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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 HP 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 ( R / R
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
R
0
2.0
120
1
1
2.0
350
1
2.0
1000
Using the Mx+B scaling function with the equations shown below will
allow you to display results directly in strain on the HP 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
B
GF
x
R
0
Temperature Effects The resistive element of a strain gage will
produce a resistance change R due to measured strain as well as
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 to the
first gage, thus responding to local temperature changes but rejecting
strain changes. Subtracting measurements from the second gage will
remove any undesirable strain errors.
D
DMM Sensitivity
R
0.24 m
0.1 m (0.4
)
0.70 m
1.0 m (1.4
)
2.0 m
1.0 m (0.5
)
1
GF
) is measured.
0
strain for common
375
8
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