User`s guide
297
Chapter 7 Tutorial
Measurement Fundamentals
4
7
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/34972A 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
0
) measurement from which strain (R / R
0
) is measured. 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 strain for common gage
factor and unstrained gage resistance values.
Using the Mx+B scaling function with the equations shown below will
allow you to display results directly in strain on the 34970A/34972A
front-panel display. You can use a custom measurement label to display
readings in “” (micro-strain) directly. The instrument will
automatically add the micro (“”) prefix based upon actual calculated
values. For more information on scaling, see page 136.
M =
B = -
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.
Strain GF
R
0
R DMM Sensitivity
1 2.0 120 0.24 m 0.1 m (0.4 )
1 2.0 350 0.70 m 1.0 m (1.4 )
1 2.0 1000 2.0 m 1.0 m (0.5 )
1
GF R
0
----------------------
1
GF
--------
34970A Refresh UG.book Page 297 Wednesday, February 17, 2010 12:34 PM