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Idiomas disponibles
Idiomas disponibles
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 DAQ970A/DAQ973A 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
(ΔR / R
) is measured. For best results, you should perform this reference measurement after the strain gage has
0
been mounted to the test body. The table below shows the resistance changes corresponding to 1 με strain for
common gage factor and unstrained gage resistance values.
Strain
1 με
1 με
1 με
Using the measurement scaling function (mX+b) with the equations shown below will allow you to display results
directly in strain on the DAQ970A/DAQ973A front-panel display. You can use a custom measurement unit to display
readings in "με" (micro-strain) directly. The instrument will automatically add the micro ("μ") prefix based upon
actual calculated values.
Temperature effects
The resistive element of a strain gage will produce a resistance change ΔR 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.
Keysight DAQ970A/DAQ973A User's Guide
GF
R
0
2.0
120 Ω
2.0
350 Ω
2.0
1000 Ω
4 Measurement Tutorials
) measurement from which strain
0
ΔR
DMM Sensitivity
0.24 mΩ
0.1 mΩ (0.4 με)
0.70 mΩ
1.0 mΩ (1.4 με)
2.0 mΩ
1.0 mΩ (0.5 με)
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