# Some questions about strain gauges

1. Mar 7, 2007

### X1088LoD

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

2. Mar 12, 2007

### FredGarvin

Sorry for missing this. I just noticed this thread.

The gauge factor $$S_G$$ has a precise definition:

$$S_G = \frac{\frac{dR}{R}}{\frac{dL}{L}} = \frac{\frac{dR}{R}}{\epsilon_x}= S_A(1-\nu * K_T)$$

where $$S_A$$ = Strain sensitivity and $$K_T$$ = Transverse Sensitivity of the gauge.

and $$\epsilon_x$$ is the strain in the direction of the gauge.

In strain gauge theory you will see the term $$\frac{dR}{R}$$ all over the place. R is the gauge resistance.

Last edited: Mar 12, 2007
3. Mar 12, 2007

### Staff: Mentor

No, no. You didn't miss it before, I just moved it from the General Engineering forum to the more appropriate Mech & Aero forum a day or so ago. I usually leave a brief note when I move a thread, but I wanted to leave it with 0 replies to help it get more views.

Thanks for the help, Fred!

4. Mar 12, 2007

### FredGarvin

There are tons of things that effect the resistance of the strain gauge. The more accurate your readings need to be, the more things you have to account for. Some of the top hitters are:

-Temperature changes
-Bridge excitation voltages
-Adhesion quality
-Lead wire lengths

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