Measurement Computing CIO-EXP-GP User Manual

Page 42

APPENDIX

9.1 About Strain Gauges

9.1.1

What Are Strain Gauges?

A Strain Gauge is a variable resistance device whose resistance changes in proportion to the

amount it is stretched or compressed. Physically it is an etched metal-foil in a grid pattern that is
glued to any surface which undergoes strain. The output is a dimensionless quantity defined as
change in length and whose symbol is

. A micro-strain of “1” means that length of the surface

of interest has changed by 1 ppm. The ratio of resistance change to strain change is known as the
Gauge Factor (GF). Typical metal foil strain gauges have Gauge Factors of 2 to 2.1. This means
that the resistance will change twice as much as the strain does. A change of 1 micro-strain
means that the resistance of the strain gauge has changed by 2 ppm or .0002% (.0001% x 2). For
a 350 ohm strain gauge with GF = 2, a 1

µε

change results in a resistance change of:

Resistance change =

[SG Resistance]

[change in length]

[Gauge Factor]

[ 350 ohm ]

[ .000001]

[ 2 ]

0.0007 ohm

9.1.2

Specification of Strain Gauges

Metal Foil gauges are available in 120, 350 and 1000 ohms. Semiconductor strain gauges exist
and have resistance of up to 10000 ohms. They can readily be used with the CIO-EXP-GP, but
may not be as linear as metal foil gauges.

Maximum strain allowed is 3% to 5% depending on type and thickness of strain gauge material.

This means a limit of 30,000 to 50,000

µε

or a maximum resistance change of 6% to 10%.

Strain Gauges are typically used to calculate a change in strain; that is, the difference between the
unstrained and the strained state.

9.2 Reference Material for Application of Strain Gauges

The Bonded Electrical Resistance Strain Gage, First Edition

by William M. Murray and William R. Miller.

1992, 424 pages ISBN: 0-19-507209-X

Available from Society for Experimental Mechanics, order # OX-2.

Strain Gage Users’ Handbook , First Edition.

1992, 424 pages. ISBN: 0-912053-36-4