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Topic:- Strain GagesTopic:- Strain Gages
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2. Strain GagesStrain Gages
By Dr. Sotiris OmirouBy Dr. Sotiris Omirou

3. Strain-Strain-
gaugegauge
The electrical resistance strain is a metal wire or metal foilThe electrical resistance strain is a metal wire or metal foil
strip which is wafer-like and can be stuck onto surfaces likestrip which is wafer-like and can be stuck onto surfaces like
a postage stamp.a postage stamp.

4. Typical bonded strain gagesTypical bonded strain gages

5. When a strain gauge is bonded to an object, and theWhen a strain gauge is bonded to an object, and the
object changes in size, the resistance of the strainobject changes in size, the resistance of the strain
gauge will change. The resistance R is given by thegauge will change. The resistance R is given by the
expression:expression:
R=R= ρρ l/Al/A
Where:Where:
 l is the length of the wire in metersl is the length of the wire in meters
 ρρ is the resistivity of the material in ohm metersis the resistivity of the material in ohm meters
 A is the cross-sectional area of the filament in mA is the cross-sectional area of the filament in m22

6. When strain gauges measure the changing dimensions of anWhen strain gauges measure the changing dimensions of an
object, they are measuringobject, they are measuring strainstrain. Strain is the ratio of the. Strain is the ratio of the
change in dimension of an object to the original dimensionchange in dimension of an object to the original dimension
Mechanical strainMechanical strain εε == ΔΔL / LL / L
When subject to strain, its resistance R changes, theWhen subject to strain, its resistance R changes, the
fractional change in resistancefractional change in resistance ΔΔR/R being proportional toR/R being proportional to
the mechanical strain i.e.the mechanical strain i.e.
Electrical strainElectrical strain ΔΔR/R= G .R/R= G . ΔΔL/LL/L
where G is the gauge factor (1.8 – 2.2)where G is the gauge factor (1.8 – 2.2)
R varies between 50R varies between 50 ΩΩ and 2Kand 2KΩΩ

7. PotentialPotential
DividerDivider

8. Potential Divider with differential amplifierPotential Divider with differential amplifier

9. Wheatstone BridgeWheatstone Bridge
1
2
4
3
R
2
R
1
R
3
R
4
V
o
Vin
The four arms of the bridgeThe four arms of the bridge
circuit are formed by thecircuit are formed by the
resistance R1 to R4.resistance R1 to R4.
The corner points 1 and 4 areThe corner points 1 and 4 are
connected to the inputconnected to the input
voltage Vvoltage Vinin
The corner points 2 and 3 areThe corner points 2 and 3 are
connected to the outputconnected to the output
voltage Vo.voltage Vo.

10. Quarter strain gauge bridge with differential amplifierQuarter strain gauge bridge with differential amplifier

11. Half strain gauge bridge with differential amplifierHalf strain gauge bridge with differential amplifier

12. Full strain gauge bridge with differential amplifierFull strain gauge bridge with differential amplifier

13. Meter readingMeter reading
Interpretation of results
(Example 1)

14. (Example 2)
Meter readingMeter reading

15. Strain Gages - reviewStrain Gages - review
Strain = Elongation / Original LengthStrain = Elongation / Original Length
Change in length = Change in electricalChange in length = Change in electrical
resistanceresistance
Electrical Resistance change is very small,Electrical Resistance change is very small,
too small to be accurately measured usingtoo small to be accurately measured using
ordinary voltmetersordinary voltmeters

16. Wheatstone BridgeWheatstone Bridge
Converts a change inConverts a change in
electrical resistanceelectrical resistance
from a strain gage to afrom a strain gage to a
change in voltagechange in voltage
Changes in strain areChanges in strain are
linearly related to alinearly related to a
change in voltagechange in voltage
outputoutput
Voltage change is tooVoltage change is too
small to be accuratelysmall to be accurately
measured and for thismeasured and for this
reason an amplifier isreason an amplifier is
usedused