Agilent Technologies 34901A User Manual page 378

Chapter 8 Tutorial
Measurement Fundamentals
Making Strain Gage Measurements A Wheatstone bridge is
commonly used to enable instruments with low-sensitivity measuring
capabilities to measure small resistance changes common in strain
measurements. Instruments with high-resolution resistance measuring
capabilities, like the 34970A internal DMM , can directly measure small
resistance changes with high precision and linearity. You should also
use the 4-wire resistance method when measuring strain gages to
eliminate system cabling errors.
An initial, unstrained, gage resistance measurement is used as the
reference (R ) measurement from which strain (
0
For best results, you should perform this reference measurement after
the strain gage has been mounted to the test body. The table below
shows the resistance changes corresponding to 1
gage factor and unstrained gage resistance values.
Strain GF
2.0
µε
1
µε
1 2.0
µε
1 2.0
Using the Mx+B scaling function with the equations shown below will
allow you to display results directly in strain on the 34970A front-panel
display. You can use a custom measurement label to display readings in
"uE" (micro-strain) directly. The instrument will automatically add the
micro ("u") prefix based upon actual calculated values. For more
information on scaling, see page 119.
1
M =
GF x R
0
Temperature Effects The resistive element of a strain gage will
produce a resistance change
changes in gage temperature. This will create an "apparent" strain
change which is undesirable. A second gage of similar type can be used
to detect temperature changes and thus remove this error source.
You should mount the second gage in close proximity and at 90
first gage, thus responding to local temperature changes but rejecting
strain changes. Subtracting measurements from the second gage will
remove any undesirable strain errors.
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∆R
R DMM Sensitivity
0
Ω Ω
120 0.24 m 0.1 m
Ω Ω
350 0.70 m 1.0 m
Ω Ω
1000 2.0 m 1.0 m
1
−
B =
GF
∆
R due to measured strain as well as
∆
R / R ) is measured.
0
µε
strain for common
Ω µε
(0.4 )
Ω µε
(1.4 )
Ω µε
(0.5 )
°
to the
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