Some questions about strain gauges

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

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Firstly, the resistivity is most definitely not a constant. Temperature is one factor - as the gauge heats up it expands, which it detects as a strain, and as the wires heat up their resistance which increase. When the gauge is under strain, one side (eg the "top") can deform more than the other side (eg the "bottom") and this can affect resistivity.

Determining Gauge Factor (GF):

GF = ( R1 / R2 ) / e

where: e is a known strain placed on the gauge R1 = change in resistance caused by this strain R2 = the undeformed resistance of the strain gauge

Hope this is helpful.

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