the thickness? The gauge is forced in the gluing plane so the third axis sees the result of both a stretch and a squeeze.]

Working out the gage factor: For double the length: deltaR / R = 2 deltaL / L = 1

Gauge Factor = (deltaR/R) / (deltaL/L) = 2/1 or about 2

This can vary with material, with temperature, with strain, and slightly from batch to batch, depending on heat treatment etc.

[This description is slightly idealized]

Brian Whatcott Altus OK

• posted

Brent-

I was doing lots of instrumentation & I wanted to really know how strain gauge load cells & their signal conditioning worked.

Unfortunately it's been years since I was really in to them, so the knowledge has faded but the knowledge of the sources remains..........

I even geeked out & bought THE book

and another

Vishay an AWESOME series of tech notes.......very valuable info from YEARS ago & they deal with your questions

There really is no shortcut to understanding strain gauges.......lot of reading & study required.

cheers Bob

• posted

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I liked this pointer specially, for the derivation of that X2 factor as a derivative.

Regards

Brian Whatcott Altus OK

• posted

Thanks for the information and advice. Indeed reading is what I have been doing much of, and unfortunately, I found the answers to my questions before I had a chance to read your response. Ironically, most of the links you provided I have stumbled upon and have been great sources of information. I appreciate your time and response.

Thanks! Brent

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